GB2613052A

GB2613052A - Vehicle drag reduction

Info

Publication number: GB2613052A
Application number: GB2213255.9A
Authority: GB
Inventors: Potts-Johnson Benjamin
Original assignee: Severn Innovation Ltd
Current assignee: Severn Innovation Ltd
Priority date: 2022-09-09
Filing date: 2022-09-09
Publication date: 2023-05-24
Also published as: GB202213255D0

Abstract

A vehicle drag reduction structure for use at the rear of a vehicle has at least one concave surface. Typically the structure has an outer flow surface 5, 8 and an inner flow surface 14, which may meet at a downwind edge to form a structure tapering in the downwind direction. The surfaces may each comprise a curved portion 7 and a flat portion 6. The structure may comprise two or three outer flow surfaces 5, 8, for example extending from side and top edges of a vehicle, and a central flow divider 13, which may taper in the downwind direction, extend rearward from a rear area of a vehicle, taper linearly or non-linearly, taper from a square, circular or polygonal base, and have at least one curved surface. The structure may comprise a fairing located proximate and parallel to an outer flow surface. The structure may be integrally formed with a door 4. A vehicle with such a structure is also provided, which may be a trailer.

Description

Vehicle drag reduction

Background

Aerodynamic drag accounts for approximately 50-60% of total energy losses for typical articulated vehicles travelling at 50-60mph. Of the sources of aerodynamic drag, the rear end of the vehicle accounts for approximately 25% of total aerodynamic drag and so approximately 12.5% of total energy losses at motorway cruising speed.

Several attempts to reduce trailer rear end drag have been made. The most effective method of reducing trailer drag is the 'boat tail' arrangement. In this arrangement three or four panels are arranged at the rear end to create a hollow tapered structure extending rearward from the rear of the vehicle. A simplified example is shown in figure 1. These panels can be folded flat against the rear door or trailer side for unloading.

Such boat tails extend 3' to 5' (0.9m to 1.5m) from the rear of the trailer so may exceed vehicle length regulations and pose a collision hazard to other vehicles and structures when manoeuvering. A deployed boat tail also increases crosswind instability. The folding and unfolding of the panels require driver training and results in wear of components such as struts, hinges, runners and rails. This solution therefore has associated operation and maintenance costs.

Another solution being investigated provides slats extending rearwards from the upper and side surface rear edges similar to the arrangement shown in figure 2. An alternative solution places a fairing above the top of the back door to intercept air flowing over the top of the trailer and direct it down into the region behind the back door. Simulations indicate this solution makes little difference to drag and may increase drag in certain flow conditions. The reason for this unsatisfactory performance is the fairing is placed beyond the cross-section of the trailer therefore increasing the effective cross-section of the vehicle and intercepting more airflow.

This arrangement also creates lift. Increased lift does reduce losses due to rolling resistance (tyre material hysteresis, bearing friction etc') but also increases drag and reduces grip making skidding and jack-knifing more likely.

Similar fairings placed just beyond the outer side edges of the back door have also been proposed. These avoid the problems associated with creating lift but similarly have little effect on overall drag as any reduction in drag is matched by increased drag due to increasing the effective cross-section of the vehicle. Simplified examples having top and side fairings are shown in figure 3 and 4.

Ducting of air from the front to the back of the vehicle has also been investigated. Ideally air scooped from the high pressure region in front of the vehicle is channelled and dumped into the low pressure region behind the vehicle thereby reducing drag. For a passive ducted system, the air flows through the duct due to this pressure difference and at a velocity greater than the air passing over the outer surfaces of the vehicle. An elongate duct has a large surface area (inner surface) to volume ratio. The ducted air experiences high skin friction drag due to its high velocity and the large inner surface area of the duct. The duct inner skin friction drag eliminates the advantages of ducting air.

In addition to passive ducting, a further alternative is pumped or forced air transfer from the high pressure region at the front of the vehicle to the low pressure region at the rear of the vehicle. Any advantage in reduced drag is offset by the energy requirement of the pumping mechanism. Also, the pumping arrangement requires additional equipment adding to complexity, weight and cost.

The present invention seeks to provide in improved solution that provides the drag reduction provided by the boat tail arrangement without the disadvantages of a substantial structure extending rearward from the vehicle. The ideal solution is also easy to operate, has minimal moving parts which may break or wear, avoids the requirement for additional equipment and does not require additional power input.

Summary of invention

The invention is defined by the associated claims.

Drawings Figures 1 to 4 show examples of prior art arrangements.

Figures 5a and 5b show a simplified representation of an articulated vehicle.

Figures 6 to 8 are enlarged views of the rear door of figure 5b.

Figures 9 to 12 show various views and cross-sections of a number of example doors incorporating the inventive concept.

Figures 13a-d are various view of an add-on drag reduction structure.

Figures 14 to 17 show various results from CFD simulations. Figures 18 to 20 show various alternative arrangements.

Description

Streamlining a moving object such as a vehicle is known to reduce drag by managing passing airflow, avoiding turbulence and a reducing adverse pressure distributions. Streamlining an articulated vehicle trailer would require a long tapered rear end; this is impractical due to vehicle length regulations, loading and unloading practicalities, and commercial requirements to maximise payload volume. The ideal trailer therefore retains the substantially rectangular shape of the conventional trailer and any drag reduction arrangement occupying a minimum of vehicle length. The ideal trailer drag reduction arrangement will offer the drag reduction provided by the conventional boat tail arrangement without the rearwardly extending structure.

A conventional streamlined tail for a vehicle provides a surface which controls the flow of air from regions around the vehicle to a point of convergence downstream. Flows meeting at this point of convergence are ideally parallel and have the same velocity. The ideal trailer drag reduction device achieves the flow structure created by the streamlined tail without the physical structure of the streamlined tail; air is directed along a path that the air would have followed if a physical streamlined tail were present.

The present invention satisfies these requirements by creating a stable circulating airflow structure extending from the rear of the vehicle. The circulating airflow structure has a size and shape comparable to that of the ideal physical streamlined tail so can be considered to be a pseudotail. This pseudotail not only controls the passing airflow to reduce drag but also reduces the transient instability due to turbulence so improves vehicle stability.

The pseudotail is formed from circulating airflows cell structures. The circulation is driven by air flowing from the upper and side surfaces The inventive concept provides flow surfaces designed to shape the airflows in the pseudotail and consequently control air flowing from around the vehicle to a region downstream of the vehicle in a way that reduces drag.

In the following examples the x direction is the direction is the direction of normal travel in straight and level cruise of the vehicle, the y direction is the horizontal direction transverse to the x direction, and the z direction is the vertical direction perpendicular to both the x and y directions. Incident air flows in the normal direction of movement are in the x direction, are upwind to downwind, and are increasing in x position. When in operation, the vehicle drag structure therefore has an upwind side and a downwind side.

S

Figures 5a and 5b show an articulated (semi) vehicle combination 1. The combination 1 includes a tractor unit (cab, rig) 2 and a trailer 3. The interior of the trailer has a substantially constant rectangular cross-section in a plane perpendicular to the x direction. The trailer is provided with two doors 4 at its rear end. The doors 4 may be any size but in this example are large enough to provide direct access to the full area of the interior cross-section. When closed, the doors 4 fully close the interior cross-section of the trailer 3. Referring to figure 6, the doors 4 have a thickness which when the doors 4 are closed extends in the x direction. The doors 4 may have any thickness but in the present example have a thickness of 1' (0.3m). This thickness is much greater than the thickness of a conventional trailer door but it is this thickness that contains the extent of the trailer drag reduction structure. In this example, the trailer drag reduction structure is integral to the doors 4.

The drag reduction structure comprises a lateral outer flow surface 5. The outer flow surface 5 comprise an angled portion 6 and a curved portion 7. The angled portion 6 is flat and is held at an angle relative to the trailer 3 side surface when the doors are closed. The curved portion 7 bridges the gap between the trailer 3 side surface and the angled portion 6. The curved portion 7 tangentially meets the trailer 3 side surface at its upwind edge 10 and tangentially meets the angled portion 6 at its downwind edge 11. The curved portion 7 therefore provides a smooth transition in airflow direction. The angled portion 6 has an upwind edge 11 coincident with the downwind edge 11 of the curved portion 7 and a downwind edge 12.

An upper outer flow surface 8 similarly provides a smooth transition in airflow direction through an angle from the trailer 3 upper surface. The door 4 also has a lower outer flow surface of a similar nature. Any outer flow surface may have an angled portion and a curved portion. These curved surfaces are not possible in folding panel arrangements.

Hollow recesses 9 are provided in the outer surface of the door. The recesses 9 of the two doors are separated by a central flow divider 13. The central flow divider 13 may take several forms. The central flow divider 13 performs the function of dividing airflow moving towards the center of the rear of the trailer towards the hollow recesses 9. The central flow divider 13 is tapered in the downwind direction. In the present example, the central flow divider is tapered at 15°. This tapering assisting in parting the incident airflow and directing it towards the downstream edges 12.

Inner flow surface 14 defines the recess 9. A portion of the inner flow surface 14 extends from the central flow divider 13 to the downwind edge 12 of the outer flow surface 5,8. A portion of the inner flow surface 14 may lie parallel to or make a small angle e'g' 5° to a portion of an outer flow surface 5,8 in the vicinity of a downwind edge 12.

The doors 4 are mounted to the trailer 3 by brackets 20 and hinge pins 21. It should be noted the hinge pin 21 is located rearmost on the door 4; this permits the door to rotate 2700 and lie flush against the trailer side. Figures 7 and 8 show the doors 4 in various stages of opening. This permits unobstructed access to the trailer interior and permits the trailer to be backed onto a loading bay.

Unlike conventional solutions, there are no moving panels that the driver has to manipulate before opening the doors. This saves on time, driver training and the inevitable maintenance cost associated with wear of moving parts. The drag reduction structure being integral to the doors 4 allows for a unitary construction which is inevitable stronger and less prone to fatigue than other solutions.

Figure 9a is a perspective of a second example of door 4 and figure 9b shows a side view of the second example. As seen in section A-A of figure 9b, a channel 22 is provided in the outer flow surface 23. A portion of the surface which defines the central flow divider 13 is curved 20. The central flow divider 13 is therefore tapered. A portion of surface 14 is curved 21 where is meets the channel 22. Channels are similarly provided in the upper and lower outer surfaces.

Figure 102 is a perspective view of a third example of door 4 and figure 10b shows a side view of the third example. As seen in section B-B of figure 10b, inner flow surface 14 has curved portions 31, 32.

Figure 11a is a perspective of a fourth example of door 4 and figure 11b shows a side view of the fourth example. As seen in section C-C of figure 11b, inner flow surface 14 has a straight portion 41 and a curved portion 42. Portion 41 is set at an angle to the downwind direction giving the central flow divider 13 a tapered form. The a portion of the inner flow surface 42 in the region of the outer flow surface angled portion 6 makes a small angle to the angled portion 6 giving a tapered cross-section.

Figures 12a and 12b are top and side views of the fourth example as seen in figures 11a and 11b. Sections E-E and D-D show some example angles for the various outer flow surfaces 5,8 angled portions band the angle of taper for the central flow divider 13. Other angles may be utilised. The optimum angle for the top and bottom outer flow surfaces 8 in section E-E may not be the same. This is due to the difference in passing airflow velocity above and below the vehicle. The angle from the outer flow surface angled portion 6 to the central inner surface 14 is related to the angle of curvature of the curved portion 7. If the angle of curvature of the curved portion 7 is 40°, the angle of the angled portion 6 to the inner surface 14 may be 50°.

The optimum angles of curvature for the angled portions 6 for a vehicle travelling at 60 mph may be: side, 40°; top, 400; bottom, 35°. The radius of curvature for the angled portions may be 8" (0.2m).

The device may be used on the rear of any form of vehicle e'g' bus, van, coach, train, aircraft etc'. The structure may be provided in the form of an add-on structure as seen in figures 13a-d which may be attached to the flat rear surface of a vehicle to provide the same drag reduction arrangement as detailed in previous examples. Figure 13a, 13b, 13c and 13d are top, side, perspective and end views respectively of such an add-on structure. Windows may be provided in the structure to correspond with windows in the vehicle.

Figures 14 to 17 show CFD simulation results for the fourth example at 25 m/s (56mph) with no cross-wind. The suspension arrangement is eliminated for efficient computation but a simplified wheel and axle arrangement is present to introduce turbulence into the flow in the underside of the vehicle. The computational domain includes the rear half of the vehicle for efficiency. It is assumed good flow smoothing and drag reduction devices are used on the upstream portion of the vehicle such that downwind turbulence is non-localised when reaching the rear of the vehicle. The computational domain coincides with the ground plane. Even with an optimised design, turbulence cannot be eliminated completely and solutions are transient. The figures indicate a single indicative flow state in an established transient pattern.

The figures show distinction recirculation regions seated in the door recesses 9. Airflow streamlines are shown in black. Figure 15 is the rear view. This shows four distinct recirculation regions 51 divided into upper/lower and left/right pairs. The flow structure has left/right symmetry but the flow is not symmetrical top to bottom. In this example the upper recirculation regions are slightly larger than the lower recirculation regions. This asymmetry in flow structure is due to the presence of running gear and proximity to ground. This vertical asymmetry varies according to vehicle speed so it is not possible to use a horizontal central flow divider to separate the upper and lower recirculation regions as the optimum height varies with that changing vehicle speed. It has been found that a non-optimally placed flow divider has a greater detrimental effect on drag performance than no flow divider. A vertical flow divider 13 is present and can be used as its optimum location is always central due to left/right flow symmetry (net symmetry); it can therefore be termed a central flow divider 13. The function of the central divider 13 can be seen in figure 16, a partial section of the lower portion of the rear end.

Figure 17 shows a close-up of simulation results in the region of an outer flow surface 6,7. Streamlines are indicated in black. The streamlines indicate a recirculation region 51 exists in the airflow in the door recess 9. Air from the recirculation region 51 travels along the inner flow surface 14 towards a curved portion 42 where it is turned to the downwind edge 12. At the downwind edge 12 airflows from the curved portion 42 and outer flow surface 6,7 meet. These two flows meet in confluence region 52. The two flows meeting in the confluence region 52 are closely aligned due to the shaping of the inner and outer flow surfaces. Curved portion 42 assists in supporting high speed flow over the inner surface 14 and minimises the difference in flow velocities of the inner and outer surface airflows. This alignment and flow velocity similarity minimises the probability of turbulence in the confluence region 52 downstream of the downwind edge 13. The effect is to create a stable recirculation flow structure minimising drag and turbulent instability. The inner and outer curved portions 42,7 respectively assist in laminar confluence.

Compared to a conventional trailer rear end having no drag reduction devices, the conventional boat tail arrangement as seen for example in figure 1 provides a 32% reduction in rear end only drag. The structure provided in the fourth example provides a 28% comparable reduction but with a substantially smaller and simpler construction.

With a 10 m/s cross-wind in addition to a 25 m/s headwind, the lateral (y direction) force on the rear end is 800N less for the fourth example than for the boat tail example. The structure of the fourth example is therefore substantially less affected by cross-wind compared to a boat tail.

In the cross-wind simulation the vertical force (lift, z direction) is +250N for the boat tail example and -600N for the fourth example. This downforce for the fourth example is not seen in zero cross-wind. The fourth example is therefore more stable and less affected by cross-wind than a boat tail arrangement.

As seen in figures 18 to 20 the central flow divider may be a cross, vertical spine, an octagonal or rectangular base pyramid or any other form of structure tapering in the downwind direction, or any combination of such structures.

The scope of the present invention includes any feature of one example of the present invention being used with in any other example. The reader may be aware that all obvious variations of the inventive concept are within the scope of the claims whether explicitly stated or not.

Claims

Claims 1. A vehicle drag reduction structure located at or suitable for locating at the rear of a vehicle and having at least one concave surface.
2. A vehicle drag reduction structure as claimed in claim 1 wherein the concave surface forms a recess in the structure.
3. A vehicle drag reduction structure as in claim 1 or claim 2 wherein the concave surface is formed from any number of flat and/or curved surfaces.
4. A vehicle drag reduction structure as claimed in any preceding claim and having an upwind side and a downwind side, and comprising an outer flow surface and an inner flow surface; wherein the outer flow surface and inner flow surface meet at a downwind edge; and the outer flow surface and inner flow surface form a structure that tapers in the downwind direction.
5. A vehicle drag reduction structure as claimed in claim 4 wherein one or both of the outer flow surface and inner flow surface comprise a curved portion.
6. A vehicle drag reduction structure as claimed in claim 4 or claim 5 wherein the outer flow surface has a curved portion and a flat portion.
7. A vehicle drag reduction structure as claimed in any of claims 4 to 6 wherein a portion of the outer flow surface and a portion of the inner flow surface extending to a downwind edge are substantially parallel.
8. A vehicle drag reduction structure as claimed in any preceding claim wherein the outer flow surface has an upwind edge and a downwind edge, and the upwind edge of the outer flow surface meets an outer surface of a vehicle body.
9. A vehicle drag reduction structure as claimed in any of claims 1 to 5 wherein the outer flow surface is a lateral outer flow surface that extends rearward from a rear portion of a vehicle outer side surface.
10. A vehicle drag reduction structure as claimed in any of claims 1 to 5 wherein the outer flow surface is an upper outer flow surface that extends rearward from a rear portion of a vehicle outer upper surface.
11. A vehicle drag reduction structure as claimed in any preceding claim comprising two or three outer flow surfaces.
12. A vehicle drag reduction structure as claimed in any previous claim comprising a central flow divider.
13. A vehicle drag reduction structure as claimed in claim 12 wherein the central flow divider tapers in the downwind direction.
14. A vehicle drag reduction structure as claimed in claim 12 or claim 13 wherein the central flow divider extends rearward from a rear area of a vehicle.
15. A vehicle drag reduction structure as claimed in any of claims 12 to 14 wherein the central flow divider tapers linearly or non-linearly.
16. A vehicle drag reduction structure as claimed in any of claims 12 to 15 wherein the central flow divider tapers from a square, circular, octagonal, polygonal, rectangular or elongate rectangular base.
17. A vehicle drag reduction structure as claimed in any of claims 12 to 16 wherein the central flow divider has at least one curved surface.
18. A vehicle drag reduction structure as claimed in any of claims 12 to 17 wherein the central flow divider is formed from or located between two flow surfaces.
19. A vehicle drag reduction structure as claimed in any of claims 12 to 18 wherein at least a portion of at least one inner flow surface extends from a central flow divider substantially to a downwind edge of an outer flow surface.
20. A vehicle drag reduction structure as claimed in any preceding claim further comprising a fairing located proximate and parallel to an outer flow surface.
21. A drag reduction structure as claimed in any preceding claim wherein the structure is integrally formed with a door.
22. A drag reduction structure as claimed in claim 21 wherein the door comprises a hinge arrangement having an axis located to permit the door to be rotated from a closed position at the rear of a vehicle to an open position; in the open position the door is located proximate and parallel to a vehicle side.
23. A vehicle drag reduction structure as claimed in any of claims 1 to 4, 8 to 10, 12 to 17 wherein the structure is attachable to the rear of a vehicle.
24. A vehicle comprising a vehicle drag reduction structure as claimed in any preceding claim.