Attachment for a haircare appliance

Abstract

Disclosed is an attachment for a haircare appliance, the attachment comprising: a nozzle having a nozzle inlet configured to receive airflow from the haircare appliance when the attachment is attached to the haircare appliance, and a nozzle outlet having a central nozzle outlet bulk flow axis; and a collar disposed around the nozzle outlet, the collar comprising a collar inlet, a collar outlet, and a hollow interior extending from the collar inlet to the collar outlet, wherein at least part of the hollow interior forms a mixing zone disposed between the nozzle outlet and the collar outlet. The nozzle outlet has a perimeter having a total length P1. The nozzle outlet has a nozzle outlet cross-sectional area A1 in a reference plane in which the nozzle outlet lies, the reference plane orthogonal to the central nozzle outlet bulk flow axis. The collar encloses a collar cross-sectional area A2 in the reference plane. A perimeter ratio PR, defined as (I), is from 2.0 to 4.0. An area ratio AR, defined as (II), is from 0.2 to 0.5. Also disclosed is a haircare appliance.

Description

[0001]ATTACHMENT FOR A HAIRCARE APPLIANCE BACKGROUND Haircare appliances may be used to dry and / or style hair. It is desirable to increase drying and / or styling performance, whilst ensuring user comfort. Entrainment of ambient air into an airflow generated by a haircare appliance may provide a beneficial airflow to a user for drying and / or styling hair. SUMMARY A first aspect provides an attachment for a haircare appliance, the attachment comprising: a nozzle having a nozzle inlet configured to receive airflow from the haircare appliance when the attachment is attached to the haircare appliance, and a nozzle outlet having a central nozzle outlet bulk flow axis; and a collar disposed around the nozzle outlet, the collar comprising a collar inlet, a collar outlet, and a hollow interior extending from the collar inlet to the collar outlet, wherein at least part of the hollow interior forms a mixing zone disposed between the nozzle outlet and the collar outlet, wherein: the nozzle outlet has a perimeter having a total length P1; the nozzle outlet has a nozzle outlet cross-sectional area A1in a reference plane in which the nozzle outlet lies, the reference plane orthogonal to the central nozzle outlet bulk flow axis; the collar encloses a collar cross-sectional area A2 in thereference plane; a perimeter ratio PR, defined as ^^ ൌ^భோଶ^గ^భ, is from 2.0 to 4.0; and an arearatio AR, defined as ^^ ൌ^భோ^భା^మ, is from 0.2 to 0.5. In use of the attachment, when attached to the haircare appliance, airflow generated by the haircare appliance is received at the nozzle inlet and discharged into the mixing zone via the nozzle outlet. Movement of the airflow through the mixing zone towards the collar outlet draws ambient air into the mixing zone via the collar inlet, which mixes with the airflow to form an entrained airflow which is discharged from the collar outlet in a bulk direction along a central collar outlet bulk flow axis of the collar outlet. The central collar outlet bulk flow axis extends through a central point of the collar outlet, in a bulk flow direction of the entrained airflow at the collar outlet. The attachment results in airflow being delivered to a user via the collar outlet that has different characteristics to the airflow generated by the haircare appliance. Accordingly, the characteristics of airflow delivered to the user may be tailored by attaching the attachment to the haircare appliance to provide particular drying and / or styling benefits, without changing operation of the haircare appliance. The entrained airflow may have a greater momentum flux than the airflow generated by the haircare appliance because the mass of the entrained airflow is the sum of the airflow generated by the haircare appliance and the ambient air drawn into the collar via the collar inlet. Increased momentum flux may correlate to reduced hair drying time, for example because the entrained airflow discharged from the collar outlet, as opposed to the airflow discharged from the haircare appliance to the nozzle inlet, may more suitably agitate a tress of hair to enable the airflow to penetrate deeper into the tress of hair without unduly disturbing the hair. The entrained airflow may have a temperature that is between a temperature of the ambient air and a temperature of the airflow generated by the haircare appliance. Such a temperature may be more comfortable to a user than a temperature generated by the haircare appliance upstream of the nozzle inlet and / or more beneficial to drying and / or styling hair. The central nozzle outlet bulk flow axis extends through a central point of the nozzle outlet, in a bulk flow direction of the airflow generated by the haircare appliance at the nozzle outlet. The bulk flow direction is a general, or average, direction of airflow at the respective nozzle or collar outlet. A perimeter ratio from 2.0 to 4.0 has been found to provide suitable momentum flux of the entrained airflow discharged from the collar outlet for optimizing hair drying time, because a perimeter ratio in this range may result in increased entrainment of ambient air in the mixing zone, compared to a perimeter ratio outside this range. The perimeter ratio PR may more preferably be from 2.0 to 3.0. An area ratio from 0.2 to 0.5 has been found to provide suitable momentum flux of the entrained airflow discharged from the collar outlet for optimizing hair drying time, because an area ratio in this range may result in increased entrainment of ambient air in the mixing zone, compared to an area ratio outside this range. The area ratio AR may more preferably be from 0.35 to 0.5. The central collar outlet bulk flow axis may be parallel to, or co-axial with, the central nozzle outlet bulk flow axis. This may reduce kinetic energy losses between the nozzle outlet and the collar outlet, which in turn may reduce a hair drying time, because airflow with higher kinetic energy may be delivered to a user compared to an outlet in which the central collar outlet bulk flow axis is not parallel to, or not co-axial with, the central nozzle outlet bulk flow axis. The nozzle, in the reference plane, may have a height and a width orthogonal to the height, wherein the height is equal to the width. The collar, in a plane at the collar outlet and orthogonal to a central collar outlet bulk flow axis of the collar outlet, may have a height and a width orthogonal to the height, wherein the height is equal to the width. The nozzle outletmay have an effective nozzle radius The collar may have an effective collar radius ^ത^ଶ,^ത1 ^ଶ ൌ^^^ ^^ ^^^^ଶ^మA radius ratio RR, ^^ ൌ ோோതభ, is from 1.2 to 1.8. It has been found that, for a nozzle and collar that each have a respective equal height and width, varying the radius ratio may impact temperature distribution of air in the entrained airflow downstream of the collar outlet. Providing a radius ratio from 1.2 to 1.8 may provide a more homogeneous temperature distribution of air in the entrained airflow at a user plane compared to a radius ratio outside this range. The user plane is located 100mm downstream of the collar outlet orthogonal to the central collar outlet bulk flow axis of the collar outlet. A more homogeneous temperature distribution at the user plane may be associated with more homogeneous hair drying and / or improved user comfort by reducing the risk a user experiencing uncomfortably hot spots within the entrained airflow. If the collar encloses a collar cross-sectional area A2 in the reference plane that results in a radius ratio of more than 1.8, mixing of the airflow discharged from the nozzle outlet with the ambient air may primarily occur only at a perimeter of the airflow discharged from the nozzle outlet such that the entrained airflow has a more non-homogenous temperature distribution. The effective nozzle radius and the effective collar radius may be referred to an a mean radius of the nozzle and the collar, respectively, in the reference plane. The mean radius being an average radius about the central nozzle outlet bulk flow axis. Providing a radius ratio from 1.2 to 1.8 in combination with the perimeter ratio and area ratio within the defined ranges may provide increased entrainment of ambient air in the entrained airflow, which may dilute the airflow generated by the haircare appliance to a more comfortable temperature and / or thrust, and improved temperature distribution of air in the entrained airflow. In combination, such ratio ranges may provide suitable entrained airflow at the user plane for reducing hair drying time compared to other ratio ranges. In the reference plane, the nozzle may comprise peripheral nozzle peaks and peripheral nozzle troughs. Providing the peripheral nozzle peaks and the peripheral nozzle troughs may increase a length of the periphery of the nozzle outlet, which may increase an amount of airflow discharged from the nozzle outlet that shears across ambient air in the collar. In turn, this may increase, in the mixing zone, an amount of entrainment of ambient air with the airflow discharged from the nozzle outlet. In addition, a comparable amount of mixing may occur in the mixing zone over a shorter distance, compared to a nozzle without axial peripheral peaks and axial peripheral troughs, which may allow for a more compact attachment to be employed.Blockage, B, at the nozzle outlet may be defined as: ^^ ൌwherein NT is a thickness of the nozzle at the nozzle outlet. The peripheral nozzle peaks and the peripheral nozzle troughs may increase the perimeter of the nozzle at the nozzle outlet whilst decreasing the area of the nozzle outlet, thereby limiting any increase in blockage caused by increasing the perimeter of the nozzle at the nozzle outlet. This may help to increase an amount of entrainment of ambient air via the collar inlet. The nozzle may comprise a hollow interior extending between the nozzle inlet and the nozzle outlet. The peripheral nozzle peaks and the peripheral nozzle troughs may define the hollow interior at the nozzle outlet, in a direction orthogonal to the central nozzle outlet bulk flow axis. The peripheral nozzle peaks are local most-distal points of the periphery of the nozzle outlet from the central nozzle outlet bulk flow axis, in a direction orthogonal to the central nozzle outlet bulk flow axis. The peripheral nozzle troughs are local most-proximal points of the periphery of the nozzle outlet to the central nozzle outlet bulk flow axis, in a direction orthogonal to the central nozzle outlet bulk flow axis. Each peripheral nozzle peak may be at least 1mm, or at least 2mm, or at least 3mm further from the central nozzle outlet bulk flow axis than an adjacent peripheral nozzle trough, in the plane orthogonal to the central nozzle outlet bulk flow axis. The peripheral nozzle peaks and the peripheral nozzle troughs may be disposed around at least half of the periphery of the nozzle outlet, around at least 80% of the periphery of the nozzle outlet, or around a full extent of the periphery of the nozzle outlet. An increased amount of the periphery of the nozzle outlet around which the peripheral nozzle peaks and the peripheral nozzle troughs are disposed may be associated with increased entrainment of ambient air with the airflow discharged from the nozzle outlet, which in turn may provide a more even entrained airflow downstream of the collar outlet. The peripheral nozzle peaks may alternate with the peripheral nozzle troughs around the periphery of the nozzle outlet. This may provide a more even mixing, in the mixing zone, of the airflow discharged from the nozzle outlet and the ambient air drawn into the mixing zone via the collar inlet. In a plane orthogonal to the central nozzle outlet bulk flow axis, the peripheral nozzle peaks and the peripheral nozzle troughs may be comprised in one of: a sinusoidal wave, a triangular wave, a square wave, and a scalloped wave that extends around the periphery of the nozzle outlet. Such wave shapes may produce vortex pairs in the airflow in the mixing zone, the vortex pairs having opposite rotation directions to one another such that the vortices interact with each other to enhance mixing in the mixing zone. The peripheral nozzle peaks may individually alternate with the peripheral nozzle troughs around at least half of the periphery of the nozzle outlet, around at least 80% of the periphery of the nozzle outlet, or around a full extent of the periphery of the nozzle outlet. An increased amount of the periphery of the nozzle outlet around which the peripheral nozzle peaks and the peripheral nozzle troughs individually alternate may be associated with more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet. Peripheral nozzle peaks in an adjacent pair of the peripheral nozzle peaks may be separated from one another by the same distance as peripheral nozzle peaks in each other adjacent pair of the peripheral nozzle peaks, in a direction around the periphery of the nozzle outlet. Peripheral nozzle troughs in an adjacent pair of the peripheral nozzle troughs may be separated from one another by the same distance as peripheral nozzle troughs in each other adjacent pair of the peripheral nozzle troughs, in a direction around the periphery of the nozzle outlet. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to an arrangement in which the peripheral nozzle peaks and / or peripheral nozzle troughs are irregularly spaced around the periphery of the nozzle outlet. Each peripheral nozzle trough may be evenly spaced between two peripheral nozzle peaks in a direction around the central nozzle outlet bulk flow axis. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to an arrangement in which a peripheral nozzle trough is offset such that it is closer to one adjacent peripheral nozzle peak than another adjacent peripheral nozzle peak in the direction around the periphery of the nozzle outlet. The peripheral nozzle peaks and the peripheral nozzle troughs may be arranged in a repeating pattern around the periphery of the nozzle outlet. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to the peripheral nozzle peaks and peripheral nozzle troughs being arranged in a non-repeating pattern around the periphery of the nozzle outlet. The repeating pattern may extend around at least half of the periphery of the nozzle outlet, around at least 80% of the periphery of the nozzle outlet, or around a full extent of the periphery of the nozzle outlet. An increased amount of the periphery of the nozzle outlet around which the repeating patterns extends may be associated with more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet. The nozzle may comprise a same number of peripheral nozzle peaks as peripheral nozzle troughs. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to an arrangement with an uneven number of peripheral nozzle peaks and peripheral nozzle troughs. The nozzle may comprise at least four peripheral nozzle peaks. Providing at least four peripheral peaks may increase the length of the perimeter of the nozzle outlet by more than if a nozzle of comparable geometry but having fewer than four peripheral peaks is provided. The nozzle may comprise no more than ten peripheral nozzle peaks. Providing more than ten peripheral peaks may increase manufacturing complexity. Further, providing more than ten peripheral peaks may not provide additional beneficial characteristics of the entrained airflow downstream of the collar outlet compared to providing ten or fewer peripheral peaks. The nozzle may comprise axial nozzle peaks and axial nozzle troughs, wherein the axial nozzle peaks and the axial nozzle troughs are disposed around a periphery of the nozzle outlet, the axial nozzle peaks are separated from the axial nozzle troughs in a direction parallel to the central nozzle outlet bulk flow axis; and the reference plane is disposed along the central nozzle outlet bulk flow axis at a mean distance between the axial nozzle peaks and the axial nozzle troughs. Providing the axial nozzle peaks and the axial nozzle troughs may increase streamwise vorticity downstream of the nozzle outlet, which may increase a rate of mixing of the airflow discharged from the nozzle outlet and the ambient air drawn into the mixing zone via the collar inlet. In turn, this may increase an amount of ambient airflow that is drawn into the mixing zone via the collar inlet, and so increase a percentage of ambient air in the entrained airflow and thus an increased momentum flux of the entrained airflow. This may result in a reduced hair drying time compared to a nozzle without axial nozzle peaks and axial nozzle troughs. In addition, a comparable amount of mixing may occur in the mixing zone over a shorter distance, compared to a nozzle without axial nozzle peaks and axial nozzle troughs, which may allow for a more compact attachment to be employed. The axial nozzle peaks are local maximal end points of the nozzle around the periphery of the nozzle outlet, in a direction parallel to the central nozzle outlet bulk flow axis and towards the nozzle outlet. The axial nozzle troughs are local minimal end points of the nozzle around the periphery of the nozzle outlet, in a direction parallel to the central nozzle outlet bulk flow axis and towards the nozzle outlet. The axial nozzle peaks may be separated from the axial nozzle troughs by at least 1mm, or at least 2mm, or at least 3mm, in the direction along the central nozzle outlet bulk flow axis. The axial nozzle peaks and the axial nozzle troughs may be disposed around at least half of the periphery of the nozzle outlet, around at least 80% of the periphery of the nozzle outlet, or around a full extent of the periphery of the nozzle outlet. An increased amount of the periphery of the nozzle outlet around which the axial nozzle peaks and the axial nozzle troughs are disposed may be associated with the ambient air being drawn into the mixing zone via the air inlet more evenly around the periphery of the nozzle outlet, and with an increased rate of mixing being induced by the axial nozzle peaks and the axial nozzle troughs, which may provide more a homogenous entrained airflow. The axial nozzle peaks may individually alternate with the axial nozzle troughs. The axial nozzle peaks may individually alternate with the axial nozzle troughs around at least half of the periphery of the nozzle outlet, around at least 80% of the periphery of the nozzle outlet, or around a full extent of the periphery of the nozzle outlet. An increased amount of the periphery of the nozzle outlet around which the axial nozzle peaks and the axial nozzle troughs are disposed may be associated with increased streamwise vorticity downstream of the nozzle outlet around the periphery of the nozzle outlet. In turn, this may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet. Axial nozzle peaks of an adjacent pair of the axial nozzle peaks may be separated from one another by the same distance as axial nozzle peaks in each other adjacent pair of the axial nozzle peaks, in a direction around the periphery of the nozzle outlet. Axial nozzle troughs of an adjacent pair of the axial nozzle troughs may be separated from one another by the same distance as axial nozzle troughs in each other adjacent pair of the axial nozzle troughs, in a direction around the periphery of the nozzle outlet. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to an arrangement in which the axial nozzle peaks and / or axial nozzle troughs are irregularly spaced around the periphery of the nozzle outlet. Each axial nozzle trough may be evenly spaced between two axial nozzle peaks in a direction around the periphery of the nozzle outlet. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to an arrangement in which an axial nozzle trough is offset such that it is closer to one adjacent axial nozzle peak than another adjacent axial nozzle peak in the direction around the periphery of the nozzle outlet. The axial nozzle peaks and the axial nozzle troughs may be arranged in a repeating pattern around the periphery of the nozzle outlet. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to the axial nozzle peaks and axial nozzle troughs being arranged in a non-repeating pattern around the periphery of the nozzle outlet. The repeating pattern may extend around at least half of the periphery of the nozzle outlet, around at least 80% of the periphery of the nozzle outlet, or around a full extent of the periphery of the nozzle outlet. An increased amount of the periphery of the nozzle outlet around which the repeating pattern is disposed may be associated with more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet. Each of the axial nozzle peaks may lie in a first plane. Each of the axial nozzle troughs may lie in a second plane. The second plane may be parallel to the first plane. The second plane may be further from the collar outlet than the first plane in a direction along the central nozzle outlet bulk flow axis. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet than the axial nozzle peaks not lying in the same plane as one another, and the axial nozzle troughs not lying in the same plane as one another. The first plane may be separated from the second plane, in the direction along the central collar outlet bulk flow axis, by at least 1mm, at least 2mm, or at least 3mm. A greater distance may be associated with increased streamwise vorticity downstream of the nozzle outlet, which may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet. The first plane and the second plane may be orthogonal to the central nozzle outlet bulk flow axis, and the reference plane may be between the first plane and the second plane. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet than if the first and second planes are not orthogonal to the central nozzle outlet bulk flow axis. The nozzle, at the periphery of the nozzle outlet, may have a steepest angle of at least 45 degrees from a plane orthogonal to the central nozzle outlet bulk flow axis. A steepest angle of at least 45 degrees may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared having a steepest angle of less than 45 degrees. The nozzle, at the periphery of the nozzle outlet, may have a steepest angle of at least 60 degrees from a plane orthogonal to the central nozzle outlet bulk flow axis. A steepest angle of at least 60 degrees may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared having a steepest angle of less than 60 degrees. An angle between a plane orthogonal to the central nozzle outlet bulk flow axis, and a line tangential to a steepest section of the nozzle at the periphery of the nozzle outlet, in a direction along the central collar outlet bulk flow axis, may be at least 45 degrees or at least 60 degrees. This may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to a lesser angle. In an axial direction of the central nozzle outlet bulk flow axis, the axial nozzle peaks and the axial nozzle troughs may be comprised in one of: a sinusoidal wave, a triangular wave, a square wave, and a scalloped wave. Such wave shapes may produce vortex pairs in the airflow in the mixing zone, the vortex pairs having opposite rotation directions to one another such that the vortices interact with each other to enhance mixing in the mixing zone. The nozzle may comprise a same number of axial nozzle troughs as axial nozzle peaks. Providing a same number of axial nozzle troughs as axial nozzle peaks may provide more homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet compared to having a different number of axial nozzle troughs to axial nozzle peaks. The nozzle may comprise at least four axial nozzle peaks. Providing fewer than four axial nozzle peaks may not provide sufficiently homogenous streamwise vorticity downstream of the nozzle outlet to provide homogenous mixing, in the mixing zone, of the airflow generated by the haircare appliance with the ambient air drawn into the mixing zone via the collar inlet. Accordingly, entrained airflow delivered to a user downstream of the collar outlet may not be sufficiently homogenous to minimise hair drying time. The nozzle comprises no more than 16 axial nozzle peaks. Providing more than 16 axial nozzle peaks may not provide additional hair drying and / or styling characteristics of the entrained compared to 16 or fewer axial nozzle peaks. The collar outlet may have a central collar outlet bulk flow axis. The collar may comprise axial collar peaks and axial collar troughs, wherein the axial collar peaks and the axial collar troughs are disposed around a periphery of the collar outlet, and the axial collar peaks are separated from the axial collar troughs in a direction parallel to the central collar outlet bulk flow axis. Providing the axial collar peaks and the axial collar troughs may induce streamwise vorticity in the entrained airflow. This may, compared to a collar without axial collar peaks and axial collar troughs, further mix the airflow and the ambient air within the entrained airflow downstream of the collar outlet and / or increase divergence of the entrained airflow downstream of the collar outlet, which may reduce a drying time of hair. The axial collar peaks are local maximal end points of the collar around the periphery of the collar outlet, in a direction parallel to the central collar outlet bulk flow axis and towards the collar outlet. The axial collar troughs are local minimal end points of the collar around the periphery of the collar outlet, in a direction parallel to the central collar outlet bulk flow axis and towards the collar outlet. The axial collar peaks may be separated from the axial collar troughs by at least 1mm, or at least 2mm, or at least 3mm, in the direction along the central collar outlet bulk flow axis. The axial collar peaks and the axial collar troughs may be disposed around at least half of the periphery of the collar outlet, around at least 80% of the periphery of the collar outlet, or around a full extent of the periphery of the collar outlet. An increased amount of the periphery of the collar outlet around which the axial collar peaks and the axial collar troughs are disposed may be associated with increased mixing of the entrained airflow that is induced by the axial collar peaks and the axial collar troughs, which may provide more homogenous mixing within the entrained airflow. The axial collar peaks may alternate with the axial collar troughs around the periphery of the collar outlet. This may increase streamwise vorticity downstream of the collar outlet compared to other arrangements of the axial collar peaks relative to the axial collar troughs. The axial collar peaks may individually alternate with the axial collar troughs. This may provide a more homogenous entrained airflow downstream of the collar outlet and / or increase streamwise vorticity downstream of the collar outlet compared to the axial collar peaks and axial collar troughs not individually alternating. The axial collar peaks may individually alternate with the axial collar troughs around at least half of the periphery of the collar outlet, around at least 80% of the periphery of the collar outlet, or around a full extent of the periphery of the collar outlet. An increased amount of the periphery of the collar outlet around which the axial collar peaks and the axial collar troughs are disposed may be associated with a more homogenous entrained airflow downstream of the collar outlet and / or increase streamwise vorticity downstream of the collar outlet. Axial collar peaks in an adjacent pair of the axial collar peaks may be separated from one another by the same distance as axial collar peaks in each other adjacent pair of the axial collar peaks, in a direction around the periphery of the collar outlet. Adjacent axial collar troughs in an adjacent pair of the axial collar troughs may be separated from one another by the same distance as axial collar troughs in each other adjacent pair of the axial collar troughs, in a direction around the periphery of the collar outlet. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to an arrangement in which the axial collar peaks and / or axial collar troughs are irregularly spaced around the periphery of the collar outlet. Each axial collar trough may be evenly spaced between two axial collar peaks in a direction around the periphery of the collar outlet. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to an arrangement in which an axial collar trough is offset such that it is closer to one adjacent axial collar peak than another adjacent axial collar peak in the direction around the periphery of the collar outlet. The axial collar peaks and the axial collar troughs may be arranged in a repeating pattern around the periphery of the collar outlet. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to the axial collar peaks and axial collar troughs being arranged in a non-repeating pattern around the periphery of the collar outlet. The repeating pattern may extend around at least half of the periphery of the collar outlet, around at least 80% of the periphery of the collar outlet, or around a full extent of the periphery of the collar outlet. An increased amount of the periphery of the collar outlet around which the repeating patterns extends may be associated with a more homogenous entrained airflow downstream of the collar outlet. Each of the axial collar peaks may lie in a first plane. Each of the axial collar troughs may lie in a second plane. The second plane may be parallel to the first plane. The second plane may be closer to the nozzle outlet than the first plane in a direction along the central collar outlet bulk flow axis. This may provide a more homogenous entrained airflow downstream of the collar outlet than the axial collar peaks not lying in the same plane as one another, and the axial collar troughs not lying in the same plane as one another. The first plane may be separated from the second plane, in the direction along the central collar outlet bulk flow axis, by at least 1mm, at least 2mm, or at least 3mm. A greater distance may be associated with increased streamwise vorticity downstream of the collar outlet, which may provide a more homogenous entrained airflow downstream of the collar outlet. The first plane and the second plane may be orthogonal to the central collar outlet bulk flow axis. This may provide a more homogenous entrained airflow downstream of the collar outlet than if the first and second planes are not orthogonal to the central collar outlet bulk flow axis. The periphery of the collar outlet may have a steepest angle of at least 45 degrees from a plane orthogonal to the central collar outlet bulk flow axis. A steepest angle of at least 45 degrees may provide a more homogenous entrained airflow downstream of the collar outlet compared having a steepest angle of less than 45 degrees. The periphery of the collar outlet may have a steepest angle of at least 60 degrees from a plane orthogonal to the central collar outlet bulk flow axis. A steepest angle of at least 60 degrees may provide a more homogenous entrained airflow downstream of the collar outlet compared having a steepest angle of less than 60 degrees. An angle between a plane orthogonal to the central collar outlet bulk flow axis, and a line tangential to a steepest section of the periphery of the collar outlet, in a direction along the central collar outlet bulk flow axis, may be at least 45 degrees or at least 60 degrees. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to a lesser angle. In an axial direction of the central collar outlet bulk flow axis, the axial collar peaks and the axial collar troughs may be comprised in one of: a sinusoidal wave, a triangular wave, a square wave, and a scalloped wave. Such wave shapes may produce vortex pairs in the airflow in the mixing zone, the vortex pairs having opposite rotation directions to one another such that the vortices interact with each other to enhance mixing in the mixing zone. The collar may comprise a same number of axial collar troughs as axial collar peaks. Providing a same number of axial collar troughs as axial collar peaks may provide a more homogenous entrained airflow downstream of the collar outlet compared to having a different number of axial collar troughs to axial collar peaks. The collar may comprise at least four axial collar peaks. Providing fewer than four axial collar peaks may not provide sufficiently homogenous streamwise vorticity downstream of the collar outlet to provide a homogenous entrained airflow to a user. The collar may comprise no more than 16 axial collar peaks. Providing more than 16 axial collar peaks may not provide additional hair drying and / or styling characteristics of the entrained compared to 16 or fewer axial collar peaks. Around the periphery of the collar outlet, in a plane orthogonal to the central collar outlet bulk flow axis, the collar may comprise peripheral collar peaks and peripheral collar troughs. Providing the peripheral collar peaks and the peripheral collar troughs may induce streamwise vorticity in the entrained airflow. This may, compared to a collar without peripheral collar peaks and peripheral collar troughs, further mix the airflow and the ambient air within the entrained airflow downstream of the collar outlet and / or increase divergence of the entrained airflow downstream of the collar outlet, which may reduce a drying time of hair. The peripheral collar peaks are local most-distal points of the periphery of the collar outlet from the central collar outlet bulk flow axis, in a direction orthogonal to the central collar outlet bulk flow axis. The peripheral collar troughs are local most-proximal points of the periphery of the collar outlet to the central collar outlet bulk flow axis, in a direction orthogonal to the central collar outlet bulk flow axis. Each peripheral collar peak may be at least 1mm, or at least 2mm, or at least 3mm further from the central collar outlet bulk flow axis than an adjacent peripheral collar trough, in the plane orthogonal to the central collar outlet bulk flow axis. The collar may comprise a hollow interior extending between the collar inlet and the collar outlet. The peripheral collar peaks and the peripheral collar troughs may define the hollow interior at the collar outlet, in a direction orthogonal to the central collar outlet bulk flow axis. The peripheral collar peaks and the peripheral collar troughs may be disposed around at least half of the periphery of the collar outlet, around at least 80% of the periphery of the collar outlet, or around a full extent of the periphery of the collar outlet. An increased amount of the periphery of the collar outlet around which the peripheral collar peaks and the peripheral collar troughs are disposed may be associated with a more homogeneous entrained airflow being induced by the peripheral collar peaks and the peripheral collar troughs, which may provide a more even entrained airflow downstream of the collar outlet. The peripheral collar peaks may alternate with the peripheral collar troughs around the periphery of the collar outlet. This may provide a more homogenous entrained airflow downstream of the collar outlet. In a direction orthogonal to the central collar outlet bulk flow axis, the peripheral collar peaks and the peripheral collar troughs may be comprised in one of: a sinusoidal wave, a triangular wave, a square wave, and a scalloped wave. Such wave shapes may produce vortex pairs in the airflow in the mixing zone, the vortex pairs having opposite rotation directions to one another such that the vortices interact with each other to enhance mixing in the mixing zone. The peripheral collar peaks may individually alternate with the peripheral collar troughs. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to the peripheral collar peaks and peripheral collar troughs not individually alternating. The peripheral collar peaks may individually alternate with the peripheral collar troughs around at least half of the periphery of the collar outlet, around at least 80% of the periphery of the collar outlet, or around a full extent of the periphery of the collar outlet. An increased amount of the periphery of the collar outlet around which the peripheral collar peaks and the peripheral collar troughs individually alternate may be associated with a more homogenous entrained airflow downstream of the collar outlet. Peripheral collar peaks in an adjacent pair of the peripheral collar peaks may be separated from one another by the same distance as peripheral collar peaks in each other adjacent pair of the peripheral collar peaks, in a direction around the periphery of the collar outlet. Peripheral collar troughs in an adjacent pair of the peripheral collar troughs may be separated from one another by the same distance as peripheral collar troughs in each other adjacent pair of the peripheral collar troughs, in a direction around the periphery of the collar outlet. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to an arrangement in which the peripheral collar peaks and / or peripheral collar troughs are irregularly spaced around the periphery of the collar outlet. Each peripheral collar trough may be evenly spaced between two peripheral collar peaks in a direction around the central collar outlet bulk flow axis. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to an arrangement in which a peripheral collar trough is offset such that it is closer to one adjacent peripheral collar peak than another adjacent peripheral collar peak in the direction around the periphery of the collar outlet. The peripheral collar peaks and the peripheral collar troughs may be arranged in a repeating pattern around the periphery of the collar outlet. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to the peripheral collar peaks and peripheral collar troughs being arranged in a non-repeating pattern around the periphery of the collar outlet. The repeating pattern extends around at least half of the periphery of the collar outlet, around at least 80% of the periphery of the collar outlet, or around a full extent of the periphery of the collar outlet. An increased amount of the periphery of the nozzle outlet around which the repeating patterns extends may be associated with more homogenous entrained airflow downstream of the collar outlet. The collar may comprise a same number of peripheral collar peaks as peripheral collar troughs. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to an arrangement with an uneven number of peripheral collar peaks and peripheral collar troughs. The collar may comprise at least four peripheral collar peaks. Providing at least four peripheral peaks has been found to provide a more homogeneous entrained airflow downstream of the collar outlet. The collar may comprise no more than ten peripheral collar peaks. Providing more than ten peripheral collar peaks may increase manufacturing complexity. Further, providing more than ten peripheral collar peaks may not provide additional beneficial characteristics of the entrained airflow downstream of the collar outlet compared to providing ten or fewer peripheral collar peaks. The collar may have rotational symmetry at the collar outlet, about the central collar outlet bulk flow axis. This may provide a more homogenous entrained airflow downstream of the collar outlet compared to an arrangement that does not have rotational symmetry about the central collar outlet bulk flow axis. In a direction along the central collar outlet bulk flow axis and towards the collar outlet, the collar may taper inwardly to the peripheral collar troughs at the collar outlet. A taper of the collar towards the collar outlet may more gradually re-shape the entrained airflow as the entrained airflow moves towards the collar outlet. A taper, as opposed to an inward step of the collar to the peripheral collar troughs, may inhibit eddying within the collar and / or may exaggerate mixing of the entrained airflow. A hollow interior of the collar may be flared outwardly towards the collar inlet. Accordingly, the collar may approximate a ‘bell mouth’ at the collar inlet, which may draw ambient air into the mixing zone via the collar inlet in a more loss-less fashion than a collar that does not flare outwardly towards the collar inlet. A maximum cross-sectional area of the collar, in a direction orthogonal to the central collar outlet bulk flow axis, may be at the collar inlet. This may help to increase an amount of ambient air that is drawn into the mixing zone via the collar inlet. A cross-sectional area of the collar may decrease from the collar inlet towards the mixing zone without any step changes. This may help to accelerate ambient air as the ambient air travels from the collar inlet to the mixing zone. The attachment may be switchable between a first configuration, in which ambient air is permitted to enter the mixing zone via the collar inlet, and a second configuration in which ambient airflow is inhibited from entering the mixing zone via the collar inlet. This may increase the functionality of the attachment, by permitting selective amounts of entrainment of ambient air with the airflow generated by the haircare appliance depending on whether the attachment is in the first configuration or the second configuration. A second aspect provides a haircare appliance comprising an air inlet, an air outlet, an airflow generator to generate an airflow from the air inlet to the air outlet, and an attachment according to any one of the preceding claims, wherein, when attached to the haircare appliance, the nozzle inlet is configured to receive airflow discharged from the air outlet. A third aspect provides a haircare appliance comprising an air inlet, a nozzle having a nozzle outlet having a central nozzle outlet bulk flow axis. an airflow generator to generate an airflow from the air inlet to the air outlet, and a collar comprising a collar inlet, a collar outlet, and a hollow interior extending from the collar inlet to the collar outlet, wherein at least part of the hollow interior forms a mixing zone disposed within a hollow interior of the collar between the nozzle outlet and the collar outlet, wherein: the nozzle outlet has a perimeter having a total length P1; the nozzle outlet has a nozzle outlet cross-sectional area A1in a reference plane in which the nozzle outlet lies, the reference plane orthogonal to the central nozzle outlet bulk flow axis; the collar encloses a collar cross-sectional area A2in thereference plane; a perimeter ratio PR, defined as ^^ ൌ^భோଶ^గ^భ, is from 2.0 to 4.0; and an arearatio AR, defined as ^^ ൌ^భோ^భା^మ, is from 0.2 to 0.5. Optional features of aspects may be equally applied to other aspects, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view of a haircare appliance according to an example; Figure 2 is a schematic section through a main body of the haircare appliance of Figure 1; Figure 3 is a perspective view of a first example attachment of the haircare appliance; Figure 4 is a side view of the first example attachment of Figure 3; Figures 5 are 6 are an axial view of the first example attachment of Figure 3; Figure 7 is a cross-sectional side view of the first example attachment of Figure 3; Figures 8a is a partial view of a nozzle of the first example attachment of Figure 3, when the nozzle is unfurled to a two-dimensional shape; Figures 8b-8d are exemplary shapes of a nozzle of alternative attachments, when the nozzle is unfurled to a two-dimensional shape; Figure 9 is a perspective view of a second example attachment of the haircare appliance; Figure 10 is an axial view of the second example attachment of Figure 9; Figure 11 is a cross-sectional plan view of the second example attachment of Figure 9; Figure 12 is a partial view of a nozzle of the second example attachment of Figure 9, when the nozzle is unfurled to a two-dimensional shape; Figure 13 is a schematic cross-sectional axial view of the second example attachment of Figure 9; Figures 14 and 15 are schematic cross-sectional side views of the second example attachment of Figure 9, in first and second configurations, respectively; and Figures 16 and 17 are plan views of a third example attachment, in first and second configurations, respectively. DETAILED DESCRIPTION A drying response of a body of hair differs from cloth or other constrained fibre mats when dried with an air jet because hair strands in the body of hair are more mobile than fibres in cloth or mats, and can therefore respond dynamically to aerodynamic loads. Air jets with higher velocities tend to expose a greater surface area of wet hair to the jet by separation of clumped hair strands and by deeper penetration into the body of hair by the jet. The footprint of the jet should be chosen so that an active evaporation zone at the body of hair provides good coverage of the head but avoids inefficiency due to the edges of the jet having a weak interaction with the hair or missing the head entirely. When conventional round nozzles are used, the optimal balance of footprint, thermal power and momentum flux can be difficult to achieve or can lead to haircare appliance designs with poor ergonomics. The ergonomics of haircare appliances may be improved by positioning a centre of mass of the haircare appliance so that strain on the wrist and arm of a user is reduced in use. More compact and light-weight haircare appliances may contribute to improved ergonomics. This can be realised without compromising drying performance if the motor-compressor is operated at a high pressure and reduced mass flow, and heater power is maintained so that a momentum flux and thermal power of the jet is preserved. In such a configuration, a nozzle attached to an outlet of the haircare appliance must preserve an optimal jet footprint in the plane of the user, while the motor-compressor operates at higher pressure and reduced flow. A conventional round nozzle tends to produce a jet that is too concentrated and results in unacceptable thermal comfort for the user. Improvements in nozzles, as described hereinafter, improve drying performance and product ergonomics compared to conventional round nozzles. A haircare appliance 10 is shown in Figures 1 and 2 and is in the form of a hair dryer. The haircare appliance 10 comprises a main body 20 and a first attachment 100 removably attached to the main body 20. The main body 20 comprises a main power supply connection 22, a housing 30, an airflow unit 40, a heating unit 50 and a control unit 60 communicatively connected to the airflow unit 40 and the heating unit 50. The housing 30 is tubular in shape and comprises an inlet 31 through which an airflow is drawn into the housing 30 by the airflow unit 40, and an outlet 32 through which the airflow is discharged from the housing 30. The housing turns by around 90 degrees between the inlet 31 and the outlet 32 such that the airflow generated by the airflow unit 40 is discharged from the outlet 32 along a main body axis 1, which is generally normal to a longitudinal axis of a portion of the housing 20 comprising the air inlet 31. The airflow unit 40 is housed within the housing 30 and comprises an impeller 41 driven by an electric motor 42. The heating unit 50 is also housed within the housing 30 and comprises eight heating elements 51 to heat the airflow. Arrangements with greater or fewer heating elements are also envisaged. The control unit 60 receives power via the main power supply connection 22, and controls the airflow unit 40 and the heating unit 50. More specifically, the control unit 60 controls the flow rate of the airflow unit 40 and a heat setting of the heating unit 50 by controlling an amount of power supplied to the airflow unit 40 and the heating unit 50, respectively. The control unit 60 comprises user controls 61 and a control module 62. The user controls 61 are provided on an external surface of the housing 30 and are used to power on and off the hair styling appliance 10. The user controls 61 may also be used to select a flow rate (e.g., high, medium, low) of the airflow unit 40, and to select a heat setting (e.g., high, medium, low, off) of the heating unit 50, which the control module 62 modifies in response to inputs at the user controls 61. In this example, each of the user controls 61 comprises buttons. However, other forms of user control may be used such as slidable switches, dials or a touchscreen. The first attachment 100 is intended to alter characteristics of the airflow generated by the haircare appliance, as will be described herein. In this example, the first attachment 100 is removably attachable to the main body 20 around a periphery of the outlet 32, and will be described in more detail with reference to Figures 3 to 7. Accordingly, a user has different experiences using the haircare appliance 10 depending on whether the first attachment 100 is attached to the outlet 32 or not. The first attachment 100 comprises a nozzle 102 and a collar 104. The collar 104 is fixed in place relative to the nozzle 102 by three screws 105 such that the collar 104 is coaxial with the nozzle 102. The three screws 105 are disposed around a periphery of the nozzle 102. The nozzle 102 has a nozzle inlet 106 disposed at a first end 108 of the nozzle 102, and a nozzle outlet 110 disposed at a second end 112 of the nozzle 102. The second end 112 is opposite the first end 108. The nozzle 102 has a generally hollow interior 114 defining a flow path between the nozzle inlet 108 and the nozzle outlet 112. The nozzle 102 extends from the nozzle inlet 106 to the nozzle outlet 112 along a central nozzle axis 2. The hollow interior 114 converges between the nozzle inlet and the nozzle outlet. The first end 108 of the nozzle 102 is releasably attachable to the main body 20 around the periphery of the outlet 32. The first end 108 defines a recess 116 configured to receive an end of the main body 20 that defines the outlet 32. The central nozzle axis 2 is coaxial with the main body axis 1 when the first end 108 of the nozzle 102 is attached to the main body 20 around the periphery of the outlet 32. The first attachment 100 is held in place via magnetic elements 118, which are attracted to respective magnets (not shown) in the main body 20 that are disposed around the periphery of the outlet 32. It will be appreciated that any other suitable way of attaching the first attachment 100 to the main body 20 may be employed in other examples. As shown in Figure 5, the nozzle outlet 110 has a maximum height Hn and a maximum width Wn, which is orthogonal to the height Hn. The maximum height Hn and the maximum width are equal to one another and, in this example, have a magnitude of around 60mm. As illustrated in Figure 7, the second end 112 of the nozzle 102 has a non-linear form in a direction along the central nozzle axis 2, and has sixteen axial nozzle peaks 120 and sixteen axial nozzle troughs 122 that individually alternate with one another around a full extent of the periphery of the nozzle outlet 110. The axial nozzle peaks 120 each lie in a first nozzle plane 3 and the axial nozzle troughs each lie in a second nozzle plane 4. The first and second nozzle planes 3, 4 are orthogonal to the central nozzle axis 2. The axial nozzle peaks 120 are further from the first end 106 of the nozzle 102 than the axial nozzle troughs 122 and the first nozzle plane 3 is separated from the second nozzle plane 4 by a distance of around 4mm, in this example. The periphery of the nozzle outlet 110, when unfurled to a 2D shape, forms a sinusoidal wave, an example of which is partially shown in Figure 8a. The axial nozzle peaks 120 are peaks, or crests, of the sinusoidal wave, and the axial nozzle troughs 122 are troughs, or local minima, of the sinusoidal wave. The sinusoidal wave is a regular wave, such that: adjacent axial nozzle peaks 120 are each an equal distance from one another along the periphery of the nozzle outlet 110, adjacent axial nozzle troughs 122 are each an equal distance from one another along the periphery of the nozzle outlet 110, and each axial nozzle trough 122 is evenly spaced between two adjacent axial nozzle peaks 120. It will be appreciated that in other examples, the sinusoidal wave that the axial nozzle peaks 120 and the axial nozzle troughs 122 form may be irregular, and / or that the axial nozzle peaks 120 and the axial nozzle troughs 122 are, respectively, local maxima and local minima of a wave of a different shape, for example the square wave shown in Figure 8b which has axial nozzle peaks 120b and axial nozzle troughs 122b, the triangular wave shown in Figure 8c which has axial nozzle peaks 120c and axial nozzle troughs 122c, or the scalloped wave shown in Figure 8d which has axial nozzle peaks 120d and axial nozzle troughs 122d. As illustrated in Figures 3 and 6, around the periphery of the nozzle outlet 110, in a plane orthogonal to the central nozzle axis 2, the second end 112 of the nozzle 102 comprises eight peripheral nozzle peaks 124 and eight peripheral nozzle troughs 126 that individually alternate with one another around a full extent of the periphery of the nozzle outlet 110. Each of the peripheral nozzle troughs 126 are a first distance from the central nozzle axis 2, and each of the peripheral nozzle peaks 124 are a second distance, greater than the first distance, from the central nozzle axis 2, such that the peripheral nozzle peaks 124 are radially outward of the peripheral nozzle troughs 126. The hollow interior 114 of the nozzle 102 tapers inwardly in a direction along the central nozzle axis 2 towards the second end 112 of the nozzle 102 to form the peripheral nozzle troughs 126, as best shown in Figure 7. The second end 112 of the nozzle 102 has rotational symmetry about the central nozzle axis 2. The peripheral nozzle peaks 124 and the peripheral nozzle troughs 126 are evenly distributed around the central nozzle axis 2 such that the adjacent peripheral nozzle peaks 120 are each an equal angle about the central nozzle axis 2 from one another, adjacent peripheral nozzle troughs 122 are each an equal angle about the central nozzle axis 2 from one another, and each peripheral nozzle trough 122 is evenly spaced between two adjacent peripheral nozzle peaks 120. The nozzle 102 has eight radially-extending lobes at the nozzle outlet 110, as viewed in a plane orthogonal to the central nozzle axis 2. Each peripheral nozzle peak 124 is a peak, or crest, of a respective one of the eight radially-extending lobes. Each of the radially- extending lobes has the same geometry as each other radially-extending lobe. Each peripheral nozzle trough 126 is a local minimum radius of the nozzle 102 between adjacent radially-extending lobes. The nozzle 102 has a constant wall thickness around the periphery of the nozzle outlet 110 such that the peripheral nozzle peaks 124 and the peripheral nozzle troughs 126 result in the second end 112 of the nozzle 102 defining channels 128 and between adjacent peripheral nozzle peaks 124. Due to the inward taper of the nozzle 102 towards the second end 112 of the nozzle 102, the channels 128 have a length that extends in a direction along the central nozzle axis 2. The channels 128 are radially disposed between the nozzle 102 and the collar 104. The collar 104 has a collar inlet 130 disposed at a first end 132 of the collar 104, and a collar outlet 134 disposed at a second end 136 of the collar 104. The second end 136 is opposite the first end 134. The collar 104 has a hollow interior 138 defining a flow path between the collar inlet 130 and the collar outlet 134. The collar 104 extends from the collar inlet 130 to the collar outlet 134 along a central collar outlet bulk flow axis 6, which is coaxial with the central nozzle axis 2, and with the main body axis 1 when the first end 108 of the nozzle 102 is attached to the main body 20 around the periphery of the outlet 32. The hollow interior 138 of the collar 104 has a circular cross-section orthogonal to the central collar outlet bulk flow axis 6 along a complete length of the collar 104, extending from the collar inlet 130 to the collar outlet 134. At the collar inlet 130, the hollow interior 138 has a first diameter of around 80mm, and at the collar outlet 134, the hollow interior 138 has a second diameter of around 62mm. The diameter of the hollow interior 138 flares radially outwardly, relative to the central collar outlet bulk flow axis 6, towards the first end 132 of the collar 104 to an angle of around 45 degrees from the central collar outlet bulk flow axis 6, forming a generally ‘bell-mouth’-shaped hollow interior 138. The second end 136 of the collar 104 has a non-linear form in a direction along the central collar outlet bulk flow axis 6, and has eight axial collar peaks 140 and eight axial collar troughs 142 that individually alternate with one another around a full extent of the periphery of the collar outlet 134. The axial collar peaks 140 each lie in a first collar plane 7 and the axial collar troughs 142 each lie in a second collar plane 8, the first and second collar planes 7, 8 being orthogonal to the central collar outlet bulk flow axis 6. The axial collar peaks 140 are further from the first end 132 of the collar 104 than the axial collar troughs 142 and the first collar plane 7 is separated from the second collar plane 8 by a distance of around 5mm. The periphery of the collar outlet 134, when unfurled to a 2D shape, forms a sinusoidal wave similar to the unfurled nozzle outlet 110 that is partially shown in Figure 8a. The axial collar peaks 140 are peaks, or crests, of the sinusoidal wave, and the axial collar troughs 142 are troughs, or local minima, of the sinusoidal wave. The sinusoidal wave is a regular wave, such that: adjacent axial collar peaks 140 are each an equal distance from one another, adjacent axial collar troughs 142 are each an equal distance from one another, and each axial collar trough 142 is evenly spaced between two adjacent axial collar peaks 140. It will be appreciated that in other examples, the sinusoidal wave that the axial collar peaks 140 and the axial collar troughs 142 form may be irregular, and / or that the axial collar peaks 140 and the axial collar troughs 142 are, respectively, local maxima and local minima of a wave of a different shape. The collar 104 surrounds the second end 112 of the nozzle 102, so that the first end 132 of the collar 104 overlaps the nozzle 102 along the central collar outlet bulk flow axis 6 to an extent that the first end 132 of the collar 104 is closer to the first end 108 of the nozzle 102 than the axial nozzle troughs 122, in a direction along the central collar outlet bulk flow axis 6. In the first and second nozzle planes 3, 4, the collar 104 is radially separated from the second end 112 of the nozzle 102. The nozzle outlet 110 is separated from the collar outlet 134 by around 25mm, along the central collar outlet bulk flow axis 6. Between the nozzle outlet 110 and the collar outlet 134 is a mixing zone 144, which is a portion of the hollow interior 138 of the collar 104, and is bounded by the collar 104. The nozzle outlet 110 is taken as lying in a reference plane 9, which is positioned along the central nozzle axis 2 at a mean distance of the periphery of the nozzle outlet 110 along the central nozzle axis 2. The reference plane 9 is orthogonal to the central nozzle axis 2 and disposed between the first nozzle plane 3 and the second nozzle plane 4. In the reference plane 9, the nozzle outlet 110 has a perimeter having a total length P1. The perimeter is greater than a circle having a diameter equal to the height Hn and the width Wn of the nozzle 102 at the nozzle outlet 110 due to the peripheral nozzle peaks 124 and the peripheral nozzle troughs 126. In the reference plane 9, the nozzle outlet 110 has a nozzle outlet cross-sectional area A1. This is a cross-sectional area of the hollow interior 114 of the nozzle 102 in the reference plane 9. A perimeter ratio PR, defined as: around 2.5. In other examples, the perimeter ratio PRis from 2.0 to 5.0. In the reference plane 9, the collar 104 encloses a collar cross-sectional area A2. This is a cross-sectional area of the hollow interior 138 of the collar 104 in the reference plane 9. In this example, the collar cross-sectional area A2is circular and can be determined from an internal diameter of the collar 104 in the reference plane 9. An area ratio AR, defined as: ^^ ൌ^భோ^భା^మ, is around 0.4. In other examples, the area ratio ARis from 0.2 to 0.5. In the reference plane 9, the nozzle outlet 110 has an effective nozzle radius ^ത^^, which is the radius of a circle having the same area as the nozzle outlet cross-sectional area A1, ^ത1 ^^ ൌ^^^ ^^ ^^^^^^భIn the reference plane 9, the collar has an effective collar radius ^ത^ଶ, which is the radius of a circle having the same area as: the collar cross-sectional area A2less the nozzle outlet cross- sectional area A1, ^ത1 ^ଶ ൌ^^^ ^^ ^^^^ଶ^మA radius ratio RR, around 1.7. In other examples, the radius ratio RR is from 1.2 to 1.8. Blockage B, in the reference plane 9, is defined as: wherein NTis a thickness of the nozzle at the nozzle outlet. The peripheral nozzle peaks 124 and the peripheral nozzle troughs 126 increase the perimeter P1of the nozzle at the nozzle outlet 110 whilst decreasing the area A1of the nozzle outlet 110 in the reference plane 9, thereby limiting any increase in blockage caused by increasing the perimeter of the nozzle at the nozzle outlet. In use, the haircare appliance 10 works to dry and / or style hair by accelerating the evaporation of water from the surface of hair. It does this by converting electrical power received via the mains power supply connection 22 into aerodynamic power and, optionally, thermal power to produce a jet of air that is directed at the wet hair. This is achieved by the control unit 60 supplying power to the electric motor 42. This causes the impeller 41 to spin to draw air into the housing 30 via the inlet 31 to generate a primary airflow. The control unit 60 also selectively supplies power to the heater 50 to heat the primary airflow upstream of the outlet 32. Air in the primary airflow is discharged from the haircare appliance 10 as the jet of air that is directed at the wet hair. Attributes of the jet that influence hair drying are a footprint of the jet in a plane of the user, thermal power of the jet and momentum flux of the jet. The footprint is the effective cross-sectional area through which the jet flows through the plane of the user, which is orthogonal to a bulk direction of the jet at a distance from the downstream-most outlet of the haircare appliance 10 that is characteristic of normal use, typically 100mm. Momentum flux of the jet is a volume flow rate that is discharged from the haircare appliance 10. Use of the first attachment 100 may modify each of these attributes when compared to use of the haircare appliance 10 absent the first attachment 100, as will be described herein. In use, the primary airflow is discharged from the outlet 32 into the nozzle 102 via the nozzle inlet 106. The primary airflow travels in a direction along the central nozzle axis 2 through the hollow interior 114 of the nozzle 102, and is discharged from the nozzle 102 via the nozzle outlet 110. The hollow interior 114 converges between the nozzle inlet 106 and the nozzle outlet 110 such that a cross-sectional area of the nozzle 102 is smaller at the nozzle outlet 110 than at the nozzle inlet 106, which causes the primary airflow to accelerate as the primary airflow travels through the hollow interior 114 of the nozzle 102 towards the nozzle outlet 110. Movement of the primary airflow from the nozzle outlet 110 towards the collar outlet 134 draws ambient air into the mixing zone 144 via the collar inlet 130 and the channels 128. The ambient air mixes with the primary airflow in the mixing zone 144 to produce an entrained airflow that is discharged from the collar outlet 134 and forms the jet of air that is directed at the hair of the user. The primary airflow having accelerated towards the nozzle outlet 110 increases an amount of ambient air that is drawn into the mixing zone 144 via the collar inlet 130. The bell mouth shape of the hollow interior 138 of the collar 104 increases an amount of ambient air that is drawn into the mixing zone 144 compared to a shallower taper being provided at the first end 132 of the collar 104 because there are fewer losses associated with the bell mouth shape. The entrained airflow has a higher momentum flux than the primary airflow because the entrained airflow is the sum of the primary airflow and the entrained ambient air. The entrained airflow has a lower average temperature than the primary airflow because the ambient air dilutes the hotter primary airflow. Accordingly, the entrained airflow is more effective at drying hair and more comfortable to a user than the primary airflow. Comfort and drying performance is dependent on a ratio of primary airflow rate and ambient air flow rate, an amount of mixing within the mixing zone 144 and a velocity and a temperature profile of the entrained airflow. The area ratio, the perimeter ratio and the radius ratio determine the amount of mixing that occurs in the mixing zone 144. If these ratios are held within the aforementioned ranges in attachments according to examples, use of the haircare appliance 10 with the first attachment 100 provides an entrained airflow with an additional thrust compared to the primary airflow, and enhanced mixing of the primary airflow and the ambient air in the mixing zone 144, which both benefit drying performance. The peripheral nozzle peaks 124 and the peripheral nozzle troughs 126 re-shape the primary airflow to the shape of the nozzle outlet 110, which increases a length over which the primary airflow shears ambient airflow in the mixing zone 144, compared to a nozzle outlet 110 devoid of peripheral nozzle peaks 124 and peripheral nozzle troughs 126. This increases a distance over which the primary airflow shears ambient air, which, in turn, increases an amount of ambient air that is entrained in the primary airflow. The peripheral nozzle peaks 124 and the peripheral nozzle troughs 126 also enhance mixing of the primary airflow because ambient air entering the mixing zone 144 via the channels is radially inward of primary airflow entering the mixing zone 144 towards the peripheral nozzle peaks 124. In turn, this provides a more homogeneous jet to the user, for example with a more homogeneous temperature distribution, which in turn provides more effective drying. The axial nozzle peaks 120 and axial nozzle troughs 122 induce streamwise vorticity in the primary airflow downstream of the nozzle outlet 110. The streamwise vortices move through the primary airflow and induce mixing with the ambient air drawn into the mixing zone 144 via the collar inlet, which can help to provide a more homogenous entrained airflow. This also encourages more thorough mixing within a shorter length along the central collar outlet bulk flow axis 6, allowing a length of the collar 102 to be reduced whilst providing equivalent hair drying performance, compared to a longer attachment without axial nozzle peaks 120 and axial nozzle troughs 122. The repeating individually alternating pattern of the axial nozzle peaks 120 and axial nozzle troughs 122 in the sinusoidal wave extending around the full extent of the periphery of the nozzle outlet 110 induces pairs of streamwise vortices that rotate in opposite directions to one another, interacting with one another to increase mixing in the mixing zone 144 around the full extent of a periphery of the hollow interior 138 of the collar 104. The amplitude of the sinusoidal wave and the number of axial nozzle peaks and axial nozzle troughs determines a strength of the vortices produced and the spacing between vortices, which, in turn, determines an amount of mixing that occurs in the mixing zone 144. The axial collar peaks 140 and axial collar troughs 142 induce streamwise vorticity in the entrained airflow downstream of the collar outlet 134 in the same manner as described above with reference to the axial nozzle peaks 120 and axial nozzle troughs 122. This provides a more even temperature distribution within the jet of air that is delivered to the plane of the user. In addition, since the entrained airflow is not constrained within the collar 104, the streamwise vortices encourage divergence of the entrained airflow downstream of the collar outlet 134, which increases the cross-sectional area, or footprint, of the jet at the plane of the user. Figures 9 to 12 show another second attachment 200 according to an example. The second attachment 200 is also for selective attachment to the main body 20 of the haircare appliance of Figures 1 around the periphery of the outlet 32. The second attachment 200 has a nozzle 202 and a collar 204. Unlike the first attachment 100, the collar 204 of the second attachment 200 is movable relative to the nozzle 202 between a first position and a second position, as will be described hereinafter. The nozzle 202 has a nozzle inlet 206 disposed at a first end 208 of the nozzle 202, and a nozzle outlet 210 disposed at a second end 212 of the nozzle 102. The second end 212 is opposite the first end 208. The nozzle 202 has a hollow interior 214 defining a flow path between the nozzle inlet 206 and the nozzle outlet 110. The nozzle 202 extends from the nozzle inlet 106 to the nozzle outlet 112 along a central nozzle axis 2a. The nozzle inlet 206 is circular in cross-section, whereas the nozzle outlet 210 is slot-shaped, having a nozzle height Hn of around 6mmand a nozzle width Wn of around 60mm. The central nozzle axis 2a is coaxial with the main body axis 1 when the first end 208 of the nozzle 202 is attached to the main body 20 around the periphery of the outlet 32. The second attachment 200 is held in place via magnetic elements (not shown), which are attracted to the respective magnets (not shown) in the main body 20. It will be appreciated that any other suitable way of attaching the second attachment 200 to the main body 20 may be employed in other examples. The second end 212 of the nozzle 202 has a non-linear form in a direction along the central nozzle axis 2a, and has thirty-two axial nozzle peaks 220 and thirty-two axial nozzle troughs 222 that individually alternate with one another around a full extent of the periphery of the nozzle outlet 210. The axial nozzle peaks 220 each lie in a first nozzle plane 3a and the axial nozzle troughs 222 each lie in a second nozzle plane 4a, the first and second nozzle planes 3a, 4a being orthogonal to the central nozzle axis 2a. The axial nozzle peaks 220 are further from the first end 206 of the nozzle 202 than the axial nozzle troughs 222 and the first nozzle plane 3a is separated from the second nozzle plane 4a by a distance of around 3mm in this example. Adjacent axial nozzle peaks 220 are each an equal distance from one another, adjacent axial nozzle troughs 222 are each an equal distance from one another, and each axial nozzle trough 222 is evenly spaced between two adjacent axial nozzle peaks 220, around the periphery of the nozzle outlet 210. It will be appreciated that in other examples, the axial nozzle peaks 220 and the axial nozzle troughs 222 may be irregularly spaced. The shape of the nozzle 202 at the periphery of the nozzle outlet 210, which is formed by the axial nozzle peaks 220 and the axial nozzle troughs 222, has a maximum angle, or steepest angle, in a direction along the central nozzle axis 2. A line tangential to that maximum angle intersects the first nozzle plane 3a or the second nozzle plane 4a at an angle 5 of around sixty degrees, as best shown in Figure 12, which shows part of the second end 212 of the nozzle 202 when unfurled to a 2D shape. Around the periphery of the nozzle outlet 210, in a plane orthogonal to the central nozzle axis 2a, the second end 212 of the nozzle 202 comprises thirty peripheral nozzle peaks 224 and thirty peripheral nozzle troughs 226 that individually alternate with one along opposing sides of the nozzle outlet 220 along the width Wn of the nozzle outlet 210. Along each side of the nozzle outlet 220, adjacent peripheral nozzle peaks 224 are each an equal distance from one another, adjacent peripheral nozzle troughs 226 are each an equal distance from one another, and each peripheral nozzle trough 224 is evenly spaced between two adjacent peripheral nozzle peaks 226. It will be appreciated that in other examples, the peripheral nozzle peaks 224 and the peripheral nozzle troughs 226 may be irregularly spaced. The hollow interior 214 of the nozzle 102 tapers inwardly in a direction along the central nozzle axis 2 towards the second end 212 of the nozzle 202 to form the peripheral nozzle troughs 226. The collar 204 has a collar inlet 230 disposed at a first end 232 of the collar 104, and a collar outlet 234 disposed at a second end 236 of the collar 104. The second end 236 is opposite the first end 234. The collar 204 has a hollow interior 238 defining a flow path between the collar inlet 230 and the collar outlet 234. The collar 204 extends from the collar inlet 230 to the collar outlet 234 along a central collar outlet bulk flow axis 6a, which is coaxial with the central nozzle axis 2a, and with the main body axis 1 when the first end 208 of the nozzle 202 is attached to the main body 20 around the periphery of the outlet 32. The collar outlet 234 is slot-shaped, having a collar height Hc of around 8mm and a collar width Wc of around 62mm. The hollow interior 238 of the collar 204 flares outwardly, relative to the central collar outlet bulk flow axis 6, towards the first end 232 of the collar 204 to an angle of around 45 degrees from the central collar outlet bulk flow axis 6, forming a generally ‘bell mouth’-shaped hollow interior 238 at the first end 232 of the collar 204. Unlike the first attachment 100, the second end 236 of the collar 204 does not have axial collar peaks or axial collar troughs. It will be appreciated that axial collar peaks and axial collar troughs may be provided in other examples. The collar 204 surrounds the second end 212 of the nozzle 202, so that the first end 232 of the collar 204 overlaps the nozzle 202 along the central collar outlet bulk flow axis 6a. The nozzle 202 has two protruding strips 246 disposed on an outer surface of the nozzle 202 at respective opposing sides of the second end 212 of the nozzle 202 and the collar 204 has two channels 248 at a periphery of the hollow interior 238 that are each positioned to receive a respective one of the two protruding strips 246, as best shown in Figure 13. The two protruding strips 246 and the two channels 248 each extend in a direction parallel to the central nozzle axis 2a and the central collar outlet bulk flow axis 6a. Each of the two channels 248 comprises an end wall stop 249 at an end of the channel 248 that is most proximal to the collar inlet 230, to limit movement of the collar 204 away from the nozzle 202. Figures 14 and 15 show schematic a slice side view of the second attachment 200 with the collar 204 in the first position and in the second position, respectively. With the collar 204 in the first position shown in Figure 14, the second attachment 200 is in a first configuration, and with the collar 204 in the second position shown in Figure 15, the second attachment 200 is in a second configuration. With the collar 204 in the first position, the collar 204 is positioned relative to the nozzle 202 such that a gap 248 is present around the periphery of the nozzle outlet 210 between the nozzle 202 and the collar 204. In the first position, the mixing zone 244 is in fluid communication with ambient air via the collar inlet 230. With the collar 204 in the second position, the collar 204 is closer to the nozzle inlet 206 along the central collar outlet bulk flow axis 6, than in the first position. A surface of the collar 204 that defines part of the hollow interior 238 towards the first end 232 of the collar 204 contacts the outer surface of the nozzle 202 around a complete periphery of the nozzle 202 towards the second end 212 of the nozzle 202. Accordingly, there is no gap around the periphery of the nozzle outlet 210 between the nozzle 202 and the collar 204 the mixing zone 244 is not in fluid communication with ambient air via the collar inlet 230. In this example, the collar 204 is movable between the first and second positions by a user pushing or pulling the collar 204 relative to the nozzle 202, along the central collar outlet bulk flow axis 6. The collar 204 is bi-stable in the first and second positions. The two protrusions 246 comprise a magnetic material, and each end of each of the two channels 249 comprises a magnet (not shown) that attracts the magnetic material to hold the respective protrusion 246 at that end of the channel 249. It will be appreciated that any other suitable way to retain the collar 204 in the first position and / or second position may alternatively be employed. Regardless of whether the collar 204 is in the first position or the second position, the nozzle outlet 210 is separated from the collar outlet 234, along the central collar outlet bulk flow axis 6. Between the nozzle outlet 210 and the collar outlet 234 is a mixing zone 244, which is a portion of the hollow interior 238 of the collar 204. The nozzle outlet 210 is taken as lying in a reference plane 9a, which is a mean distance of the periphery of the nozzle outlet 210 along the central nozzle axis 2. The reference plane 9a is orthogonal to the central nozzle axis 2a and between the first nozzle plane 3a and the second nozzle plane 4a. In the reference plane 9a, the nozzle outlet 310 has a perimeter having a total length P1. The perimeter is greater than a circle having a diameter equal to the height Hn and the width Wn of the nozzle 202 at the nozzle outlet 210 due to the peripheral nozzle peaks 224 and the peripheral nozzle troughs 226. In the reference plane 9a, the nozzle outlet 210 has a nozzle outlet cross-sectional area A1. This is a cross-sectional area of the hollow interior 214 of the nozzle 102 in the reference plane 9. A perimeter ratio PR, defined as: around 4.0. In other examples, the perimeter ratio PRis from 2.0 to 5.0. In the reference plane 9a, with the collar 204 in the first position, the collar 204 encloses a collar cross-sectional area A2. An area ratio AR, defined as: around 0.25. In other examples, the area ratio AR is from 0.2 to 0.5. The second attachment 200, when attached to the outlet 32 of the main body 20, has dual functionality depending on whether the collar 204 is in the first position or the second position. With the collar 204 in the first position, the second attachment 200 is operable in much the same manner as the first attachment 100 described above with reference to Figures 3 to 7, and provides better hair drying efficiency than when the collar 204 is in the second position. This is because ambient air A is drawn into the mixing zone 244 via the collar inlet 230 by the primary airflow P, and an entrained airflow E is discharged from the collar outlet 234 as the jet that is delivered to the plane of the user. The entrained airflow E has a higher momentum flux and lower average temperature than the primary airflow P so that the entrained airflow E is more effective at drying hair and more comfortable to a user than the primary airflow P, as described previously. With the collar 204 in the second position, the second attachment 200 is operable as a more conventional concentrator attachment and provides better hair styling performance than when the collar 204 is in the first position. The shape of the nozzle outlet 210 causes the primary airflow to accelerate downstream of the outlet 32 so that the jet of air delivered to the plane of the user has a higher velocity than the primary airflow. With the collar 204 in the first position, more ambient air A from the collar inlet is entrained in the primary airflow P than with the collar in the second position, for the same primary airflow P characteristics. With the collar 204 in the first position, the airflow unit 40 operating at a power of 105W and the heating unit 50 operating on the ‘high’ heat setting, a total flow of around 48 L / s is delivered to the plane of the user, with the flow having a peak velocity of around 24 m / s and a peak temperature of around 53 degrees Centigrade. With the collar 204 in the second position, the airflow unit 40 operating at a power of 105W and the heating unit 50 operating on the ‘high’ heat setting, a total flow of around 23 L / s is delivered to the plane of the user, with the flow having a peak velocity of around 39 m / s and a peak temperature of around 80 degrees Centigrade. The axial and peripheral nozzle peaks and troughs 220, 222, 224, 226 help to mix air within the primary airflow in the mixing zone, which provides a more homogeneous jet or air to the plane of the user than if the axial and / or peripheral nozzle peaks and troughs 220, 222, 224, 226 were omitted. However, no ambient air is mixed with the primary airflow in the mixing zone 244 because the collar inlet 230 is closed off from the mixing zone 244. Accordingly, the airflow PM discharged from the collar outlet 234 is a more homogeneously mixed state of the primary airflow, which has a lower momentum flux and higher average temperature than the entrained airflow E that is discharged from the collar outlet 234 when the collar 204 is in the first position, for the same haircare appliance settings. An increased temperature of the jet that is delivered to the user plane may raise a temperature of the hair to a temperature at which the hair is more receptive to styling, compared to a hair temperature received when the second attachment 200 is in the first configuration. In other examples, the collar 204 is movable between the first and second positions by a user pushing the collar 204 relative to the nozzle 202, along the central collar outlet bulk flow axis 6. The collar 204 is biased to the first position by a spring (not shown) of the second attachment 200. The user pushes the collar 204 away from the first position, along the central collar outlet bulk flow axis 6 towards the nozzle 202, to the second position. A latch of the second attachment 200 releasably engages the collar 204 to hold the collar 204 in the second position. A subsequent push of the collar 204 away from the second position, along the central collar outlet bulk flow axis 6 towards the nozzle 202, causes the latch to release the collar 204, allowing the collar 204 to move along the central collar outlet bulk flow axis 6, in a direction away from the nozzle 202 under a biasing force of the spring, to the first position. In still other examples, the second attachment 200 comprises an electrically-powered actuator which receives electrical power from the main body 20, via an electrical interface, when the second attachment 200 is attached to the main body 20. Actuation of the electrically-powered actuator, for example in response to a user input at the user controls 61, causes the collar 204 to move between the first and second positions. Figures 16 and 17 show plan views of an third attachment 300 according to an example. The third attachment 300 is similar to the second attachment 200 described with reference to Figures 9 to 13. The third attachment 300 has a nozzle 302, a collar 304 and a sleeve 350. The nozzle 302 is generally the same as the nozzle 202 of the second attachment 200 described with reference to Figures 9 to 13 and like features have the same reference number but increased by 100. The collar 304 is similar to the collar 204 of the second attachment 200 described with reference to Figures 9 to 13, except that the collar 304 is fixed relative to the nozzle 302 in a position in which a mixing zone is in fluid communication with ambient air via a collar inlet 330 of the collar 304. Further, a hollow interior 338 at a first end 332 of the collar 304 does not flare outwardly. The sleeve 350 surrounds the collar 304 and is movable, in a direction along a central collar outlet bulk flow axis 6b, towards and away from the nozzle 302 to switch the third attachment 300 between a first configuration and a second configuration. The sleeve 350 defines two channels (not shown) on an internal surface thereof, and the collar 304 has two protrusions (not shown) on an outer surface thereof. The two channels receive the two protrusions, and end walls of each of the two channels limit movement of the sleeve 350 along the central collar outlet bulk flow axis 6b. A first position of the sleeve 350 is shown in Figure 16, in which the third attachment 300 is in the first configuration. The sleeve 350 is positioned relative to a nozzle inlet 230 of the nozzle 302 such that the mixing zone is in fluid communication with ambient air via the collar inlet 330. In the first position, the sleeve 350 does not overlap axial nozzle peaks 320 and axial nozzle troughs 322 disposed around a periphery of the nozzle outlet, along the central collar outlet bulk flow axis 6a. A second position of the sleeve 350 is shown in Figure 17, in which the third attachment 300 is in the second configuration. The sleeve 350 is closer to a nozzle inlet 306 of the nozzle 302 along the central collar outlet bulk flow axis 6a, than in the first position, and closes off the collar inlet 330 from the mixing zone. In the second position, the sleeve 350 overlaps the axial nozzle peaks 320 and the axial nozzle troughs 322 around the periphery of the nozzle outlet, along the central collar outlet bulk flow axis 6a. In this example, the sleeve 350 is movable between the first and second positions by a user pushing or pulling the sleeve 350 relative to the nozzle 302, along the central collar outlet bulk flow axis 6. Since the sleeve 350 is disposed around the outside of the collar 304, the sleeve 350 does not heat up, in use, to as high a temperature as the collar 304. Accordingly, moving the sleeve 350, as opposed to moving the collar 304 to switch the third attachment 300 between the first and second configurations is more comfortable for the user. The sleeve 350 is bi-stable in the first and second positions, as described above with reference to the collar 204 of the second attachment 200 described with reference to Figures 9 to 13. It will be appreciated that any other suitable way to retain the collar 204 in the first position and / or second position may alternatively be employed, for example with magnetic elements. With the sleeve 350 in the first position, the third attachment 300 is operable in generally the same manner as when the collar 204 of the second attachment 200 described with reference to Figures 9 to 13 is in the first position, and provides better hair drying efficiency than when the sleeve 350 is in the second position, as described previously. With the sleeve 350 in the second position, the third attachment 300 is operable in generally the same manner as when the collar 204 of the second attachment 200 described with reference to Figures 9 to 13 is in the second position, and provides better hair styling performance than when the sleeve 350 is in the first position, as described above. In other examples, the third attachment 300 comprises an electrically-powered actuator which receives electrical power, via an electrical interface, from the main body 20 when the third attachment 300 is attached to the main body 20. Actuation of the electrically-powered actuator, for example in response to a user input at the user controls 61, causes the sleeve 350 to move between the first and second positions. It will be appreciated that other example attachments may be provided which fall within the scope of the disclosure. For example, although not shown, the collar 104, 204, 304 of any of the described attachments 100, 200, 300 may also have peripheral collar peaks and troughs, akin to those described with reference to the peripheral nozzle peaks and troughs 124, 224, 126, 226. It will be appreciated that in other examples, the axial nozzle peaks 120, 220, 320 and the axial nozzle troughs 122, 222, 322 or the axial collar peaks 140 and the axial collar troughs 142 may be irregularly spaced from one another and / or may not extend around a full extent of the periphery of the nozzle outlet 110, 210, 310 or collar outlet 134, respectively. It will be appreciated that in other examples, the nozzle 102, 202, 302 may comprise a different number of axial nozzle peaks 120, 220 to axial nozzle troughs 220, 222. It will be appreciated that in other examples, the collar 104, 204, 304 may comprise a different number of axial collar peaks 140 to axial collar troughs 142. It will be appreciated that in other examples, the axial nozzle peaks 120, 220 may not all line in the same plane as one another and the axial nozzle troughs 122 may not all lie in the same plane as one another. It will be appreciated that in other examples, peaks and troughs of a wave of a different shape such as the square wave shown in Figure 8b, the triangular wave shown in Figure 8c, or the scalloped wave shown in Figure 8d may be formed by the axial nozzle peaks 120 and the axial nozzle troughs 122 or the axial collar peaks 140 and the axial collar troughs 142, and that the wave of a different shape may have a maximum angle of at least 45 degrees or at least 60 degrees to the central nozzle axis 2. It will be appreciated that a different component may be provided to selectively permit and inhibit ambient air from entering the mixing zone via the collar inlet 230, 330 and / or that the collar 204 or the sleeve 350 may be movable in a different direction to that described herein to switch the attachment between the first and second configurations.

Claims

CLAIMS 1. An attachment for a haircare appliance, the attachment comprising: a nozzle having a nozzle inlet configured to receive airflow from the haircare appliance when the attachment is attached to the haircare appliance, and a nozzle outlet having a central nozzle outlet bulk flow axis; and a collar disposed around the nozzle outlet, the collar comprising a collar inlet, a collar outlet, and a hollow interior extending from the collar inlet to the collar outlet, wherein at least part of the hollow interior forms a mixing zone disposed between the nozzle outlet and the collar outlet, wherein: the nozzle outlet has a perimeter having a total length P1; the nozzle outlet has a nozzle outlet cross-sectional area A1 in a reference plane in which the nozzle outlet lies, the reference plane orthogonal to the central nozzle outlet bulk flow axis; the collar encloses a collar cross-sectional area A2 in the reference plane; aperimeter ratio PR, defined as, is from 2.0 to 4.0; and an area ratio AR, defined as ^^ோfrom 0.2 to 0.5.

2. The attachment of claim 1, wherein, in the reference plane: the nozzle, in the reference plane, has a height and a width orthogonal to the height, wherein the height is equal to the width; the collar, in a plane at the collar outlet and orthogonal to a central collar outlet bulk flow axis of the collar outlet, has a height and a width orthogonal to the height, wherein the height is equal to the width; the nozzle outlet has an effective nozzle radius R1, ^ത^ ൌ1 ^ ^^^ ^^ ^^^^^^భthe collar has an effective collar radius R2, ^ത1 ^ଶ ൌ^^^ ^^ ^^^^ଶ^మand a radius ratio RR, ^^ோ ൌis from 1.2 to 1.

8.

3. The attachment according to claim 1 or claim 2, wherein, in the reference plane, the nozzle comprises peripheral nozzle peaks and peripheral nozzle troughs.

4. The attachment of claim 3, wherein the peripheral nozzle peaks individually alternate with the peripheral nozzle troughs around a full extent of the periphery of the nozzle outlet.

5. The attachment of claim 3 or claim 4, wherein peripheral nozzle peaks in an adjacent pair of the peripheral nozzle peaks are separated from one another by the same distance as peripheral nozzle peaks in each other adjacent pair of the peripheral nozzle peaks, in a direction around the periphery of the nozzle outlet, and peripheral nozzle troughs in an adjacent pair of the peripheral nozzle troughs are separated from one another by the same distance as peripheral nozzle troughs in each other adjacent pair of the peripheral nozzle troughs, in a direction around the periphery of the nozzle outlet.

6. The attachment of any one of claims 3 to 5, wherein each peripheral nozzle trough is evenly spaced between two peripheral nozzle peaks in a direction around the central nozzle outlet bulk flow axis.

7. The attachment of any one of claims 3 to 6, wherein the peripheral nozzle peaks and the peripheral nozzle troughs are arranged in a repeating pattern around the periphery of the nozzle outlet.

8. The attachment of claim 7, wherein the repeating pattern extends around a full extent of the periphery of the nozzle outlet.

9. The attachment of any one of claims 3 to 8, wherein the nozzle comprises a same number of peripheral nozzle peaks as peripheral nozzle troughs.

10. The attachment of any one of claims 3 to 9, wherein one or more of: the nozzle comprises at least four peripheral nozzle peaks; andthe nozzle comprises no more than ten peripheral nozzle peaks.

11. The attachment of any one of the preceding claims, wherein: the nozzle comprises axial nozzle peaks and axial nozzle troughs; the axial nozzle peaks and the axial nozzle troughs are disposed around a periphery of the nozzle outlet; the axial nozzle peaks are separated from the axial nozzle troughs in a direction parallel to the central nozzle outlet bulk flow axis; and the reference plane is disposed along the central nozzle outlet bulk flow axis at a mean distance between the axial nozzle peaks and the axial nozzle troughs.

12. The attachment of claim 11, wherein the axial nozzle peaks individually alternate with the axial nozzle troughs around a full extent of the periphery of the nozzle outlet.

13. The attachment of any one of claim 11 or claim 12, wherein axial nozzle peaks of an adjacent pair of the axial nozzle peaks are separated from one another by the same distance as axial nozzle peaks in each other adjacent pair of the axial nozzle peaks, in a direction around the periphery of the nozzle outlet, and axial nozzle troughs of an adjacent pair of the axial nozzle troughs are separated from one another by the same distance as axial nozzle troughs in each other adjacent pair of the axial nozzle troughs, in a direction around the periphery of the nozzle outlet.

14. The attachment of any one of claims 11 to 13, wherein each axial nozzle trough is evenly spaced between two axial nozzle peaks in a direction around the periphery of the nozzle outlet.

15. The attachment of any one of claims 11 to 14, wherein the axial nozzle peaks and the axial nozzle troughs are arranged in a repeating pattern around the periphery of the nozzle outlet.

16. The attachment of claim 15, wherein the repeating pattern extends around a full extent of the periphery of the nozzle outlet.

17. The attachment of any one of claims 11 to 16, wherein: each of the axial nozzle peaks lie in a first plane and each of the axial nozzle troughs lie in a second plane; and the second plane is parallel to the first plane and is further from the collar outlet than the first plane in a direction along the central nozzle outlet bulk flow axis.

18. The attachment of claim 17, wherein the first plane and the second plane are orthogonal to the central nozzle outlet bulk flow axis, and the reference plane is between the first plane and the second plane.

19. The attachment of any one of claims 11 to 18, wherein the nozzle, at the periphery of the nozzle outlet, has a steepest angle of at least 45 degrees from a plane orthogonal to the central nozzle outlet bulk flow axis.

20. The attachments of any one of claims 11 to 19, wherein, in an axial direction of the central nozzle outlet bulk flow axis, the axial nozzle peaks and the axial nozzle troughs are comprised in one of: a sinusoidal wave, a triangular wave, a square wave, and a scalloped wave.

21. The attachment of any one of the preceding claims, wherein the collar outlet has a central collar outlet bulk flow axis, and the collar comprises axial collar peaks and axial collar troughs, wherein the axial collar peaks and the axial collar troughs are disposed around a periphery of the collar outlet, and the axial collar peaks are separated from the axial collar troughs in a direction parallel to the central collar outlet bulk flow axis.

22. The attachment of any one of the preceding claims, wherein the collar outlet has a central collar outlet bulk flow axis, and around the periphery of the collar outlet, in a plane orthogonal to the central collar outlet bulk flow axis, the collar comprises peripheral collar peaks and peripheral collar troughs.

23. The attachment of any one of the preceding claims, wherein the attachment is switchable between a first configuration, in which ambient air is permitted to enter the mixing zone via the collar inlet, and a second configuration in which ambient airflow is inhibited from entering the mixing zone via the collar inlet.

24. A haircare appliance comprising an air inlet, an air outlet, an airflow generator to generate an airflow from the air inlet to the air outlet, and an attachment according to any one of the preceding claims, wherein, when attached to the haircare appliance, the nozzle inlet is configured to receive airflow discharged from the air outlet.

25. A haircare appliance comprising an air inlet, a nozzle having a nozzle outlet having a central nozzle outlet bulk flow axis, an airflow generator to generate an airflow from the air inlet to the air outlet, and a collar comprising a collar inlet, a collar outlet, and a hollow interior extending from the collar inlet to the collar outlet, wherein at least part of the hollow interior forms a mixing zone disposed within a hollow interior of the collar between the nozzle outlet and the collar outlet, wherein: the nozzle outlet has a perimeter having a total length P1; the nozzle outlet has a nozzle outlet cross-sectional area A1 in a reference plane in which the nozzle outlet lies, the reference plane orthogonal to the central nozzle outlet bulk flow axis; the collar encloses a collar cross-sectional area A2 in the reference plane; aperimeter ratio PR, defined as ^^ ൌ^భோଶ^గ^భ, is from 2.0 to 4.0; and an area ratio AR, defined as ^^ ൌ^భோ^భା^మ, is from 0.2 to 0.5.

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