Vehicle comprising a drag reduction system

Abstract

The present disclosure relates to a drag reduction system (1) for a vehicle (5). The drag reduction system (1) comprises a cylinder (11) and a drive system configured to drive rotation of the cylinder (11) about its longitudinal axis. Also disclosed is a vehicle (5) comprising a vehicle body (517) and the drag reduction system (1). The vehicle body (517) comprises a rear face (518) and the cylinder (11) is arranged behind the rear face (518). The drive system may be configured to drive rotation of the cylinder (11) such that the velocity ratio of the cylinder (11) is greater than 2.0. The drive system may comprise a turbine impeller configured to be driven by a surrounding fluid to drive rotation of the cylinder (11). The drag reduction system 1 may comprise a guide vane arranged to guide a surrounding fluid towards the cylinder (11) in use. Also disclosed is a method of operating a drag reduction system 1 for a vehicle (5).

Description

[0001] VEHICLE COMPRISING A DRAG REDUCTION SYSTEM

[0002] TECHNICAL FIELD

[0003] The present invention relates to a drag reduction system for a vehicle, a vehicle comprising a drag reduction system, and a method of operating a drag reduction system for a vehicle.

[0004] BACKGROUND

[0005] Drag is defined as a force acting opposite to the relative motion of an object with respect to a surrounding fluid. For example, a vehicle is subject to drag when it is moving through air or water. The greater the drag on a vehicle, the greater the motive force required to overcome the drag, and therefore the greater the amount of energy required to produce the motive force. It is desirable to reduce the overall amount of energy used by a vehicle, for example to reduce the environmental impact of the vehicle and to make the vehicle more economic to operate. It is therefore desirable to reduce the drag force acting on a vehicle in motion.

[0006] The problem of reducing drag is particularly significant in the field of commercial vehicles. Such vehicles include vans, lorries, trailers, and buses. These vehicles typically have aerodynamically unfavourable body shapes, including a non -streamlined front face, relatively sharp edges, and a flat vertical rear face. It is perceived wisdom that the biggest contributor to the total drag on a typical commercial vehicle is the rear face of the vehicle. The large, substantially vertical, and substantially flat rear face creates an area of low pressure behind the vehicle during travel, which results in a force that acts opposite to the direction of motion of the vehicle.

[0007] There is a need to address the problem of how to reduce the contribution of the rear face of a vehicle to the total drag on the vehicle by reducing the area of low pressure behind the vehicle during travel.

[0008] SUMMARY

[0009] A first aspect provides a drag reduction system for a vehicle, comprising a cylinder and a drive system configured to drive rotation of the cylinder about its longitudinal axis. A second aspect provides a vehicle comprising a vehicle body and the drag reduction system of the first aspect. The vehicle body comprises a rear face. The cylinder of the drag reduction system is arranged behind the rear face of the vehicle body.

[0010] The aspects and examples described herein have been found by the inventors to significantly reduce drag on a vehicle as a result of the area of low pressure behind the vehicle during travel, thereby significantly reducing the overall drag on the vehicle and increasing the overall energy efficiency of the vehicle. When the vehicle is in motion, surrounding fluid attaches to the circumferential surface of the cylinder and rotation of the cylinder forces the attached fluid towards the rear face of the vehicle body. This causes an increase in pressure in the area behind the rear face of the vehicle body, thereby reducing drag caused by the area of low pressure behind the rear face of the vehicle body. The effect is caused by a combination of the Magnus and Coanda effects resulting from rotation of the cylinder, as will be understood by those skilled in the art.

[0011] The following statements may apply to either the first or second aspect as appropriate.

[0012] The drive system may be configured to drive rotation of the cylinder in a direction of rotation which causes the cylinder to entrain the surrounding fluid when the vehicle is in motion to inhibit recirculation of the surrounding fluid at the rear of the vehicle. This in turn causes an increase in pressure in the area behind the rear face of the vehicle body, thereby reducing drag caused by the area of low pressure behind the rear face of the vehicle body.

[0013] The drive system may be configured to drive rotation of the cylinder in a clockwise direction of rotation as viewed from the left-hand side of the vehicle with respect to a normal forward direction of travel of the vehicle.

[0014] A velocity ratio of the cylinder may be defined as a ratio of the magnitude of the tangential velocity of the cylinder to the magnitude of the freestream velocity of a surrounding fluid when the vehicle is in motion. The drive system may be configured to drive rotation of the cylinder such that the velocity ratio of the cylinder is greater than 2.0, preferably within the range of 2.5-7.0, preferably greater than 3.0, and more preferably within the range of 3.0-5.0. The drive system may comprise any suitable means to drive rotation of the cylinder to achieve any of the values of velocity ratio described herein . The drive system may comprise a passive drive system, for example comprising one or more turbines as described further below, or an active drive system, for example comprising one or more electric motors, a flow sensor, and a controller as described further below. In other examples, the drive system may comprise a transmission for transmitting torque from a powertrain of a vehicle on which the drag reduction system is installed in use to the cylinder to drive rotation of the cylinder, with the transmission configured to achieve any of the values of the velocity ratio of the cylinder described herein.

[0015] The skilled person will appreciate that the velocity ratio of the cylinder, in use, can be directly and positively verified, for example by measuring the angular velocity of the cylinder using a suitable sensor, calculating the tangential velocity using the measured angular velocity, measuring the freestream velocity of a surrounding fluid using a suitable sensor, such as a pitot tube, and determining the velocity ratio using the calculated tangential velocity of the cylinder and the measured freestream velocity of the surrounding fluid.

[0016] The inventors have found that velocity ratios of greater than 2.0 are particularly beneficial. The inventors have found that velocity ratios of less than 2.0 may mean that the cylinder does not effectively entrain the surrounding fluid when the vehicle is in motion and is therefore ineffective in reducing drag as described above.

[0017] The drive system may be configured to drive rotation of the cylinder such that the velocity ratio of the cylinder is equal to 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, or 9.5, or within a range defined between any of these values.

[0018] The drive system may be configured to drive rotation of the cylinder such that the velocity ratio of the cylinder according to any value described herein is achieved when the freestream velocity of the surrounding fluid is above a predetermined threshold. The predetermined threshold may be equivalent to the freestream velocity of the surrounding fluid of a vehicle when the vehicle is travelling at 0 m / s, 2 m / s, 5 m / s, 10 m / s, 20 m / s, or 30 m / s, or at a speed within a range defined between any two of these values . The drive system may be configured to drive rotation of the cylinder such that the velocity ratio of the cylinder according to any value described herein is achieved at any value of freestream velocity equivalent to the freestream velocity of the surrounding fluid of a vehicle when the vehicle is travelling at any given speed, such as any given speed up to the maximum speed of the vehicle.

[0019] The drag reduction system may be arranged such that the cylinder protrudes beyond an edge of the rear face of the vehicle body in a radial direction by a maximum distance, H. The radial direction may be parallel to the rear face of the vehicle body. H may be greater than or equal to 1 / 6D, where D is the diameter of the cylinder. H may be less than or equal to 1 / 2D. H may be equal to 1 / 2D, 1 / 3D, 1 / 6D, or 0D, or may be within a range defined between any of these values. The distance H may be measured beyond the edge of the rear face, between the edge and the furthest point on the circumference of the cylinder in a radial direction. The radial direction may be parallel to the rear face of the vehicle body. An arc may be defined by the circumference of the cylinder beyond the edge and a chord defined by a line extending from the edge perpendicularly to the rear face of the vehicle body, wherein the distance H is equal to the length of the sagitta of the arc beyond the edge.

[0020] The cylinder may be arranged horizontally. The cylinder may be arranged across the back of the vehicle body in a spanwise direction. The cylinder may be arranged to extend between the side edges of the rear face of the vehicle body.

[0021] The cylinder may be arranged proximate to a top edge of the rear face of the vehicle body. The cylinder may be arranged such that the longitudinal axis of the cylinder is closer to the top edge than a lower edge of the rear face of the vehicle body.

[0022] The cylinder may not protrude above the top edge of the rear face of the vehicle body. The top of the cylinder may be aligned with the top edge. A horizontal tangential plane of the cylinder may be aligned with the top edge. The top edge may lie within a horizontal tangential plane of the cylinder. A distance between the longitudinal axis of the cylinder and the top edge in the radial direction may be equal to the radius of the cylinder.

[0023] The drag reduction system may be arranged such that the cylinder protrudes above the top edge of the rear face of the vehicle body in a radial direction by a maximum distance, H. The radial direction may be parallel to the rear face of the vehicle body. H may be greater than or equal to 1 / 6D, where D is the diameter of the cylinder. H may be less than or equal to 1 / 2D. H may be equal to 1 / 2D, 1 / 3D, 1 / 6D, or OD, or may be within a range defined between any of these values. The distance H may be measured above the top edge, between the top edge and the furthest point on the circumference of the cylinder in a radial direction parallel to the rear face of the vehicle body. An arc may be defined by the circumference of the cylinder above the top edge and a chord defined by a line extending from the top edge perpendicularly to the rear face of the vehicle body, wherein the distance H is equal to the length of the sagitta of the arc above the top edge.

[0024] The cylinder may be separated from the rear face of the vehicle body by a minimum distance, L. L may be greater than or equal to 1 / 6D, where D is the diameter of the cylinder. L may be greater than or equal to 1 / 2D. L may be within any range defined between any of 1 / 6D, 1 / 3D, or 1 / 2D.

[0025] The combination of a horizontal cylinder as described above, a velocity ratio within the range of 2.0-5.0, a distance H greater than or equal to 1 / 6D and less than or equal to 1 / 2D, and a distance L greater than or equal to 1 / 6D has been found to be particularly effective in reducing drag. The inventors have found that this combination of parameters can substantially reduce the drag on the rear face of the vehicle body as a result of reducing the area of low pressure behind the vehicle. In some cases, the inventors have found that the drag on the rear face of the vehicle body can be reduced in this manner by up to 40%, which may reduce the total drag on the vehicle by up to 30% . However, any combination of parameters described herein have been found to provide at least some benefits in terms of drag reduction.

[0026] The length of the cylinder may be substantially equal to the width of the rear face of the vehicle body. In other examples, the length of the cylinder may be less than the width of the rear face of the vehicle body.

[0027] The cylinder may be substantially longer than its diameter. An aspect ratio of the length of the cylinder to the diameter of the cylinder may be greater than 4. The aspect ratio may be greater than 4, greater than 5, greater than 6, greater than 7, greater than 8, greater than 9, or greater than 10, or may be in a ranged defined between any of these values.

[0028] The cylinder may be arranged vertically. The cylinder may be arranged vertically at a side of the rear face of the vehicle body. The cylinder may be arranged proximate to a side edge of the rear face of the vehicle body. The cylinder may be arranged such that the longitudinal axis of the cylinder is closer to the side edge than a side edge of the rear face of the vehicle body on an opposite side of the rear face. The cylinder may not protrude beyond the side edge. A vertical tangential plane of the cylinder may be aligned with the side edge. The side edge may lie within a vertical tangential plane of the cylinder. A distance between the longitudinal axis of the cylinder and the side edge in the radial direction may be equal to the radius of the cylinder.

[0029] The drag reduction system may be arranged such that the cylinder protrudes beyond the side edge of the rear face of the vehicle body in a radial direction by a maximum distance, H. The radial direction may be parallel to the rear face of the vehicle body. H may be greater than or equal to 1 / 6D, where D is the diameter of the cylinder. H may be less than or equal to 1 / 2D. H may be equal to 1 / 2D, 1 / 3D, 1 / 6D, or 0D, or may be within a range defined between any of these values. The distance H may be measured beyond the side edge, between the side edge and the furthest point on the circumference of the cylinder in the radial direction. An arc may be defined by the circumference of the cylinder beyond the side edge and a chord defined by a line extending parallel to the side edge, wherein the distance H is equal to the length of the sagitta of the arc beyond the side edge.

[0030] The length of the cylinder may be substantially equal to the height of the rear face of the vehicle body. In other examples, the length of the cylinder may be less than the height of the rear face of the vehicle body. The length of the cylinder may be less than 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, or 10% of the height of the rear face of the vehicle body.

[0031] The drive system may comprise a turbine impeller configured to be driven by a surrounding fluid when the vehicle is in motion to drive rotation of the cylinder. The turbine impeller may convert the energy from the surrounding fluid when the vehicle is in motion into rotational kinetic energy of the cylinder. The turbine impeller may comprise a centripetal flow configuration.

[0032] The turbine impeller may be configured to be driven by a surrounding fluid when the vehicle is in motion to drive rotation of the cylinder such that the velocity ratio of the cylinder is greater than 2.0, or any other value described herein. This may be achieved through suitable design of the geometry and configuration of the turbine impeller blades, or by any other suitable means. The diameter of the turbine impeller may be within the range of 0.5D-1.5D, where D is the diameter of the cylinder. The width of the turbine impeller may be within the range of 0.2D-1.0D, where D is the diameter of the cylinder. The number of blades of the turbine impeller may be within the range of 6 to 24.

[0033] The turbine impeller advantageously provides a means of driving rotation of the cylinder that does not require energy from a finite energy source, such as fuel or a battery. In addition, the turbine impeller advantageously provides passive control of the rotation of the cylinder, in that the turbine impeller is configured to rotate at a predetermined angular velocity with respect to the velocity of the surrounding fluid, i.e., with respect to the velocity of the vehicle, such that it drives rotation of the cylinder to achieve the velocity ratio values described herein. This may be achieved by configuring the size, orientation, and / or number of the blades of the turbine impeller, or any other suitable design parameter of the turbine impeller. The turbine impeller may therefore remove the need for an active control system, utilising controllers and processors and the like, to control rotation of the cylinder.

[0034] The turbine impeller is preferably the only means the drive system comprises to drive rotation of the cylinder. In other examples, the drive system may alternatively or additionally comprise electrical, mechanical, electromechanical, hydraulic, pneumatic, or any other suitable means configured to drive rotation of the cylinder.

[0035] The drive system may comprise a turbine housing arranged to house the turbine impeller. The turbine housing may comprise one or more fluid inlets arranged to allow a surrounding fluid to flow into the turbine housing via the one or more fluid inlets to drive rotation of the turbine impeller when the vehicle is in motion. The turbine housing may comprise one or more fluid outlets arranged to allow the surrounding fluid to exit the turbine housing via one or more fluid outlets the downstream of the turbine impeller when the vehicle is in motion.

[0036] The one or more fluid inlets may be arranged to allow a surrounding fluid to flow into the turbine housing via one or more fluid inlets the in a first direction. The one or more fluid outlets may be arranged to allow fluid to exit the turbine housing via the one or more fluid outlets in a second direction. The first direction and the second direction may be substantially parallel. The first direction and the second direction may lie substantially in the same plane. The first direction and the second direction may be the same direction. The one or more fluid outlets may be arranged to allow the surrounding fluid to exit the turbine housing via the one or more fluid outlets in a direction parallel to the freestream of the surrounding fluid when the vehicle is in motion. This is advantageous in minimising the detrimental aerodynamic effects of the fluid exiting the turbine housing.

[0037] The cylinder may be mounted to the vehicle body via the turbine housing. The turbine housing may comprise one or more mounting points for mounting the turbine housing to the vehicle body. The mounting points may be for mounting the turbine housing to rear face of the vehicle body.

[0038] The cylinder may comprise two parts. The turbine impeller may be configured to drive rotation of both parts of the cylinder. The turbine impeller may be arranged between the two parts of the cylinder. The turbine impeller may be configured to drive rotation of both parts of the cylinder at an end of each part of the cylinder.

[0039] The turbine impeller may be a first turbine impeller. The drive system may comprise one or more further turbine impellers. The one or more further turbine impellers may comprise any of the features of the first turbine impeller described herein. The first turbine impeller and the one or more further turbine impellers may be the only means the drive system comprises to drive rotation of the cylinder. The drive system may comprise one or more further turbine housings arranged to house the one or more further turbine impellers. The one or more further turbine housings may comprise any of the features of turbine housing described herein. The one or more further turbine impellers may comprise a second turbine impeller. The first turbine impeller and the second turbine impeller may be arranged at opposite ends of the cylinder.

[0040] The drive system may comprise one or more electric motors configured to drive rotation of the cylinder. The drive system may comprise a controller. The controller may be configured to receive one or more signals indicative of the velocity of a surrounding fluid. The one or more signals indicative of the velocity of the surrounding fluid may comprise a signal indicative of the speed of a vehicle. For example, in use, the controller may be configured to receive a signal indicative of the speed of the vehicle on which the drag reduction system is installed. The controller may be configured to receive a signal indicative of the speed of a vehicle from an electronic control unit of the vehicle.

[0041] The controller may be configured to receive a signal indicative of a wind direction and speed. For example, in use, the controller may be configured to receive a signal indicative of a wind direction and speed surrounding the vehicle on which the drag reduction system is installed. The signal indicative of a wind direction and speed may be received via an internet connection of the vehicle, or from any other suitable source.

[0042] As will be understood by the skilled person, the velocity of the surrounding fluid can be calculated using or inferred from the speed of the vehicle and / or the wind direction and speed.

[0043] The one or more signals indicative of the velocity of the surrounding fluid may comprise a signal from a flow sensor. The drive system may comprise a flow sensor configured to determine the velocity of a surrounding fluid. The flow sensor may comprise a pitot tube or any other suitable sensor. In use, when the drag reduction system is installed on a vehicle, the flow sensor may be configured to determine the velocity of a surrounding fluid when the vehicle is in motion. The flow sensor may be configured to output a signal indicative of the velocity of the surrounding fluid. The controller may be configured to receive the signal indicative of the velocity of the surrounding fluid from the flow sensor.

[0044] The controller may be configured to determine a target angular velocity in dependence on the one or more signals indicative of the velocity of the surrounding fluid and a target velocity ratio as defined herein. The controller may be configured to control the one or more electric motors to drive rotation of the cylinder at a target angular velocity to achieve the target velocity ratio. The target velocity ratio may be any value of velocity ratio described herein.

[0045] The drag reduction system may comprise a guide vane arranged to guide the surrounding fluid towards the cylinder when the vehicle is in motion. The guide vane may be arranged to direct the surrounding fluid onto the cylinder when the vehicle is in motion. The guide vane may be arranged between the rear face of the vehicle body and the cylinder. The guide vane may be mounted to the rear face of the vehicle body. The guide vane may be arranged to encourage entrainment of the surrounding fluid by the cylinder. The guide vane may be arranged to inhibit separation of the surrounding fluid from the vehicle body before the surrounding fluid reaches the cylinder.

[0046] The guide vane may be arranged to direct the surrounding fluid onto the cylinder when the vehicle is in motion to encourage entrainment of the surrounding fluid by the cylinder and thereby inhibit separation of fluid from the cylinder before the fluid is drawn towards the rear face of the vehicle body. The guide vane may be arranged to direct the surrounding fluid onto the cylinder when the vehicle is in motion to optimise entrainment of the surrounding fluid by the cylinder. Optimising entrainment of the surrounding fluid by the cylinder may comprise maximising a ratio of a proportion of the surrounding fluid entrained by the cylinder to a proportion of the surrounding fluid not entrained by the cylinder.

[0047] In use, after a surrounding fluid has been entrained by the cylinder, the fluid may have a tendency to then separate from the cylinder before it has been drawn towards the rear face of the vehicle body. The guide vane may be arranged to direct the surrounding fluid onto the cylinder when the vehicle is in motion in such a way as to inhibit this.

[0048] The guide vane may be arranged to encourage separation of fluid from the cylinder after the fluid has been drawn towards the rear face of the vehicle body. The guide vane may be arranged to inhibit at least a portion of the fluid entrained by the cylinder from being expelled towards the freestream of the surrounding fluid after the fluid has been entrained by the cylinder and drawn towards the rear face of the vehicle body. The guide vane may comprise a first surface. The first surface may be arranged to guide the surrounding fluid towards the cylinder when the vehicle is in motion. The first surface may be arranged to direct the surrounding fluid onto the cylinder when the vehicle is in motion. The first surface may be arranged to direct the surrounding fluid onto the cylinder to encourage or optimise entrainment of the surrounding fluid by the cylinder and thereby inhibit separation of fluid from the cylinder before the fluid is drawn towards the rear face of the vehicle body. The first surface may be arranged to extend parallel to the freestream of the surrounding fluid when the vehicle is in motion. In other examples, the first surface may be inclined to the direction of the freestream of the surrounding fluid when the vehicle is in motion.

[0049] The first surface of the guide vane may be aligned with a tangent of the cylinder. The cylinder may not extend beyond the first surface. In other examples, the cylinder may extend above the first surface. The first surface of the guide vane may lie at least partially within a first surface plane. The first surface plane may be parallel with the longitudinal axis of the cylinder. In other examples, the first surface plane may be angled to the longitudinal axis of the cylinder. A tangent of the cylinder may lie within the first surface plane. The cylinder may not extend beyond the first surface plane. In other examples, the cylinder may extend beyond the first surface plane.

[0050] The guide vane may comprise a second surface. The second surface may be arranged to encourage separation of fluid from the cylinder after the fluid has been drawn towards the rear face of the vehicle body. The second surface may be arranged to inhibit at least a portion of the fluid entrained by the cylinder from being expelled towards the freestream of the surrounding fluid after the fluid has been entrained by the cylinder and drawn towards the rear face of the vehicle body. The second surface may extend in a direction having a component parallel to the circumference of the cylinder. The second surface may be arranged to extend parallel to the circumference of the cylinder. In other examples, the second surface may be arranged to extend non -parallel to the circumference of the cylinder. The second surface may be arranged such that a gap between the second surface and the cylinder increases in a direction parallel to the circumference of the cylinder. The second surface may have a length arranged to extend in a direction having a component parallel to the circumference of the cylinder. The length may be between 20% and 50% of the circumference of the cylinder. The guide vane may be arranged such that the second surface extends away from the first surface. The guide vane may be arranged such that the second surface extends in a direction away from the nearest edge of the rear face of the vehicle body. The guide vane may be arranged such that the second surface extends in a direction away from the freestream of the surrounding fluid when the vehicle is in motion. The second surface may be arranged such that the gap between the second surface and the cylinder increases in a direction away from the first surface. The second surface may be arranged such that the gap between the second surface and the cylinder increases in a direction away from the nearest edge of the rear face of the vehicle body. The second surface may be arranged such that the gap between the second surface and the cylinder increases in a direction away from the freestream of the surrounding fluid when the vehicle is in motion.

[0051] Where the cylinder is arranged such that it protrudes beyond an edge of the rear face of the vehicle body in a radial direction, the guide vane may be arranged such that the first surface of the guide vane is also beyond the edge of the rear face of the vehicle body or the first surface of the guide vane extends at least partially beyond the edge of the rear face of the vehicle body. Where the cylinder is arranged such that a tangential plane of the cylinder is aligned with an edge of the rear face of the vehicle body, the guide vane may be arranged such that the first surface of the guide vane is aligned with the edge of the rear face of the vehicle body.

[0052] The guide vane may extend in a direction parallel to the length of the cylinder. The length of the guide vane may be equal to the length of the cylinder.

[0053] A maximum gap between the second surface of the guide vane and the cylinder may be within the range of 1 / 8D to 1 / 2D, where D is the diameter of the cylinder. The maximum gap may be 1 / 8D, 1 / 4D, 3 / 4D, or 1 / 2D, or within a range between any of these values. A minimum gap between the second surface of the guide vane and the cylinder may be within the range of 1 / 16D to 1 / 2D. The guide van may comprise a tip, wherein the first and second surfaces of the guide vane meet at the tip of the guide vane. A minimum gap between the tip of the guide vane and the cylinder may be within the range of 1 / 16D to 1 / 2D.

[0054] If the gap between the second surface of the guide vane, or the tip of the guide vane, and the cylinder is too small, then the friction on the cylinder caused by the flow of fluid through the gap may be too high such that the torque required to drive rotation of the cylinder to achieve the velocity ratios described herein may not be readily achievable. If the gap is too big, then the advantageous effects described above with respect to inhibiting separation of fluid from the cylinder before the fluid is drawn towards the rear face of the vehicle body, inhibiting at least a portion of the fluid entrained by the cylinder from being expelled towards the freestream of the surrounding fluid after the fluid has been entrained by the cylinder and drawn towards the rear face of the vehicle body, and / or encouraging separation of the fluid from the cylinder after the fluid has been drawn towards the rear face of the vehicle body may be reduced.

[0055] The cylinder may be a first cylinder. The drag reduction system may comprise one or more further cylinders. The one or more further cylinders may comprise any of the features of the first cylinder described herein.

[0056] The one or more further cylinders may be arranged behind the rear face of the vehicle body. The one or more further cylinders may be arranged vertically behind the rear face of the vehicle body. The one or more further cylinders may be arranged at a side of the rear face of the vehicle body. The one or more further cylinders may be arranged proximate to a side edge of the rear face of the vehicle body.

[0057] The drive system may be configured to drive rotation of each of the one or more further cylinders about the longitudinal axis of the respective further cylinder.

[0058] The drive system may be a first drive system. The drag reduction system may comprise one or more further drive systems. The one or more further drive systems may comprise any of the features of the first drive system described herein.

[0059] Each of the one or more further drive systems may be configured to drive rotation of one of the one or more further cylinders about the longitudinal axis of the respective further cylinder. Each of the one or more further drive systems may be configured to drive rotation of a different one of the one or more further cylinders about the longitudinal axis of the respective further cylinder.

[0060] The one or more further cylinders may comprise two further cylinders. The two further cylinders may be arranged behind the rear face of the vehicle body. The two further cylinders may be arranged vertically behind the rear face of the vehicle body. The two further cylinders may be arranged at either side of the rear face of the vehicle body. One of the two further cylinders may be arranged proximate to a side edge of the rear face of the vehicle body and the other of the two further cylinders may be arranged proximate to a side edge of the rear face of the vehicle body on an opposite side of the rear face. The first cylinder and the two further cylinders may be arranged to form a three -sided aero-frame.

[0061] The one or more further drive systems may comprise two further drive systems. One of the two further drive systems may be configured to drive rotation of one of the two further cylinders about the longitudinal axis of the respective further cylinder and the other of the two further drive systems may be configured to drive rotation of the other of the two further cylinders about the longitudinal axis of the respective further cylinder.

[0062] The two further drive systems may be configured to drive rotation of one of the two further cylinders in a first direction of rotation and drive rotation of the other of the two further cylinders in a second direction, wherein the second direction is opposite the first direction. For example, the two further drive systems may be configured to drive rotation of one of the two further cylinders in a clockwise direction as viewed from a first direction and drive rotation of the other of the two further cylinders in an anti clockwise direction as viewed from the first direction.

[0063] The inventors have found that the arrangement of a first horizontal cylinder arranged at the top of the rear face of the vehicle body and two further cylinders arranged at either side of the rear face of the vehicle body is particularly effective in reducing drag. This arrangement may substantially reduce the drag on the rear face of the vehicle, in some cases by up to an additional 20% compared to having a single horizontal cylinder alone.

[0064] Where the drag reduction system comprises two or more drive systems, two of the drive systems may share a common turbine housing. The common turbine housing may be arranged to house both a turbine impeller of one of the drive systems and a turbine impeller of the other drive system. The guide vane described above may be a first guide vane. Where the drag reduction system comprises one or more further cylinders, the drag reduction system may comprise one or more further guide vanes arranged to guide a surrounding fluid towards the one or more further cylinders when the vehicle is in motion. The one or more further guide vanes may comprise any of the features of the first guide van described herein. The one or more further guide vanes may comprise a further guide for each of the one or more further cylinders, wherein each further guide vane is arranged to guide the surrounding fluid towards the respective further cylinder when the vehicle is in motion. The one or more further guide vanes may comprise any of the features of the first guide van described herein.

[0065] The vehicle may be a commercial vehicle. The rear face of the vehicle body may be substantially flat. The surface of the rear face may comprise minor indentations or protrusions, but the rear face is preferably devoid of any major contours. The vehicle may be any vehicle comprising a vehicle body with a flat rear face. The vehicle may be a van, a bus, trailer, or any other vehicle or trailer that features a flat rear face.

[0066] It will be appreciated that the term ‘commercial’ is used to refer to a particular class of vehicle and is not intended to limit the invention to vehicles for a particular use, be that commercial or otherwise.

[0067] A third aspect provides a drag reduction system for a vehicle, comprising a first cylinder, one or more further cylinders, and a drive system configured to drive rotation of each of the first cylinder and one or more further cylinders about its longitudinal axis.

[0068] A fourth aspect provides a vehicle comprising a vehicle body and the drag reduction system of the third aspect. The vehicle body comprises a rear face. The first cylinder and one or more further cylinders of the drag reduction system are arranged behind the rear face of the vehicle body.

[0069] A fifth aspect provides a method of operating a drag reduction system for a vehicle . The method comprises providing a drag reduction system. The drag reduction system may comprise any drag reduction system described herein, the drag reduction system of any vehicle described herein, any other drag reduction system within the scope of the appended claims, or the drag reduction system of any other vehicle within the scope of the appended claims. The method further comprises operating the drive system of the drag reduction system to drive rotation of the cylinder such that a ratio of the magnitude of the tangential velocity of the cylinder to the magnitude of the freestream velocity of a surrounding fluid is greater than 2.0, or any other value described herein.

[0070] Any of the features described above with reference to the first or second aspects may equally apply to the third, fourth, or fifth aspects. For example, the first cylinder of the third or fourth aspects may comprise any of the features of the cylinder of the first or second aspects. The one or more further cylinders of the third or fourth aspects may comprise any of the features of the one or more further cylinders of the first or second aspects. The drag reduction system of the fifth aspect may be the drag reduction system of the first or third aspects.

[0071] It will be understood that references to ‘above’, ‘below’, ‘horizontal’, ‘vertical’, etc. are with respect to the vehicle in its normal orientation. For example, where the vehicle comprises one or more wheels, the normal orientation of the vehicle may be the orientation of the vehicle when all of the wheels of the vehicle are in contact with the ground.

[0072] BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Implementations will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0074] Figure la shows a schematic isometric view of a drag reduction system for a vehicle ; Figure lb shows a further schematic isometric view of the drag reduction system of Figure la;

[0075] Figure 1c shows a schematic rear view of the drag reduction system of Figure la; Figure Id shows a schematic side view of the drag reduction system of Figure la; Figure le shows a schematic exploded isometric view of part of the drag reduction system of Figure la;

[0076] Figure If shows a schematic exploded rear view of part of the drag reduction system of Figure la;

[0077] Figure 1g shows a further schematic exploded rear view of part of the drag reduction system of Figure la; Figure 2a shows a schematic isometric view of a drag reduction system according to another example;

[0078] Figure 2b shows a schematic isometric view of part of the drag reduction system of Figure 2a;

[0079] Figure 2c shows a schematic side view of part of the drag reduction system of Figure 2a;

[0080] Figure 2d shows a close-up schematic side view of part of the drag reduction system of Figure 2a;

[0081] Figure 2e shows a schematic isometric view of the guide vane of the drag reduction system of Figure 2a in isolation;

[0082] Figure 2f shows a further close-up schematic side view of part of the drag reduction system of Figure 2a;

[0083] Figure 3a shows a schematic isometric view of a drag reduction system according to another example;

[0084] Figure 3b shows a schematic isometric view of part of the drag reduction system of Figure 3a;

[0085] Figure 3c shows a schematic side view of part of the drag reduction system of Figure 3a;

[0086] Figure 3d shows a close-up schematic side view of part of the drag reduction system of Figure 3a;

[0087] Figure 3e shows a schematic isometric view of the guide vane of the drag reduction system of Figure 3a in isolation;

[0088] Figure 3f shows a close-up schematic side view of part of the drag reduction system of

[0089] Figure 3a;

[0090] Figure 4 shows a schematic side view of a drag reduction system according to another example;

[0091] Figure 5a shows a schematic isometric view of a vehicle comprising the drag reduction system of Figure la;

[0092] Figure 5b shows a further schematic isometric view of the vehicle of Figure 5a;

[0093] Figure 5c shows a schematic plan view of the vehicle of Figure 5a;

[0094] Figure 5d shows a close-up schematic isometric view of the vehicle of Figure 5a;

[0095] Figure 5e shows a close-up schematic side view of the vehicle of Figure 5a;

[0096] Figure 5f shows a close-up schematic rear view of the vehicle of Figure 5a;

[0097] Figure 5g shows a further close-up schematic side view of the vehicle of Figure 5a;

[0098] Figure 5h shows a further close-up schematic side view of the vehicle of Figure 5a; Figure 5i shows a further schematic side view of the vehicle of Figure 5a when travelling in a normal forward direction of travel;

[0099] Figure 6a shows a schematic side view of the vehicle of Figure 5a showing the velocity of the surrounding fluid when the vehicle is travelling in a normal forward direction of travel if the drag reduction system was absent ;

[0100] Figure 6b shows a schematic side view of the vehicle of Figure 5a showing the velocity of the surrounding fluid when the vehicle is travelling in the normal forward direction of travel with the drag reduction system present;

[0101] Figure 7a shows a schematic side view of the vehicle of Figure 5a showing the pressure of the surrounding fluid when the vehicle is travelling in a normal forward direction of travel if the drag reduction system was absent ;

[0102] Figure 7b shows a schematic side view of the vehicle of Figure 5a showing the pressure of the surrounding fluid when the vehicle is travelling in the normal forward direction of travel with the drag reduction system present;

[0103] Figure 8a shows a schematic isometric view of a drag reduction system according to another example;

[0104] Figure 8b shows a further schematic isometric view of the drag reduction system of Figure 8a;

[0105] Figure 8c shows a schematic rear view of the drag reduction system of Figure 8a;

[0106] Figure 9a shows a close-up schematic isometric view of a vehicle comprising the drag reduction system of Figure 8a;

[0107] Figure 9b shows a further close-up schematic isometric view of the vehicle of Figure 9a;

[0108] Figure 9c shows a close-up schematic rear view of the vehicle of Figure 9a;

[0109] Figure 9d shows a close-up schematic plan view of the vehicle of Figure 9a;

[0110] Figure 9e shows a close-up schematic plan view of the vehicle of Figure 9a;

[0111] Figure 9f shows a further schematic side view of the vehicle of Figure 9a when travelling in a normal forward direction of travel with the drag reduction system absent ; Figure 9g shows a further schematic side view of the vehicle of Figure 9a when travelling in the normal forward direction of travel with the drag reduction system present;

[0112] Figure 10a shows a close-up schematic isometric view of a vehicle comprising two drag reduction systems of Figure la;

[0113] Figure 10b shows a further close-up schematic isometric view of the vehicle of Figure 10a; Figure 10c shows a schematic rear view of the vehicle of Figure 10a;

[0114] Figure I la shows a schematic plan view of the vehicle of Figure 10a showing the velocity of the surrounding fluid when the vehicle is travelling in a normal forward direction of travel if the drag reduction systems were absent ;

[0115] Figure 11b shows a schematic side view of the vehicle of Figure 10a showing the velocity of the surrounding fluid when the vehicle is travelling in the normal forward direction of travel with the drag reduction systems present;

[0116] Figure 12a shows a schematic isometric view of a drag reduction system for a vehicle according to another example;

[0117] Figure 12b shows a further schematic isometric view of the drag reduction system of Figure 12a;

[0118] Figure 12c shows a schematic rear view of the drag reduction system Figure 12a; Figure 12d shows a schematic side view of the drag reduction system Figure 12a; Figure 12e shows a close-up schematic exploded rear view of part of the drag reduction system Figure 12a;

[0119] Figure 12f shows a schematic exploded rear view of the drag reduction system Figure 12a;

[0120] Figure 13a shows a schematic isometric view of a vehicle comprising the drag reduction system of Figure 12a;

[0121] Figure 13b shows a schematic side view of the vehicle of Figure 13a;

[0122] Figure 13c shows a further schematic isometric view of the vehicle of Figure 13a;

[0123] Figure 13d shows a schematic rear view of the vehicle of Figure 13a; and

[0124] Figure 15 illustrates a method of operating a drag reduction system for a vehicle .

[0125] DETAILED DESCRIPTION

[0126] Figure la shows a schematic isometric view of a drag reduction system 1 for a vehicle. Figure lb shows a further schematic isometric view of the drag reduction system 1. Figure 1c shows a schematic rear view of the drag reduction system 1. Figure Id shows a schematic side view of the drag reduction system 1. In use, the drag reduction system 1 may be mounted to a body of a vehicle. The drag reduction system 1 may be mounted to a rear face of a body of a vehicle.

[0127] The drag reduction system 1 comprises a cylinder 11 and a drive system 12 configured to drive rotation of the cylinder 11 about its longitudinal axis. In this example, an aspect ratio of the length of the cylinder 11 to the diameter of the cylinder is greater than 4. The drive system 12 comprises first and second turbine impellers, a first turbine housing 13a arranged to house the first turbine impeller, and a second turbine housing 13b arranged to house the second turbine impeller. The first and second turbine impellers and, respectively, the first and second turbine housings 13a, 13b, are arranged at opposite ends of the cylinder 11. In other examples, the second turbine impeller and second turbine housing 13b may be absent.

[0128] The first turbine housing 13a comprises a fluid inlet 14a arranged to allow fluid to flow into the first turbine housing 13a and a fluid outlet 15a arranged to allow fluid to exit the first turbine housing 13a downstream of the first turbine impeller. As shown in Figure Id, the fluid inlet 14a and the fluid outlet 15a are arranged such that, in use, fluid exits the turbine housing 13a via the fluid outlet 15a in a direction, 17. parallel to the direction, fi, in which fluid enter the turbine housing 13a via the fluid inlet 14a.

[0129] The first turbine housing 13a further comprises a plurality of mounting points 16a for mounting the first turbine housing 13a to a rear face of a vehicle body. For clarity, not all of the mounting points 16a are labelled. In this example, the plurality of mounting points 16a comprises six threaded bores for receiving corresponding threaded screws extending through a rear face of a vehicle body; however, in alternative examples, any suitable number or type of mounting points may be provided. Features of the second turbine housing 13b in common with the first turbine housing 13a are indicated with the same reference numerals followed by ‘b’ in place of ‘a’ .

[0130] Figure le shows a schematic exploded isometric view of part of the drag reduction system 1. Figure If shows a schematic exploded rear view of part of the drag reduction system 1. The cylinder 11 comprises a first end plate 17a and a cylindrical body 18. In this example, the cylindrical body 18 is hollow and the first end plate 17a is attached to a first end of the cylindrical body 18 by means of an interference fit. In other examples, the first end plate 17a may be formed integrally with the hollow cylindrical body 18, or the cylindrical body 18 may not be hollow and the first end plate 17a may be formed by a first end face of the cylindrical body 18.

[0131] Figures le and If show the first turbine impeller 19a of the drive system. The drive system further comprises a first drive shaft 110a. The first turbine impeller 19a is mounted to the first drive shaft 110a for rotation with the first drive shaft 110a. The first drive shaft 110a extends axially through the centre of the first turbine impeller 19a. The drive system further comprises a first outer bearing I l la and a first inner bearing 112a arranged on the first drive shaft 110a. The first turbine impeller 19a is mounted within the first turbine housing via the first drive shaft 110a, the first outer bearing I l la, and the first inner bearing 112a such that the first turbine impeller 19a and the first drive shaft 110a are configured to rotate relative to the first turbine housing about the longitudinal axis of the first drive shaft 110a. The first drive shaft 110a extends from the first turbine impeller 19a, out of the first turbine housing (as shown in Figure 1g described below), and through a hole in the centre of the first end plate 17a of the cylinder 11 . The first drive shaft l lOa is fixed to the first end plate 17a for rotation with the cylinder 11.

[0132] In this example, the first turbine impeller 19a comprises a centripetal flow configuration. Such turbine impeller configurations will be known to those skilled in the art. In use, a flow of fluid enters the first turbine housing 13a via the fluid inlet 14a and is directed by the blades of the first turbine impeller 19a towards the centre of the first turbine impeller 19a. This causes the first turbine impeller 19a to rotate before the fluid flows from the centre of the first turbine impeller 19a in an axial direction. The flow then exits the first turbine housing 13a via the fluid outlet 15a. In this way, the first turbine impeller 19a is configured to drive rotation of the cylinder about the longitudinal axis of the cylinder 11.

[0133] Referring to Figure If, in this example, the diameter, Dt, of the first turbine impeller 19a is equal to 0.78D, where D is the diameter of the cylinder 11. The width of the first turbine impeller 19a, Wt, is equal to 0.28D, where D is the diameter of the cylinder 11. The number of blades of the first turbine impeller 19a is equal to 16. In other examples, the diameter, width, and / or number of blades of the first turbine impeller 19a may different, for example as described elsewhere herein.

[0134] The second turbine impeller operates in the same manner as the first turbine impeller 19a. The cylinder comprises a second end plate attached to a second end of the cylindrical body 18, and the drive system comprises a second drive shaft, a second outer bearing, and a second inner bearing. The second turbine impeller, second drive shaft, second end plate, second outer bearing, second inner bearing, and second turbine housing are arranged as described above with respect to the first turbine impeller 19a, the first drive shaft 110a, first end plate 17a, first outer bearing I l la, first inner bearing 112a, and first turbine housing. In other examples, the second turbine impeller may comprise a different configuration to the first turbine impeller 19a. In some examples, one or both of the first turbine impeller 19a and the second turbine impeller may comprise a different configuration to that of Figure le.

[0135] Figure 1g shows a further schematic exploded rear view of part of the drag reduction system 1. Figure 1g shows the first turbine housing 13a with the first turbine impeller housed within the first turbine housing 13a.

[0136] It will be appreciated that the drive system 12 of the drag reduction system 1 of Figures la-g is merely illustrative, and that in other examples, any other suitable drive system configured to drive rotation of the cylinder about its longitudinal axis may be additionally or alternatively provided. It will also be appreciated that the drag reduction system 1 of Figures la-g as a whole is merely illustrative and that the drag reduction system may take any form within the scope of the appended claims.

[0137] Figure 2a shows a schematic isometric view of a drag reduction system 2 according to another example. Features of the drag reduction system 2 in common with the drag reduction system 1 of Figures la-g are indicated with the same reference numerals with the first number being ‘2’ instead of ‘ 1 ’ .

[0138] The drag reduction system 2 comprises a guide vane 213. In use, with the drag reduction system 2 mounted to a body of a vehicle, the guide vane 213 is arranged to guide the surrounding fluid towards the cylinder when the vehicle is in motion .

[0139] Figure 2b shows a schematic isometric view of part of the drag reduction system 2. Figure 2b shows the drag reduction system 2 with the first turbine housing , the first turbine impeller, the first drive shaft, the first outer bearing, the first inner bearing, and the first endplate of the cylinder removed. The guide vane 213 extends in a direction parallel to the length of the cylinder 21 and the length of the guide vane 213 is substantially equal to the length of the cylinder 21. In other examples, the guide vane 213 may be shorter or longer. Figure 2c shows a schematic side view of part of the drag reduction system 2. Figure 2c shows the drag reduction system 2 with the first turbine housing, the first turbine impeller, the first drive shaft, the first outer bearing, the first inner bearing, and the first endplate of the cylinder removed. Figure 2c additionally shows the second end plate 27b of the cylinder 21. The guide vane 213 comprises a first surface 214 and a second surface 215. In use, the first surface 214 is arranged to extend parallel to the freestream of a surrounding fluid when the vehicle is in motion (see Figure 3f described in further detail below). The first surface 214 lies within a first surface plane, the first surface plane being parallel with the longitudinal axis of the cylinder 21. A tangent of the body 28 of the cylinder 21 lies within the first surface plane. In other words, as viewed in Figure 2c, the top of the body 28 of the cylinder 21 is aligned with the first surface 214. The second surface 215 is arranged to extend parallel to the circumference of the cylinder 21. In this example, the second surface 215 is arranged to extend parallel to around 20% of the circumference of the cylinder 21. In other examples, the second surface 215 may be arranged to extend parallel to more or less the circumference of the cylinder 21.

[0140] Figure 2d shows a close-up schematic side view of part of the drag reduction system 2. A gap, G, between the second surface 215 of the guide vane 213 and the cylinder 21 is shown. In this example, the gap, G, is equal to around 1 / 8D, where D is the diameter of the cylinder 21. In other examples, the gap, G, may be smaller or bigger.

[0141] Figure 2e shows a schematic isometric view of the guide vane 213 of the drag reduction system 2 in isolation.

[0142] Figure 2f shows a close-up schematic side view of part of the drag reduction system 2. Figure 2f shows an example flow of a surrounding fluid in use. The arrows in Figure 2f indicate the flow of fluid, including a freestream, F, of the fluid. The surrounding fluid flows over the guide vane 213. The surrounding fluid is then directed onto the cylinder 21 by the first surface 214 of the guide vane 213 before the surrounding fluid is entrained by the cylinder 21. As illustrated, fluid from the freestream, F, is entrained by the cylinder 21 and then separates from the cylinder 21. The guide vane 213 is arranged such that the second surface 215 extends away from the first surface 214 in a direction away from the freestream, F. The first surface 214 of the guide vane 213 is arranged to direct the surrounding fluid onto the cylinder 21 in such a way as to encourage entrainment of the surrounding fluid by the cylinder 21 , thereby inhibiting separation of the surrounding fluid from the cylinder 21 before the surrounding fluid is drawn towards the rear face of the vehicle body, as indicated by arrows Fr. The first surface 214 of the guide vane 213 is also arranged to direct the surrounding fluid onto the cylinder 21 in such a way as to encourage separation of the surrounding fluid from the cylinder 21 after the surrounding fluid has been drawn towards the rear face of the vehicle body, as indicated by arrows Ft. The second surface 215 of the guide vane 213 is arranged to inhibit flow of at least a portion of the fluid back towards the freestream, F, after the fluid has been entrained by the cylinder 21 and drawn towards the rear face of the vehicle body, as indicated by arrow Fs.

[0143] It will be appreciated that the drag reduction system 2 of Figures 2a -f is merely illustrative and that the drag reduction system may take any form within the scope of the appended claims.

[0144] Figure 3a shows a schematic isometric view of a drag reduction system 3 according to another example. Features of the drag reduction system 3 in common with the drag reduction system 2 of Figures 2a-f are indicated with the same reference numerals with the first number being ‘3’ instead of ‘2’ .

[0145] Figure 3b shows a schematic isometric view of part of the drag reduction system 3. Figure 3b shows the drag reduction system 3 with the first turbine housing, the first turbine impeller, the first drive shaft, the first outer bearing, the first inner bearing, and the first endplate of the cylinder removed. The guide vane 313 extends in a direction parallel to the length of the cylinder 31 and the length of the guide vane 313 is substantially equal to the length of the cylinder 31. In other examples, the guide vane 313 may be shorter or longer.

[0146] Figure 3c shows a schematic side view of part of the drag reduction system 3. Figure 3c shows the drag reduction system 3 with the first turbine housing, the first turbine impeller, the first drive shaft, the first outer bearing, the first inner bearing, and the first endplate of the cylinder removed. Figure 3c additionally shows the second end plate 37b of the cylinder 31. The guide vane 313 comprises a first surface 314 and a second surface 315. In use, the first surface 314 is arranged to be inclined to the direction of the freestream of a surrounding fluid when the vehicle is in motion (see Figure 3f described in further detail below). As shown, the body 38 of the cylinder 31 extends beyond the first surface 314, i.e., as viewed in Figure 3c, the cylinder 31 protrudes above the first surface 314. When installed on a vehicle in use, the first surface 314 extends beyond the rear face of the vehicle body.

[0147] The second surface 315 is arranged to extend non-parallel to the circumference of the cylinder 31. In this example, the second surface 315 has a length arranged to extend in a direction having a component parallel to the circumference of the cylinder 31 , the length being equal to around 10% of the circumference of the cylinder 31. In other examples, the length may be equal to a greater or smaller proportion of the circumference of the cylinder 31.

[0148] Figure 3d shows a close-up schematic side view of part of the drag reduction system 3. A maximum gap, G2, between the second surface 315 of the guide vane 313 and the cylinder 31 and a minimum gap, Gi, between the second surface 315 of the guide vane 313 and the cylinder 31 is shown. The second surface 315 is arranged such that the gap G increases in a direction away from the freestream of the surrounding fluid in use (see Figure 3f described in further detail below). In this example, the maximum gap, G2, is equal to around 1 / 8D and the minimum gap, Gi, is equal to around 1 / 16D, where D is the diameter of the cylinder 31. In other examples, the maximum or minimum gap, G, may be smaller or bigger.

[0149] Figure 3e shows a schematic isometric view of the guide vane 313 of the drag reduction system 3 in isolation.

[0150] Figure 3f shows a close-up schematic side view of part of the drag reduction system 3. Figure 3f shows an example flow of a surrounding fluid in use. The arrows in Figure 3f indicate the flow of fluid, including a freestream, F, of the fluid. The surrounding fluid flows over the guide vane 313. The surrounding fluid is then directed onto the cylinder 31 by the first surface 314 of the guide vane 313 before the surrounding fluid is entrained by the cylinder 31. As illustrated, fluid from the freestream, F, is entrained by the cylinder 31 and then separates from the cylinder 31. The guide vane 313 is arranged such that the second surface 315 extends away from the first surface 314 in a direction away from the freestream, F.

[0151] The first surface 314 of the guide vane 313 is arranged to direct the surrounding fluid onto the cylinder 31 in such a way as to encourage entrainment of the surrounding fluid by the cylinder 31 , thereby inhibiting separation of the surrounding fluid from the cylinder 31 before the surrounding fluid is drawn towards the rear face of the vehicle body, as indicated by arrows Fr. The first surface 314 of the guide vane 313 is also arranged to direct the surrounding fluid onto the cylinder 31 in such a way as to encourage separation of the surrounding fluid from the cylinder 31 after the surrounding fluid has been drawn towards the rear face of the vehicle body, as indicated by arrows Ft. The second surface 315 of the guide vane 313 is arranged to inhibit flow of the surrounding fluid back towards the freestream, F, after the surrounding fluid has been entrained by the cylinder 31 and drawn towards the rear face of the vehicle body, as indicated by arrow Fs.

[0152] It will be appreciated that the drag reduction system 3 of Figures 3a-f is merely illustrative and that the drag reduction system may take any form within the scope of the appended claims.

[0153] Figure 4 shows a schematic side view of a drag reduction system 4 according to another example. Features of the drag reduction system 4 in common with the drag reduction system 1 of Figures la-g are indicated with the same reference numerals with the first number being ‘4’ instead of ‘ 1 ’ . The drag reduction system 4 differs from the drag reduction system 1 in that the first turbine impeller 19a is replaced by a first electric motor 416a and the first turbine housing 13a is replace by a first motor housing 417a arranged to house the first electric motor 416a. The electric motor 416 is configured to drive rotation of the first drive shaft 410a, and in turn drive rotation of the cylinder 41 via the first end plate 47a.

[0154] The drive system 42 may comprise a second electric motor and a second motor housing in place of the second turbine impeller and the second turbine housing 13b of the drag reduction system 1 of Figures la-g. In such examples, as in the drag reduction system 1 of Figures la-g, the cylinder 41 may comprise a second end plate attached to a second end of the cylindrical body 48, and the drive system may comprise a second drive shaft, a second outer bearing, and a second inner bearing. The second electric motor, second drive shaft, second end plate, second outer bearing, second inner bearing, and second motor housing may be arranged as described above with respect to the first electric motor 416a, the first drive shaft 410a, first end plate 47a, first outer bearing 411a, first inner bearing 412a, and first motor housing 417a.

[0155] The drag reduction system 4 further comprises a flow sensor 423 configured to determine the velocity of a surrounding fluid. In this example, the flow sensor 423 comprises a pitot tube, but in other examples the flow sensor 423 may comprise any other suitable flow sensor. In use, when the drag reduction system 4 is installed on a vehicle, the flow sensor 423 is configured to determine the freestream velocity of a surrounding fluid when the vehicle is in motion.

[0156] The drag reduction system 4 further comprises a controller 424. The controller 424 is in communication with the first electric motor 416a and the flow sensor 423, as indicated by the dotted lines in Figure 4. Where the drag reduction system 4 comprises a second electric motor as described above, the controller 424 may also be in communication with the second electric motor. The flow sensor 423 is configured to output a signal indicative of the velocity of the surrounding fluid and the controller 424 is configured to receive the signal. The controller 424 is configured to determine a target angular velocity in dependence on the signal indicative of the velocity of the surrounding fluid and a target velocity ratio as defined herein. The controller 424 is configured to control the electric motor 416a, and, where present, optionally the second electric motor, to drive rotation of the cylinder 48 at a target angular velocity to achieve the target velocity ratio. The target velocity ratio may be any value of velocity ratio described herein.

[0157] It will be appreciated that the drag reduction system 4 of Figure 4 is merely illustrative and that the drag reduction system may take any form within the scope of the appended claims.

[0158] Figure 5a shows a schematic isometric view of a vehicle 5. Figure 5b shows a further schematic isometric view of the vehicle 5. Figure 5c shows a schematic plan view of the vehicle 5. The vehicle 5 comprises a vehicle body 517. The vehicle body 517 comprises a rear face 518. In this example, the vehicle 5 is a van and the rear face 518 of the vehicle body 517 is substantially flat. The vehicle 5 further comprises the drag reduction system 1 of Figures la-g. In other examples, the vehicle 5 may comprise the drag reduction system of Figures 2a-f, the drag reduction system of Figure 3 , and / or the drag reduction system of Figures 3a-f, in place of or in addition to the drag reduction system 1 of Figures la-g.

[0159] In this example, the drag reduction system 1 is mounted to the rear face 518 of the vehicle body 517 via the mounting points of the first and second turbine housings 13a, 13b. The drag reduction system 1 is arranged such that the cylinder 11 extends horizontally between the side edges of the rear face 518 of the vehicle body 517 proximate the top edge of the rear face 518 of the vehicle body 517. The length of the cylinder 11 is substantially equal to the width of the rear face 518 of the vehicle body 517.

[0160] Figure 5d shows a close-up schematic isometric view of the vehicle 5. Figure 5e shows a close-up schematic side view of the vehicle 5. In this example, the fluid inlets 14a, 14b of the first and second turbine housings 13a, 13b are arranged above the top edge of the rear face 518 of the vehicle body 517.

[0161] Figure 5f shows a close-up schematic rear view of the vehicle 5.

[0162] Figure 5g shows a further close-up schematic side view of the vehicle 5. For clarity, only the vehicle body 517 and the cylinder 11 of the drag reduction system 1 are shown. The cylinder 11 protrudes above the top edge of the rear face 518 of the vehicle body 517 in a radial direction, r, such that the cylinder 11 protrudes in the radial direction, r, into the surrounding fluid when the vehicle 5 is in motion. In this example, radial direction, r, is the parallel to the rear face 518 of the vehicle body 517.

[0163] The drag reduction system is arranged such that the cylinder 11 protrudes in the radial direction, r, above the top edge of the rear face 518 of the vehicle body 517 by a maximum distance, H. In this example, H is measured between the top edge of the rear face 518 of the vehicle body 517 and the furthest point on the circumference of the cylinder 11 in the radial direction, r. In this example, H is 1 / 4D, where D is the diameter of the cylinder 11. In other examples, H may be smaller or larger. The cylinder 11 is separated from the rear face 518 of the vehicle body 517 by a minimum distance, L. In this example, the minimum distance, L, is 1 / 2D, where D is the diameter of the cylinder 11. In other examples, L may be smaller or larger.

[0164] Figure 5h shows a further close-up schematic side view of the vehicle 5. For clarity, only the vehicle body 517 and the cylinder 11 of the drag reduction system 1 are shown. The drive system is configured to drive rotation of the cylinder 11 in a clockwise direction, R, as viewed from the left-hand side of the vehicle with respect to a normal forward direction of travel, d, of the vehicle. In this example, the drive system is configured to drive rotation of the cylinder 11 such that a velocity ratio of the cylinder 11 , defined as a ratio of the magnitude of the tangential velocity of the cylinder 11 to the magnitude of the freestream velocity of a surrounding fluid when the vehicle 5 is in motion, is greater than 2.0.

[0165] Figure 5i shows a further schematic side view of the vehicle 5 when travelling in the normal forward direction of travel, d. Figure 5i shows the freestream, F, of the surrounding fluid of the vehicle 5, the flow, A, of the surrounding fluid as influenced by the vehicle body 517, and the flow, B, of the surrounding fluid as influenced by the drag reduction system 1 with the cylinder rotating as shown in Figure 5h. As illustrated by the flow, B, rotation of the cylinder causes the cylinder to entrain the surrounding fluid and inhibit recirculation of the surrounding fluid at the rear of the vehicle 5.

[0166] It will be appreciated that the vehicle 5 of Figures 5a-i is merely illustrative and that the vehicle may take any form within the scope of the appended claims.

[0167] Figure 6a shows a schematic side view of the vehicle 5 of Figures 5a-i showing the ratio of the velocity to the surrounding fluid to the free stream velocity of the surrounding fluid when the vehicle 5 is travelling in the normal forward direction of travel, d, if the drag reduction system was absent. Figure 6b shows a schematic side view of the vehicle 5 of Figures 5a-i showing the velocity of the surrounding fluid when the vehicle 5 is travelling in the normal forward direction of travel, d, with the drag reduction system 1 present. Figure 7a shows a schematic side view of the vehicle 5 of Figures 5a-i showing the pressure of the surrounding fluid when the vehicle 5 is travelling in the normal forward direction of travel, d, if the drag reduction system was absent. Figure 7b shows a schematic side view of the vehicle 5 of Figures 5a-i showing the pressure of the surrounding fluid when the vehicle 5 is travelling in the normal forward direction of travel, d, with the drag reduction system 1 present.

[0168] As illustrated by Figures 6a to 7b, the drag reduction system 1 has the effect of increasing the volume of the region of low velocity flow of the surrounding fluid immediately behind the rear face 518 of the vehicle body 517, which in turn increases the pressure of this region of flow. In turn, this reduces drag resulting from the rear face 518 of the vehicle body 517.

[0169] Figure 8a shows a schematic isometric view of a drag reduction system 8 according to another example. Figure 8b shows a further schematic isometric view of the drag reduction system 8. Figure 8c shows a schematic rear view of the drag reduction system 8. In use, the drag reduction system 8 may be mounted to a body of a vehicle. The drag reduction system 8 may be mounted to a rear face of a body of a vehicle.

[0170] The drag reduction system 8 comprises a first cylinder 81a, a second cylinder 81b, and a third cylinder 81c. Each of the first, second, and third cylinders 81a, 81b, 81c share all of the features of the cylinder of the drag reduction system of Figures la-g. In other examples, one or more of the first, second, and third cylinders 81a, 81b, 81c may comprise different features. In this example, the second and third cylinders 81b, 81c extend perpendicularly to the first cylinder 81a from opposite ends of the first cylinder 81a. As shown in Figures 8a to 8c, the first cylinder 81a extends horizontally and the second and third cylinders 81b, 81c extend vertically from opposite ends of the first cylinder 81a.

[0171] The drag reduction system 8 further comprises a first drive system 82a configured to drive rotation of the first cylinder 81a about its longitudinal axis, a second drive system 82b configured to drive rotation of the second cylinder 81b about its longitudinal axis, and a third drive system 82c configured to drive rotation of the third cylinder 81c about its longitudinal axis. The first drive system 82a and the second drive system 82b comprise a first common turbine housing 819a. The first drive system 82a and the third drive system 81c share a second common turbine housing 819b. The first drive system 82a comprises a first turbine impeller housed within the first common turbine housing 819a and a second turbine impeller housed within the second common turbine housing 819b. The first and second turbine impellers of the first drive system 82a and, respectively, the first and second common turbine housings 819a, 819b, are arranged at opposite ends of the first cylinder 81a.

[0172] The first common turbine housing 819a comprises a first fluid inlet 820a arranged to allow fluid to flow into the first common turbine housing 819a upstream of the first turbine impeller of the first drive system 82a. The first common turbine housing 819a further comprises a second fluid inlet 821a arranged to allow fluid to flow into the first common turbine housing 819a upstream of the first turbine impeller of the second drive system 82a. The first common turbine housing 819a further comprises a fluid outlet 822a arranged to allow fluid to exit the first common turbine housing 819a downstream of both the first turbine impeller of the first drive system 82a and the first turbine impeller of the second drive system 82a.

[0173] The second common turbine housing 819b comprises a first fluid inlet 820b arranged to allow fluid to flow into the second common turbine housing 819b upstream of the second turbine impeller of the first drive system 82a. The second common turbine housing 819b further comprises a second fluid inlet 821b arranged to allow fluid to flow into the second common turbine housing 819b upstream of the first turbine impeller of the third drive system 82c. The second common turbine housing 819b further comprises a fluid outlet 822b arranged to allow fluid to exit the second common turbine housing 819b downstream of both the second turbine impeller of the first drive system 82a and the first turbine impeller of the third drive system 82c.

[0174] Each of the first and second common turbine housings 819a, 819b comprise a plurality of mounting points 827a, 827b, respectively, for mounting the respective common turbine housing 819a, 819b to a rear face of a vehicle body. For clarity, not all of the mounting points are labelled. In this example, the mounting points 827a, 827b are as described above with respect to the mounting points of the turbine housings of the drive system of Figure la.

[0175] The second drive system 82b further comprises a second turbine housing 83a and a second turbine impeller housed within the second turbine housing 83a. The first and second turbine impellers of the second drive system 82b and, respectively, the first common turbine housing 819a and the second turbine housing 83a of the second drive system 82b are arranged at opposite ends of the second cylinder 81b. Features of the second turbine housing 83a of the second drive system 82b in common with the first and second turbine housings of the drag reduction system 1 of Figures la-g are indicated with the same reference numerals with the first number being ‘ 8’ instead of ‘ 1 ’ .

[0176] The third drive system 82c further comprises a second turbine housing 83b and a second turbine impeller housed within the second turbine housing 83b. The first and second turbine impellers of the second drive system 82b and, respectively, the second common turbine housing 819b and the second turbine housing 83b of the third drive system 82c are arranged at opposite ends of the third cylinder 81c. Features of the second turbine housing 83b of the third drive system 82c in common with the first and second turbine housings of the drag reduction system 1 of Figures la-g are indicated with the same reference numerals with the first number being ‘ 8’ instead of ‘ 1 ’ .

[0177] Each of the first, second, and third drive systems 82a, 82b, 82c comprises a first and second drive shaft, a first and second outer bearing, and a first and second inner bearing as described above with reference to Figures le and If. The first and second turbine impellers of the first, second, and third drive systems 82a, 82b, 82c are arranged with respect to the respective first and second drive shaft, first and second outer bearing and first and second inner bearing, and the respective first, second, and third cylinders 81a, 81b, 81c, to drive rotation of the respective first, second, and third cylinders 81a, 81b, 81c as described above with reference to Figures le and If. In other examples, one or more of the first, second, and third drive systems 82a, 82b, 82c may be configured differently.

[0178] It will be appreciated that the drag reduction system 8 of Figures 8a-c is merely illustrative and that the drag reduction system may take any form within the scope of the appended claims.

[0179] Figure 9a shows a close-up schematic isometric view of a vehicle 9 according to another example. Figure 9b shows a further close-up schematic isometric view of the vehicle 9. Figure 9c shows a close-up schematic rear view of the vehicle 9. The vehicle 9 comprises a vehicle body 917. The vehicle body 917 comprises a rear face 918. In this example, the vehicle 9 is a van and the rear face 918 of the vehicle body 917 is substantially flat. The vehicle 9 further comprises the drag reduction system 8 of Figures 8a-c. In this example, the drag reduction system 8 is mounted to the rear face 918 of the vehicle body 917 via the mounting points of the first and second common turbine housings 819a, 819b, the mounting points of the second turbine housing 83a of the second drive system 82b, and the mounting points of the second turbine housing 83b of the third drive system 82c.

[0180] The drag reduction system 8 is arranged such that the first cylinder 81a extends horizontally between the side edges of the rear face 918 of the vehicle body 917 proximate the top edge of the rear face 918 of the vehicle body 917. The length of the first cylinder 81a is substantially equal to the width of the rear face 918 of the vehicle body 917. The second and third cylinders 81b, 81c extend vertically between the top and lower edges of the rear face 918 of the vehicle body 917 proximate opposite side edges of the rear face 918 of the vehicle body 917. In this example, the first fluid inlets 820a, 820b of the first and second common turbine housings 819a, 819b are arranged above the top edge of the rear face 918 of the vehicle body 917. The second fluid inlets 821a, 821b of the first and second common turbine housings 819a, 819b and the fluid inlets 84a, 84b of the second turbine housings 83a, 83b of the second and third drive systems 82b, 82c are arranged beyond the respective side edges of the rear face 918 of the vehicle body 917.

[0181] The first cylinder 81a is arranged with respect to the top edge of the rear face 918 of the vehicle body 917 as described above with reference to the cylinder of Figure 5g. The first drive system 82a is configured to drive rotation of the first cylinder 81a as described above with reference to the drive system and the cylinder of Figure 5g.

[0182] Figure 9d shows a close-up schematic plan view of the vehicle 9. For clarity, only the vehicle body 917 and the second and third cylinders 81b, 81c are shown. The second and third cylinders 81b, 81c protrude beyond the respective side edges of the rear face 918 of the vehicle body 917 in a radial direction, r, parallel to the rear face 918 of the vehicle body 917, such that the cylinders 81b, 81c protrude in the radial direction, r, into the surrounding fluid when the vehicle 9 is in motion. The second and third cylinders 81b, 81c protrude in the radial direction, r, beyond the respective side edges of the rear face 918 of the vehicle body 917 by a maximum distance, H. In this example, H is measured between the respective side edge of the rear face 918 of the vehicle body 917 and the furthest point on the circumference of the respective cylinder 81b, 81c in the radial direction, r. In this example, H is 1 / 4D, where D is the diameter of the respective cylinder 81b, 81c. In other examples, the maximum distance may be smaller or larger.

[0183] The second and third cylinders 81b, 81c are separated from the rear face 918 of the vehicle body 917 by a minimum distance, L. In this example, L is 1 / 2D, where D is the diameter of the respective cylinder 81b, 81c. In other examples, L may be smaller or larger.

[0184] Figure 9e shows a close-up schematic plan view of the vehicle 9. For clarity, only the vehicle body 917 and the second and third cylinders 81b, 81c are shown. The second and third drive systems 82b, 82c are configured to drive rotation of the second and third cylinders 81b, 81c, respectively, about the longitudinal axis of the respective cylinder 81b, 81c, in opposite directions. As viewed directly from above, as shown in Figure 9e, the second drive system is configured to drive rotation of the second cylinder 81b about its longitudinal axis in the anti-clockwise direction and the third drive system 82c is configured to drive rotation of the third cylinder 81c in the clockwise direction. In this example, the second and third drive systems 82b, 82c are configured to drive rotation of the respective cylinder 81b, 81c such that a velocity ratio of the respective cylinder 81b, 81c, defined as a ratio of the magnitude of the tangential velocity of the respective cylinder 81b, 81c, to the magnitude of the freestream velocity of a surrounding fluid when the vehicle 4 is in motion, is greater than 2.0. In other examples, the second and third drive systems 82b, 82c may be configured to drive rotation of the respective cylinder 81b, 81c such that a velocity ratio of the respective cylinder 81b, 81c is according to any value of velocity ratio described herein.

[0185] Figure 9f shows a further schematic side view of the vehicle 9 when travelling in the normal forward direction of travel, d, with the drag reduction system absent. Figure 9f shows the freestream, F, of the surrounding fluid of the vehicle 9 and the flow, Bi, of the surrounding fluid as influenced by the rear face 918 of the vehicle body 917. Figure 9g shows a further schematic side view of the vehicle 9 when travelling in the normal forward direction of travel, d, with the drag reduction system 8 present. Figure 9g shows the freestream, F, of the surrounding fluid of the vehicle 9 and the flow, B2, of the surrounding fluid as influenced by the drag reduction system 8 with the first, second, and third cylinders rotating as described above. As illustrated by the flows, B i, B2, rotation of the cylinders causes the cylinders to entrain the surrounding fluid and inhibit recirculation of the surrounding fluid at the rear of the vehicle 9.

[0186] It will be appreciated that the vehicle 9 of Figures 9a-f is merely illustrative and that the vehicle may take any form within the scope of the appended claims.

[0187] Figure 10a shows a close-up schematic isometric view of a vehicle 10 according to another example. Figure 10b shows a further close-up schematic isometric view of the vehicle 10. Figure 10c shows a schematic rear view of the vehicle 10. The vehicle 10 comprises a vehicle body 1017. The vehicle body 1017 comprises a rear face 1018. In this example, the vehicle 10 is a van and the rear face 1018 of the vehicle body 1017 is substantially flat. The vehicle 10 further comprises two drag reduction systems la, lb of Figures la-g. In other examples, the vehicle 10 may comprise two drag reduction systems of Figures 2a-f, two drag reduction systems of Figures 3a-f, and / or two drag reduction systems of Figure 4 in place of or in addition to the two drag reductions system la, lb.

[0188] In this example, each drag reduction system la, lb is mounted to the rear face 1018 of the vehicle body 1017 via the mounting points of the respective first s 13aa, 13ab and second turbine housings 13ba, 13bb. The drag reduction system la, lb are arranged such that the cylinders I l a, 11b extend vertically between the top and lower edges of the rear face 1018 of the vehicle body 1017 proximate opposite side edges of the rear face 1018 of the vehicle body 1017. In this example, the fluid inlets 14aa, 14ab of the first turbine housings 13aa, 13ab and the fluid inlets 14ba, 14bb of the second turbine housings 13ba, 13bb are arranged beyond the respective side edges of the rear face 1018 of the vehicle body 1017. The cylinders I l a, 11b are arranged with respect to the side edges of the rear face 1018 of the vehicle body 1017 as described above with reference to the cylinders of Figure 9d. The respective drive system 12a, 12b of the first and second drag reduction systems la, lb is configured to drive rotation of the respective cylinder 11 as described above with reference to the drive systems and cylinders of Figure 9e.

[0189] Figure I la shows a schematic plan view of the vehicle 10 of Figures lOa-c showing the velocity of the surrounding fluid when the vehicle 10 is travelling in the normal forward direction of travel, d, if the drag reduction systems were absent. Figure 11b shows a schematic side view of the vehicle 10 of Figures lOa-c showing the velocity of the surrounding fluid when the vehicle 10 is travelling in the normal forward direction of travel, d, with the drag reduction systems la, lb present.

[0190] As illustrated by Figures I la and 11b, the drag reduction systems la, lb have the effect of increasing the volume of the region of low velocity flow of the surrounding fluid behind the rear face 1018 of the vehicle body 1017, which in turn increases the pressure of this region of flow. In turn, this reduces drag resulting from the rear face 1018 of the vehicle body 1017.

[0191] Figure 12a shows a schematic isometric view of a drag reduction system 11 for a vehicle according to another example. Figure 12b shows a further schematic isometric view of the drag reduction system 11. Figure 12c shows a schematic rear view of the drag reduction system 11. Figure 12d shows a schematic side view of the drag reduction system 11. In use, the drag reduction system 11 may be mounted to a body of a vehicle. The drag reduction system 11 may be mounted to a rear face of a body of a vehicle. Features of the drag reduction system 11 in common with the drag reduction system 1 of Figures la-g are indicated with the same reference numerals with the first number being ‘ 11 ’ instead of ‘ 1 ’ .

[0192] The cylinder 111 of the drag reduction system 11 comprises two parts 111-1 , 111-2. In this example, the two parts 111-1 , 111-2 are equal in length, but may have different lengths in other examples. The drive system 112 comprises a single turbine impeller and a single turbine housing 113 arranged to house the turbine impeller. The turbine housing 113 and, respectively, the turbine impeller, is arranged between the two parts 111-1 , 111-2 of the cylinder 111. The fluid outlet 115 of the turbine housing 113 is arranged on an underside of the turbine housing 113.

[0193] The drag reduction system 11 further comprises first and second mounting brackets 1125a, 1125b arranged at opposite ends of the cylinder 111. The first mounting bracket 1125a is arranged at the opposite end of the first part 111-1 of the cylinder 111 to the turbine housing 113 and the second mounting bracket 1125b is arranged at the opposite end of the second part 111-2 of the cylinder 111 to the turbine housing 113. The turbine housing 113 and the mounting brackets 1125a, 1125b comprise mounting points for mounting the drag reduction system 11 to a rear face of a vehicle body. Figure 12e shows a close-up schematic exploded rear view of part of the drag reduction system 11. Figure 12f shows a schematic exploded rear view of the drag reduction system 11. Figure 12e shows the drive system 112 with the turbine housing removed and Figure 12f shows the drive system 112 with the turbine housing 113 in place. The drive system 112 comprises first and second bearings 1111 , 1112 arranged on the drive shaft 1110. The turbine impeller 119 is mounted with the turbine housing 113 via the drive shaft 1110 and the first and second bearings 1111 , 1112 such that the turbine impeller 119 and the drive shaft 1110 are configured to rotate relative to the turbine housing 113 about the longitudinal axis of the drive shaft 1110.

[0194] As shown in Figure 12f, the drag reduction system 11 comprises first and second further bearings 1126a, 1126b. The first end plate 117-la of the first cylinder part 111-1 is mounted to the first mounting bracket 1125a via the first further bearing 1126a to allow for rotation of the first cylinder part 111-1 relative to the first mounting bracket 1125a. The second end plate 117- 1 b of the second cylinder part 111-2 is mounted to the second mounting bracket 1125b via the second further bearing 1126b to allow for rotation of the second cylinder part 111-2 relative to the second mounting bracket 1125b.

[0195] Figure 13a shows a schematic isometric view of a vehicle 12 according to another example. Figure 13b shows a schematic side view of the vehicle 12. Figure 13c shows a further schematic isometric view of the vehicle 12. Figure 13d shows a schematic rear view of the vehicle 12. The vehicle 12 comprises a vehicle body 1217. The vehicle body 1217 comprises a rear face 1218. In this example, the vehicle 12 is a van and the rear face 1218 of the vehicle body 1217 is substantially flat. The vehicle 12 further comprises the drag reduction system 11 of Figures 12a-f.

[0196] In this example, the drag reduction system 11 is mounted to the rear face 1218 of the vehicle body 1217 via the mounting points of the turbine housing 113 and the first and second mounting brackets 1125a, 1125b. The drag reduction system 11 is arranged such that the cylinder 111 extends horizontally between the side edges of the rear face 1218 of the vehicle body 1217 proximate the top edge of the rear face 1218 of the vehicle body 1217. The length of the cylinder 111 is substantially equal to the width of the rear face 1218 of the vehicle body 1217. The cylinder 111 is arranged with respect to the top edge of the rear face 1218 of the vehicle body 1217 as described above with reference to the cylinder of Figure 5g. The drive system 112 is configured to drive rotation of the cylinder 111 as described above with reference to the drive system and the cylinder of Figure 5g. Figure 14 illustrates a method 13 of operating a drag reduction system for a vehicle. The method 13 comprises providing 1301 a drag reduction system comprising a cylinder and a drive system configured to drive rotation of the cylinder about its longitudinal axis. The drag reduction system may comprise any drag reduction system described herein, the drag reduction system of any vehicle described herein, any other drag reduction system within the scope of the appended claims, or the drag reduction system of any other vehicle within the scope of the appended claims.

[0197] The method 13 further comprises operating 1302 the drive system to drive rotation of the cylinder such that a ratio of the tangential velocity of the cylinder to the freestream velocity of a surrounding fluid is greater than 2.0, or any other value described herein.

[0198] Any variations of any of the specific examples described above are possible within the scope of the appended claims.

Claims

CLAIMS1. A vehicle comprising a vehicle body and a drag reduction system, the vehicle body comprising a rear face, the drag reduction system comprising: a cylinder arranged behind the rear face of the vehicle body; and a drive system configured to drive rotation of the cylinder about its longitudinal axis.

2. The vehicle of claim 1 , wherein a velocity ratio of the cylinder is defined as a ratio of the magnitude of the tangential velocity of the cylinder to the magnitude of the freestream velocity of a surrounding fluid when the vehicle is in motion, wherein the drive system is configured to drive rotation of the cylinder such that the velocity ratio of the cylinder is greater than 2.0.

3. The vehicle of claim 1 or claim 2, wherein the drag reduction system is arranged such that the cylinder protrudes beyond an edge of the rear face of the vehicle body in a radial direction, H, wherein H is greater than or equal to 1 / 6D, where D is the diameter of the cylinder.

4. The vehicle of any preceding claim, wherein H is less than or equal to 1 / 2D.

5. The vehicle of any preceding claim, wherein the drive system is configured to drive rotation of the cylinder such that the velocity ratio is in the range of 2.5-7.0.

6. The vehicle of any preceding claim, wherein the drive system is configured to drive rotation of the cylinder such that the velocity ratio is in the range of 3.0-5.0.

7. The vehicle of any preceding claim, wherein the cylinder is separated from the rear face of the vehicle body by a minimum distance, L, wherein L is greater than or equal to 1 / 6D, where D is the diameter of the cylinder.

8. The vehicle of any preceding claim, wherein L is greater than or equal to 1 / 3D.

9. The vehicle of any preceding claim, wherein the cylinder is arranged horizontally.

10. The vehicle of claim 9, wherein the cylinder is arranged proximate to a top edge of the rear face of the vehicle body.

11. The vehicle of any of claims 1 to 8, wherein the cylinder is arranged vertically.

12. The vehicle of claim 11 , wherein the cylinder is arranged proximate to a side edge of the rear face of the vehicle body.

13. The vehicle of any preceding claim, wherein the length of the cylinder is substantially equal to the width of the rear face of the vehicle body.

14. The vehicle of any preceding claim, wherein an aspect ratio of the length of the cylinder to the diameter of the cylinder is greater than 4.

15. The vehicle of any preceding claim, wherein the drive system comprises a turbine impeller configured to be driven by a surrounding fluid when the vehicle is in motion to drive rotation of the cylinder.

16. The vehicle of claim 15, wherein the turbine impeller is configured to be driven by a surrounding fluid when the vehicle is in motion to drive rotation of the cylinder such that the velocity ratio of the cylinder is greater than 2.0.

17. The vehicle of any preceding claim, wherein the drag reduction system comprises a guide vane arranged to guide a surrounding fluid towards the cylinder when the vehicle is in motion.

18. The vehicle of claim 17, wherein the guide vane comprises a surface arranged to extend parallel to the circumference of the cylinder, wherein the surface is arranged to extend parallel to between 20% and 50% of the circumference of the cylinder.

19. The vehicle of claim 17, wherein the guide vane comprises a surface arranged to extend parallel to the circumference of the cylinder, or the vehicle of claim 17, wherein a maximum gap between the surface and the cylinder is within the range of 1 / 8D to 1 / 2D, where D is the diameter of the cylinder.

20. The vehicle of any preceding claim, wherein the cylinder is a first cylinder and the drag reduction system comprises one or more further cylinders arranged behind the rear face of the vehicle body.

21. The vehicle of claim 20, wherein the drive system is a first drive system and the drag reduction system comprises one or more further drive systems, wherein each of the one or more further drive systems is configured to drive rotation of one of the one or more further cylinders about the longitudinal axis of the respective further cylinder.

22. The vehicle of claim 20 or claim 21 , wherein the one or more further cylinders are arranged vertically behind the rear face of the vehicle body.

23. The vehicle of any of claims 20 to 22, wherein the one or more further cylinders comprises two further cylinders arranged vertically behind the rear face of the vehicle body.

24. The vehicle of claim 23, wherein the two further cylinders are arranged at either side of the rear face of the vehicle body.

25. The vehicle of any preceding claim, wherein the rear face of the vehicle body is flat.

26. A method of operating a drag reduction system for a vehicle, the drag reduction system comprising a cylinder and a drive system configured to drive rotation of the cylinder about its longitudinal axis, the method comprising operating the drive system to drive rotation of the cylinder such that a ratio of the magnitude of the tangential velocity of the cylinder to the magnitude of the freestream velocity of a surrounding fluid is greater than 2.0.

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