Aerodynamic element with chevron trailing edge for motor vehicles

Abstract

AERODYNAMIC ELEMENT WITH CHEVRON TRAILTING EDGE FOR A MOTOR VEHICLE The invention relates to an external element (12) of a motor vehicle, comprising a guiding surface (14) for a primary airflow (2) caused by the movement of the vehicle, said guiding surface (14) having a trailing edge (16) separating the primary airflow (2) from a secondary airflow (4) downstream of the external element (12). The trailing edge (16) has, within the extent of the guiding surface (14), a chevron profile (16.1) pointing downstream so as to form, downstream of each of said chevrons (16.1), between the primary (2) and secondary (4) airflows, two counter-rotating vortices (18, 20). (Figure to be published with the abbreviation: Figure 4)

Description

Title of the invention: AERODYNAMIC ELEMENT WITH CHEVRON TRAILING EDGE FOR VEHICLE CAR Technical field

[0001] The invention relates to the field of aerodynamics, more particularly the aerodynamics of motor vehicles. Prior art

[0002] The aerodynamic drag of a body moving in a fluid, in this case air, is the result of the forces exerted by the fluid flow, which opposes the movement of the body. It includes friction drag linked to the friction of the fluid along the surfaces of the body and form drag when the flow streamlines are not parallel at all points to the surface of the body. In the case of a motor vehicle, form drag is the main drag. It is also the result of depression zones due to air recirculation caused by flow separation along abrupt variations in the exterior surface of the vehicle. This phenomenon is present, for example, at the wheel arches, exterior mirrors, underbody and especially at the rear of the vehicle. It is commonly accepted that the rear of a motor vehicle contributes 25 to 30% of the total drag of the vehicle.

[0003] Motor vehicles having a tailgate, i.e. a rear trunk lid, comprising a window, commonly called a rear window, have the general disadvantage that an air recirculation zone is formed on the rear face of the vehicle, causing a depression and therefore increasing the drag. It is common to provide a spoiler located at the rear of the roof in order to fix a point of separation of the flow and the pressure at this point, thus imposed upstream and downstream, reducing the intensity of the recirculation.

[0004] Published patent document FR 3 059 964 A1 discloses a rear spoiler for a motor vehicle, comprising retractable scoops, configured to, when deployed, guide a portion of the airflow at the rear of the vehicle roof directly to the top of the rear window, so as to clean it.

[0005] Published patent document US 9,371,098 B discloses a rear spoiler equipped with two air passages at each of the two lateral ends, so as to counteract the lateral vortices contributing to aerodynamic drag.

[0006] Published patent document GB 2 486 517 B2 discloses a rear spoiler consisting of two separate elements arranged on each lateral side of the rear hatch and each comprising an air inlet and a corresponding air outlet intended to counteract the lateral vortices contributing to aerodynamic drag.

[0007] The published patent document WO 2018 / 163528 A1 discloses an aerodynamic accessory arranged on the rear sides of a motor vehicle and comprising fins intended to reduce the lift effect generated by the depression on the upper part of the rear face of the vehicle. Statement of the invention

[0008] The invention aims to overcome at least one of the drawbacks of the aforementioned state of the art. More particularly, the invention aims to reduce the drag caused by a trailing edge separating a main fluid flow from a secondary recirculating fluid flow.

[0009] The subject of the invention is an exterior element of a motor vehicle, comprising a surface for guiding a main air flow caused by the movement of the vehicle, said guide surface having a trailing edge separating the main air flow from a secondary air flow downstream of the exterior element; remarkable in that the trailing edge has, in the extent of the guide surface, a chevron profile pointing downstream so as to form, downstream of each of said chevrons, between the primary and secondary air flows, two counter-rotating vortices.

[0010] The counter-rotating vortices are around a main direction extending along a main direction of the main air flow.

[0011] The exterior element is an aerodynamic element for the bodywork or any other exterior part of the motor vehicle, such as in particular the wheels, the underbody or the suspension arms of the wheels.

[0012] According to an advantageous embodiment of the invention, there are at least 3 chevrons, preferably at least 4.

[0013] According to an advantageous embodiment of the invention, each chevron forms two portions of the trailing edge, converging downstream and forming an average angle of less than 180°, preferably less than 100°.

[0014] According to an advantageous embodiment of the invention, the trailing edge comprises transverse portions connecting the downstream ends of the chevrons.

[0015] According to an advantageous embodiment of the invention, the guide surface and the trailing edge extend transversely to the main air flow, between two transverse ends and between two planes separated by less than 20% of the distance between said two transverse ends.

[0016] According to an advantageous embodiment of the invention, said external element is configured to form a cavity under the trailing edge, capable of forming the secondary air flow.

[0017] According to an advantageous embodiment of the invention, said exterior element is a rear roof spoiler of the motor vehicle.

[0018] The invention also relates to a motor vehicle comprising at least one exterior element, remarkable in that the at least one exterior element is according to the invention.

[0019] According to an advantageous embodiment of the invention, the vehicle comprises a body and a base, and the at least one exterior element is an aerodynamic accessory fixed to the body or to the base.

[0020] According to an advantageous embodiment of the invention, the motor vehicle comprises a rear hatch with a window inclined relative to the horizontal, the at least one bodywork element being a rear roof spoiler adjacent to an upper edge of the inclined window.

[0021] The measures of the invention are interesting in that they make it possible to reduce the aerodynamic drag formed essentially at the level of the secondary flow, downstream of the trailing edge. The trailing edges of a primary flow, followed by a step (below, i.e. moving away transversely from the primary flow) where a secondary flow with recirculation is formed are numerous on a motor vehicle as well as on any object of complex shape subject to a fluid flow, either by displacement of said object relative to the fluid, or by displacement of the fluid relative to the object, or even by a combination of displacements of the fluid and the object. Such a step has a divergent profile relative to the guide surface of the primary flow and the trailing edge. Brief description of the drawings

[0022] [Fig. 1] is a schematic representation of the rear face of a vehicle and the aerodynamic phenomena causing drag;

[0023] [Fig.2] is a side view of the rear of a motor vehicle equipped with a rear spoiler;

[0024] [Fig.3] is a perspective view of a motor vehicle equipped with a rear spoiler according to the invention;

[0025] [Fig.4] is a detail view of a portion of the rear spoiler of [Fig.3], illustrating the aerodynamic phenomena;

[0026] [Fig.5] is a double rear and side view of the rear face of the motor vehicle of Figures 3 and 4, illustrating, in the left part, the distribution of the pressure coefficient Cp, and in the right part, the distribution of the average pressure forces;

[0027] [Fig.6] is a geometric representation of a bodywork element, according to the invention;

[0028] [Fig.7] is a geometric representation of a bodywork element, according to a variant of the invention. Detailed description

[0029] [Fig.l] schematically illustrates the flow of air along the rear face of a vehicle.

[0030] It is observed that the main flow 2 along the roof of the vehicle detaches from the rear window due to the abrupt geometric variation and then generates a secondary vortex flow 4. The latter tends to form an air recirculation along the rear window. The depression formed by this air recirculation promotes the formation of the two main vortices 6 and 8 from the lateral flows along the sides of the vehicle. These two main vortices 6 and 8 are counter-rotating in that they rotate in opposite directions. They are quite powerful and increase the drag at the rear face of the vehicle.

[0031] The phenomenon described above is particularly harmful for rear window inclination angles of between 30° and 40°, these angles being prized from a practical and aesthetic point of view. Indeed, for smaller angles (<30°), the separation of the main flow 2 with respect to the rear window is limited, meaning that the secondary recirculation flow 4 is limited and the intensification of the two counter-rotating vortices is limited. For larger angles (>40°), the separation of the main flow 2 with respect to the rear window is significant and the secondary recirculation flow 4 is more towards the rear, so that the intensification of the two main counter-rotating vortices is also limited.

[0032] [Fig. 2] is a side view of the rear part of a motor vehicle equipped with a rear roof spoiler, i.e. located at the rear of the roof and above the rear window. Such a spoiler makes it possible to extend the guide surface formed by the roof and, therefore, to increase the angle of inclination seen by the flow to a value ô greater than the geometric angle q> of inclination of the rear window.

[0033] [Fig. 3] illustrates in perspective a motor vehicle 2 equipped with a rear roof spoiler 12 according to the invention. The spoiler 12 is fixed to the rear tailgate of the motor vehicle, it being understood, however, that this spoiler can also be fixed to the roof of the vehicle. The spoiler 12 comprises a guide surface 14 for the main flow along the roof, and a trailing edge 16 forming a rear edge of the guide surface 14. This trailing edge 16 has a profile, in the extent of the guide surface and the edge in question, forming chevrons.

[0034] [Fig.4] is a detailed view of a portion of the rear spoiler of [Fig.3]. It can be seen that each chevron 16.1 forms two portions 16.2 and 16.3 of the trailing edge 16, which converge downstream. The main flow 2 represented by the arrows Vo then meets the secondary flow 4, for each chevron 16.1, via the two portions 16.2 and 16.3 of the trailing edge corresponding to this chevron. These two portions are inclined relative to the direction Vo of the main flow 2, and in an opposite manner. This meeting between these two flows via these inclined portions of the trailing edge will generate two counter-rotating secondary vortices 18 and 20. The secondary vortex 18, seen from downstream, rotates clockwise while the secondary vortex 20, also seen from downstream, rotates counter-clockwise. This phenomenon takes place downstream of each chevron 16.1, as can be seen in [Fig.4]. Advantageously, the end portions 16.4 of the trailing edge 16 are configured to form only one secondary vortex 22 rotating in a direction opposing the corresponding main vortex 6 (see [Fig.l]), so as to oppose it. [Fig.4] shows only the left end portion (in the direction of forward vehicle travel) 16.4 of the trailing edge 16, it being understood that the right end portion of the trailing edge is symmetrical and generates a secondary vortex rotating in the opposite direction to the corresponding main vortex 8 (see [Fig.l]).

[0035] The formation of secondary vortices which has just been described has essentially two effects.

[0036] The first effect relates to the two end secondary vortices 22. Each of these two secondary vortices 22 opposes the adjacent main vortex 6 or 8 (see [Fig.l]) and thus allows them to be reduced. Their reduction makes it possible to significantly reduce the drag at the rear face of the vehicle.

[0037] The second effect relates to the pairs of counter-rotating secondary vortices 18 and 20 formed at each chevron, between the end secondary vortices. These pairs of counter-rotating secondary vortices 18 and 20 modify the entrainment of the mixing layer between the main flow 2 and the secondary flow 4, downstream of the trailing edge 16, and thus reduce its thickness. This reduction in the thickness of the mixing layer reduces the separation of the main flow 2 and, therefore, reduces the size of the recirculation bubble of the secondary flow 4.

[0038] These phenomena and effects have been confirmed by simulations and experimental measurements of flow along a vehicle equipped with a conventional rear roof spoiler and the same vehicle equipped with a spoiler according to the invention. Tomographic representations of longitudinal vorticity clearly show the formation of secondary vortices as well as their position relative to the two main vortices. Tomographic representations of total pressure in the wake of the vehicle show a lowering of the curves of the same total pressure, following the wake, corresponding to a reduction in the separation of the main flow. Similarly, tomographic representations of vorticity in the wake of the vehicle show a shortening of the recirculation bubble in the flow secondary.

[0039] The left view of [Fig.5] illustrates the distribution of the difference in pressure coefficient Cp on the rear face of the vehicle, between a configuration according to the invention and a configuration where the trailing edge of the spoiler is conventional, i.e. generally straight. The pressure coefficient Cp is a dimensionless aerodynamic coefficient commonly used in the study and graphical representation of the distribution of pressures around bodies placed in an incompressible fluid flow.The pressure coefficient Cp is 1 when the local pressure, for example along a disturbance, is a stopping pressure, that is to say it is equal to the static pressure plus the dynamic pressure upstream of the disturbance, 0 when the local pressure, for example along the disturbance, is equal to the static pressure associated with the conditions at infinity upstream of the disturbance and is negative when the local pressure, for example along the disturbance, is lower than the static pressure upstream of the disturbance.

[0040] More particularly, the left view is a front view of the rear face of the vehicle, showing by gray levels the distribution of the difference in pressure coefficient Cp between a vehicle according to the invention and a conventional vehicle. The very light or white areas correspond to negative differences (or delta A) and the dark areas correspond to positive differences. This means that the invention reduces the pressure coefficient Cp on the upper part but increases it on the lower part.

[0041] The right-hand view of [Fig. 5] is a side view of the rear face of the vehicle showing two curves of average pressure forces along said rear face, in a longitudinal plane. The arrows are a vector representation of the average pressure forces. The broken line curve corresponds to the vehicle with the conventional spoiler while the solid line curve corresponds to the vehicle with the spoiler according to the invention, as illustrated in Figures 3 and 4. In the upper part, it can be seen that the pressure coefficient Cp, negative, is greater in absolute value, with the invention, which means that the resulting force is greater. On the other hand, in the lower part, it can be seen that the pressure coefficient Cp, also negative, is smaller in absolute value, with the invention, which means that the resulting force is lower. The total force on the rear face turns out to be lower with the invention.This results from the difference in the distribution of the pressure coefficient Cp as well as the profile of the rear face. In fact, the upper part, corresponding to the rear window, has a significant inclination compared to the vertical, meaning that the horizontal component of the pressure is more reduced than on the lower part which is significantly less inclined.

[0042] It should also be noted that the drag reduction achieved by the chevron trailing edge allows the inclination of the spoiler to be modified, particularly upwards in order to reduce lift, without unacceptably increasing drag at the rear of the vehicle.

[0043] The phenomena and advantages which have just been described in relation to the rear roof spoiler of a vehicle apply to other bodywork elements as well as any exterior element comprising a guide surface and a trailing edge downstream of said surface, separating a main flow from a secondary flow induced by the main flow downstream of the trailing edge.These may include the edges of wings in front of the front and / or rear wheel arches, the trailing edges of exterior mirror housings, connections between two body panels, such as between two adjacent doors, the connection areas between the rear flanks and the rear face, spoiler corners, the trailing edges of screens under the vehicle body and / or on suspension elements (such as the arms connecting the wheels to the body), the trailing edges of wheel spokes or wheel trims, and / or the trailing edges of the fan housing.

[0044] [Fig.6] illustrates different geometric parameters of a bodywork element according to the invention, applicable to the spoiler as illustrated above or to any other application, in particular as listed above.

[0045] Each chevron 16.1 forms two portions 16.2 and 16.3 of the trailing edge 16. These two portions 16.2 and 16.3 are in this case schematized by rectilinear portions, it being understood however that they can be curved, in particular with inflection points and changes in convexity. They converge towards the rear or downstream and are inclined at an angle α and [3, respectively, relative to the direction of the main flow, which is in this case perpendicular to a main direction of the trailing edge 16. Advantageously the angles α and [3 are equal. Each of the angles α and [3 is advantageously between 10° and 80°, more advantageously between 20° and 50°. The angle a+[3 between the two portions 16.2 and 16.3 of the trailing edge 16, formed by a chevron 16.1, is advantageously between 20° and 160°, more advantageously between 30° and 100°. The two portions 16.2 and 16.3 of the trailing edge 16 do not necessarily converge to form a point, a rounded or flat portion 16.5 as illustrated in [Fig.6] may be provided. The same applies to the hollows between the chevrons 16.1. Radii of curvature R2, R3 and R4, or any other form of connection (such as a portion of an ellipse, a parabolic profile, etc.) may be provided in order to form rounded edges. The downstream extent of the chevrons 16.1, expressed by the lengths Li and L2, their width 1 and their pitch e may take different values ​​depending in particular on the total size of the bodywork element and the flow conditions which will prevail there.

[0046] [Fig.7] illustrates a bodywork element according to a variant of the car element rosserie of [Fig.6], in accordance with the invention and applicable to the spoiler as illustrated above or to any other application, in particular as listed above. The reference signs of [Fig.6] are used to designate identical or corresponding elements, these signs being however supplemented by a sign " ' " (prime) for the purpose of distinguishing between these two embodiments of the invention. Reference is also made to the description of these elements in relation to [Fig.6].

[0047] The bodywork element 12' of [Fig.7] differs from that of [Fig.6] in that the downstream ends of the rafters 16'.1 are connected to each other by transverse wall portions 16'.5. These transverse portions 16'.5 have a width LrL2 less than LB advantageously less than 50%, more advantageously less than 20%, of the length L2 expressing the extent, towards the downstream, of the inclined portions 16'.2 and 16'.3 of the trailing edge 16'. The transverse portions have the advantage of stabilizing and / or reinforcing the downstream ends of the rafters and of avoiding the formation of sharp or blunt zones. These 16'.5 transverse portions may also have a section profiled from an aerodynamic point of view, such as with rounded edges, an elliptical section or even a section in the profile of an aircraft wing, so as to reduce its drag.

[0048] Generally, the bodywork element formed by the primary flow guide surface and the trailing edge of said guide surface is advantageously generally planar or has one or more curvatures while remaining generally planar, namely between two planes distant from each other by a distance less than 20% of the distance between said two transverse ends.

Claims

Claims

1. Exterior element (12; 12') of a motor vehicle (10), comprising a guide surface (14; 14') for a main air flow (2) caused by the movement of the vehicle (10), said guide surface (14; 14') having a trailing edge (16; 16') separating the main air flow (2) from a secondary air flow (4) downstream of the exterior element (12; 12'); the trailing edge (16; 16') having, in the extent of the guide surface (14; 14'), a chevron profile (16.1; 16'.1) pointing downstream so as to form, downstream of each of said chevrons (16.1; 16'.1), between the primary (2) and secondary (4) air flows, two counter-rotating vortices (18, 20), characterized in that each chevron (16.1; 16'.1) forms two portions (16.2, 16.3; 16'.2, 16'.3) of the trailing edge (16; 16'), converging downstream and forming an average angle (a+[3) less than 100°.

2. External element (12; 12') according to claim 1, characterized in that the chevrons (16.1; 16'.1) are at least 3 in number.

3. External element (12') according to one of claims 1 or 2, characterized in that the trailing edge (16') comprises transverse portions (16'.5) connecting the downstream ends of the rafters (16'.1).

4. External element (12; 12') according to one of claims 1 to 3, characterized in that the guide surface (14; 14') and the trailing edge (16; 16') extend transversely to the main air flow (2), between two transverse ends and between two planes separated by less than 20% of the distance between said two transverse ends.

5. Exterior element (12; 12') according to one of claims 1 to 4, characterized in that said bodywork element is configured to form a step under the trailing edge (16; 16'), capable of forming the secondary air flow (4).

6. Exterior element (12; 12') according to one of claims 1 to 5, characterized in that said exterior element is a rear roof spoiler of the motor vehicle (10).

7. Motor vehicle (10) comprising at least one exterior element, characterized in that the at least one exterior element (12; 12') is according to one of claims 1 to 6.

8. Motor vehicle (10) according to claim 7, characterized in that the vehicle comprises a body and an underbody, and the at least one exterior element is an aerodynamic accessory fixed to the car-

9. roof or underbody. Motor vehicle (10) according to one of claims 7 and 8, characterized in that said motor vehicle comprises a rear hatch with a window inclined relative to the horizontal, the at least one exterior element being a rear roof spoiler adjacent to an upper edge of the inclined window.

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