FR3109358A1 - AERODYNAMIC ELEMENT WITH CHEVRONS TRAILING EDGE FOR MOTOR VEHICLES - Google Patents

Abstract

The invention relates to an external element (12) of a motor vehicle, comprising a guide surface (14) of a main air flow (2) caused by the displacement of the vehicle, said guide surface (14) having a trailing edge (16) separating the main air flow (2) from a secondary air flow (4) downstream of the outer member (12) . The trailing edge (16) has, in the extent of the guide surface (14), a herringbone profile (16.1) pointing downstream so as to form, downstream of each of said rafters (16.1), between the primary (2) and secondary (4) air flows, two contra-rotating vortices (18, 20). (Figure to be published with the abstract: Figure 4)

Description

AERODYNAMIC ELEMENT WITH HERRINGBONE TRAILING EDGE FOR MOTOR VEHICLES

The invention relates to the field of aerodynamics, more particularly the aerodynamics of motor vehicles.

The aerodynamic drag of a moving body 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 the frictional drag related to the friction of the fluid along the surfaces of the body and the shape drag when the streamlines of the flow are not parallel at all points to the surface of the body. In the case of a motor vehicle, the shape drag is the main drag. It is also the result of areas of depression due to recirculation of air caused by a separation of the flow along abrupt variations of the exterior surface of the vehicle. This phenomenon is present, for example, in the wheel arches, the exterior mirrors, the underbody and especially the rear face of the vehicle. It is commonly accepted that the rear face of a motor vehicle contributes 25 to 30% of the total drag of the vehicle.

Motor vehicles having a tailgate, that is to say a rear trunk lid, comprising a window, commonly called a rear window, have the general disadvantage that an air recirculation zone forms on the rear face of the vehicle, causing depression and therefore increasing 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, lowering the intensity of the recirculation.

The published patent document FR 3 059 964 A1 discloses a rear spoiler for a motor vehicle, comprising retractable scoops, configured to, when they are deployed, guide part of the air flow at the rear of the roof of the vehicle. straight up the rear window to clean it.

The 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 participating in the aerodynamic drag.

The 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 tailgate and each comprising an air inlet and a corresponding air outlet intended to counteract the lateral vortices participants in aerodynamic drag.

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 .

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

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

Counter-rotating vortices are around a main direction extending along a main direction of the main airflow.

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

According to an advantageous mode of the invention, the chevrons are at least 3 in number, preferably at least 4.

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°.

According to an advantageous embodiment of the invention, the trailing edge comprises transverse portions connecting the downstream ends of the rafters.

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.

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

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

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

According to an advantageous mode of the invention, the vehicle comprises a bodywork and an underbody, and the at least one external element is an aerodynamic accessory fixed to the bodywork or to the underbody.

According to an advantageous embodiment of the invention, the motor vehicle comprises a tailgate 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 .

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, that is to say moving away transversely from the primary flow) where a secondary flow is formed with recirculation 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 of the object . Such a step presents a divergent profile with respect to the guiding surface of the primary flow and the trailing edge.

is a schematic representation of the rear face of a vehicle and the aerodynamic phenomena causing drag;

is a side view of the rear of a motor vehicle equipped with a rear spoiler;

is a perspective view of a motor vehicle equipped with a rear spoiler according to the invention;

is a detail view of part of the rear spoiler of FIG. 3, illustrating the aerodynamic phenomena;

is a double rear and side view of the rear face of the motor vehicle of FIGS. 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;

is a geometric representation of a bodywork element, according to the invention;

is a geometric representation of a bodywork element, according to a variant of the invention.

detailed description

Figure 1 schematically illustrates the airflow along the rear face of a vehicle.

It is observed that the main flow 2 along the roof of the vehicle separates from the rear window due to the abrupt geometric variation and then generates a secondary vortex flow 4. The latter tends to form a recirculation of air along the rear window. The depression formed by this recirculation of air favors 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 side of the vehicle.

The phenomenon described above is particularly harmful for angles of inclination of the rear screen of between 30° and 40°, these angles being valued 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 further to the rear, so that the intensification of the two counter-rotating main vortices is also limited.

Figure 2 is a profile view of the rear part of a motor vehicle equipped with a rear roof spoiler, that is to say 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, consequently, to cause the angle of inclination seen by the flow to pass to a value δ greater than the geometric angle φ of inclination of the rear window .

Figure 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 hatch 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 of 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.

Figure 4 is a detail view of part of the rear spoiler of Figure 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 V ₀ 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 with respect to the direction V ₀ of the main flow 2, and this in the opposite way. 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, turns anti-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 a single secondary vortex 22 rotating in a direction opposing the corresponding main vortex 6 (see FIG. 1), so as to oppose it. Figure 4 shows only the left end portion (in the direction of movement of the vehicle forward) 16.4 of the trailing edge 16, it being understood that the right end portion of the trailing edge is symmetrical and generates a vortex secondary rotating in the opposite direction to the corresponding main tourbillon 8 (see FIG. 1).

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

The first effect relates to the two secondary end vortices 22. Each of these two secondary vortices 22 opposes the adjacent main vortex 6 or 8 (see FIG. 1) and thus makes it possible to reduce them. Their reduction makes it possible to significantly reduce the drag at the level of the rear face of the vehicle.

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 decreases the separation of the main flow 2 and, therefore, reduces the size of the recirculation bubble of the secondary flow 4.

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 decrease 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 secondary flow.

The left-hand view of FIG. 5 illustrates the distribution of pressure coefficient difference C _p 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. ie generally right. The pressure coefficient C _p is an adimensional aerodynamic coefficient commonly used in the study and the graphic representation of the distribution of pressures around bodies placed in an incompressible fluid flow. The pressure coefficient C _p is equal to 1 when the local pressure, for example along a disturbance, is a stop pressure, i.e. equals 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.

More particularly, the left view is a front view of the rear face of the vehicle, showing by shades of gray the distribution of the difference in pressure coefficient C _p between a vehicle according to the invention and a conventional vehicle. Very light or white areas correspond to negative differences (or delta Δ) and dark areas correspond to positive differences. This means that the invention reduces the pressure coefficient C _p on the upper part but increases it on the lower part.

The right view of Figure 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 average pressure forces. The curve in broken line corresponds to the vehicle with the conventional spoiler whereas the curve in continuous line corresponds to the vehicle with the spoiler according to the invention, as illustrated in FIGS. 3 and 4. In the upper part, it is observed that the coefficient of pressure C _p , 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 is observed that the pressure coefficient C _p , 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 distribution of the pressure coefficient C _p as well as from the profile of the rear face. Indeed, the upper part, corresponding to the rear window, has a significant inclination with respect to the vertical, meaning that the horizontal component of the pressure is more reduced than on the lower part which is substantially less inclined.

It should also be noted that the drag reduction obtained by the herringbone trailing edge makes it possible to modify the inclination of the spoiler, in particular upwards in order to reduce the lift, without however increasing the drag at the level of the rear side of the vehicle.

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 external 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 in particular the edges of the fenders at the front of the front and/or rear wheel arches, the trailing edges of exterior mirror shells, the connections between two bodywork panels, such as between two adjacent doors , the connection zones between the rear flanks and the rear face, the spoiler corners, the trailing edges of screens under the body of the vehicle and/or on the suspension elements (such as the connecting arms of the wheels to the body ), the trailing edges of rim spokes or wheel trims, and/or the trailing edges of the fan motor housing.

FIG. 6 illustrates various 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.

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 however understood that they may be curved, in particular with points of inflection and convexity changes. They converge towards the rear or downstream and are inclined at an angle α and β, respectively, with respect to the direction of the main flow, which in this case is perpendicular to a main direction of the trailing edge 16. Advantageously the angles α and β are equal. Each of the angles α and β is advantageously between 10° and 80°, more advantageously between 20° and 50°. The angle α+β 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 can be provided. The same applies to the hollows between the rafters 16.1. Radii of curvature R ₁ , R ₂ , R ₃ and R ₄ , or any other form of connection (such as a portion of an ellipse, a parabolic profile, etc.) can be provided in order to form roundings. The downstream extent of the rafters 16.1, expressed by the lengths L ₁ and L ₂ , their width l and their pitch e can take on different values depending in particular on the total size of the bodywork element and the conditions of flow that will reign there.

FIG. 7 illustrates a bodywork element according to a variant of the bodywork element of FIG. 6, in accordance with the invention and applicable to the spoiler as illustrated above or even to any other application, in particular as listed above. The reference signs in FIG. 6 are used to designate identical or corresponding elements, these signs being however supplemented by a "'" (prime) sign 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 Figure 6.

The bodywork element 12' of FIG. 7 differs from that of FIG. 6 in that the downstream ends of the rafters 16'.1 are interconnected by transverse wall portions 16'.5. These transverse portions 16′.5 have a width L ₁ -L ₂ less than L ₁ , advantageously less than 50%, more advantageously less than 20%, of the length L ₂ expressing the extent, downstream, of the portions inclined 16'.2 and 16'.3 from 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 transverse portions 16′.5 may moreover have a profiled section from an aerodynamic point of view, such as in particular with rounded edges, an elliptical section or even an airplane wing profile section, so as to reduce its streak.

In general, the bodywork element formed by the surface for guiding the primary flow and the trailing edge of said guide surface is advantageously generally flat or has one or more curvatures while remaining generally flat, namely between two planes spaced from each other by a distance less than 20% of the distance between said two transverse ends.

Claims (10)

Exterior element (12; 12') of the 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 (14; 14') having a trailing edge (16; 16') separating the main air flow (2) from a secondary air flow (4) downstream of the outer element (12); 12'); characterized in that the trailing edge (16; 16') has, in the extent of the guide surface (14; 14'), a herringbone profile (16.1; 16'.1) pointing towards 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). External element (12; 12') according to claim 1, characterized in that the chevrons (16.1; 16'.1) are at least 3 in number. External element (12; 12') according to one of Claims 1 and 2, 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 (α+β) of less than 100°. Outer element (12') according to one of Claims 1 to 3, characterized in that the trailing edge (16') comprises transverse portions (16'.5) connecting the downstream ends of the rafters (16'.1) . Outer element (12; 12') according to one of Claims 1 to 4, 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. Exterior element (12; 12') according to one of Claims 1 to 5, characterized in that the said bodywork element is configured to form a step under the trailing edge (16; 16'), able to form the flow secondary air (4). Exterior element (12; 12') according to one of Claims 1 to 6, characterized in that the said exterior element is a rear roof spoiler of the motor vehicle (10). Motor vehicle (10) comprising at least one outer element, characterized in that the at least one outer element (12; 12') is according to one of Claims 1 to 7. Motor vehicle (10) according to Claim 8, characterized in that the vehicle comprises a bodywork and an underbody, and the at least one external element is an aerodynamic accessory fixed to the bodywork or to the underbody. Motor vehicle (10) according to one of Claims 8 and 9, characterized in that the said motor vehicle comprises a tailgate 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 sloped pane.