AERODYNAMIC TRAILER FAIRING

Abstract: A fairing (1) for attachment to or formed integral with the front face (2) of a vehicle so as to reduce aerodynamic drag. The fairing has a leading surface (4) which faces generally in the intended direction of travel of the vehicle. Assuming the fairing has a rectangular peripheral shape, the top (5) and two opposed side adge portions (6) of the leading surface are each curved so as to merge with a respective plane extending rearwardly from the respective adjacent edge of the vehicle front face. The radius of curvature of each of the curved edge portions is selected so that the Reynolds number for air flow around that edge portion at a particular vehicle speed exceeds the critical value of the Reynolds numbers for that speed.

Description: AERODYNAMIC TRAILER FAIRING This invention relates to fairings for attachment to cargo carrying vehicles such as trailers so as to reduce the aero¬ dynamic drag. The invention has been particularly developed for use with trailers and it will be convenient to hereinafter describe the invention in that use, but it is to be understood that- the invention is applicable to other vehicles such as railway trucks or unitary vans having high bluff front walls of the cargo carrying compartment. Much of the aerodynamic drag in prime-movφr trailer combinations arises at the large flat surface presented to the airstream by the front face of the trailer particularly above and at the sides of the cab ,of the prime mover. As the trailer travels through the air, the air flow over the trailer separates from the trailer at the top and vertical leading edges of the trailer creating a much higher pressure on this front face compared to the ideal case where no air flow separati exists. This problem of air flow separation and the creation. ' of eddies is illustrated schematically in figures 1 and 2 of the accompanying drawings. Several methods have been employed to alleviate the high pressure build up which creates substantial drag, but most of those methods tend to give less than satisfactory results in the cross-wind conditions, which trailable vehicles generally experience. Deflectors, whilst working satisfactorily when the vehicle is heading directly into the wind, give negligible savings under high cross-wind conditions. Existing fairings which are fitted tothe -front face of the trailer, such as those described in Australian Patent Specification 491,158, improve this situation but do not fully stop flow separations especially under cross-wind conditions. It is an o ject of the present invention to provide an aerodynamic fairing for attachment to the front face of a trailer or other vehicle and which is operative to substantially reduce aerodynamic drag on the trailer or vehicle even under high cross-wind conditions. According to the present invention there is provided a fairing for attachment to the front face of a vehicle, including a leading surface which faces generally in the • intended direction of travel of the vehicle, said leading surface having at least one continuously curved edge portion arranged to substantially merge with a plane extending rearwardly from an edge of said front face, and said curved edge portion has a radius of curvature which is selected so that the Reynolds number for air flow around the curbed edge portion at a particular vehicle speed exceeds the critical value of the Reynolds number for that speed. With the foregoing arrangement, the drag coefficient is significantly less than when the Reynolds number value is below the critical value. In most cases the front face of the vehicle will be rectangular so that th.e fairing has a peripheral shape such, that side edge portions of the fairing merge with the side walls of the vehicle and a top edge portion merges with a top wall of the vehicle. The vehicle walls may not be walls as such, but may be the outside surface of cargo carried by the vehicle. The essential features of the invention, and further optional features, are described in detail in the following passages of the specification which refer to the accompanying drawings. The drawings however, are merely illustrative of how nv , form and arrangement of the features (whether they be essential or optional features) shown is not to be understood as limiting on the invention. In the drawings; Figure.1 is a diagrammatic plan, view of a vehicle to which the present invention may be applied; Figure 2 is a diagrammatic side elevational view of the vehicle shown in Figure 1; Figure 3 illustrates the air flow pattern over a circular cylinder when the critical Reynolds number is not achieved; Figure 4 is a view similar to Figure 3 but showing the air flow pattern when the critical Reynolds number is exceeded; Figure 5 is a graph illustrating the relationship of drag coefficient to the Reynolds number in the situations illustrated by Figures 3 and 4; Figure 6 is a semi-diagrammatic perspective view of a fairing according to one embodiment of the invention; Figure 7 is a side elevational view of the fairing shown in Figure 6; Figure 8 is a view taken along line VIII-VIII of Figure 7; Figure 9 is a view taken along 1_ IX-IX of Figure 7; Figure 10 is a view similar to Figure 6 but showing another embodiment; Figure 11 is a side elevational view of the fairing shown in Figure 10; Figure 12 is a. view taken along line XII-XII of Figure OMPI 11 ; Figure 13 is a. view taken along line XIII-XIII of Figure 11; Figure 14 is a semi-diagrammatic perspective view showing an embodiment of the invention applied to a vehicle. Significant reduction in-the drag coefficient occurs when the critical Reynolds number is exceeded because at such relatively high values of the Reynolds number the air flow around the curved surface of the fairing and along the associated trailer wall undergoes a change in behaviour. In particular, there is a dramatic reduction in the area of separated flow. The following explanation of the background aerodynamics, may assist in reaching an understanding of what is involved. If a cylinder is placed in a uniform airstream with its axis at right angles to the direction of the airstream (see Figures 2 and 3) it is found that the drag coefficient will remain at a fairly constant high value, typically about 1.1 (Figure 5) , for a large range of values of the dimensionless quantity known as the Reynolds number. The Reynolds number (Re) is defined by the expression Re = 2VR/^_\ where V is the relative airstream velocity, R is the surface radius of curvature and μ is the kinematic viscosity of air. Upon increasing the Reynolds number, the flow behaviour changes to ' give a significantly lower drag coefficient, for example 0.4 - see Figure 5. That is, when a critical value of the Reynolds number is reached a basic change in the flow behaviour occurs resulting in a much smaller area of separated flow. In the lower Reynolds number range the flow experiences an early separation from the cylinder surface at an angle θ to the oncoming flow of . reached, the separation is delayed such that the angle Q of separation is approximately. 100° to the initial flow direction (Figure 4) - i.e., the flow is past the point of maximum diameter before separation occurs ^ Consideration will now be given to application of the foregoing principles to an example fairing 1 as shown in Figures 6 to 9 which is adapted to be attached to the front face 2 of a trailer 3 as showndiagrammatically in Figures 1 and 2 Although the fairing 1 is described as being attached to the trailer 3, that is to be understood as embracing a situation in which the fairing is formed integral with the trailer 3 or a part of the trailer 3 such as the front wall. The example fairing 1 shown in Figures 6 to 9 is generally rectangular in front elevation and has a leading • surface 4 which, in use, faces in the intended direction of travel of the trailer 3. The surface 4 has a top edge portion 5 and oppositely located side edge portions 6, and each of those edge portions 5 and 6 is curved so as to smoothly merge with a respective wall 7 or 8 of the trailer 3 as best seen in Figure 14. Applying the principles of the invention to the edge portions 5 and 6, the radius of curvature "R u of each curved edge portion 5 and 6 is chosen so as to be large enough to enable the critical Reynolds number for the expected trailer velocity to be exceeded so that the type of flow illustrated in Figure 4 dominates, thus alleviating flow separations which normally occur along the side and top walls 8 and 7 of the trailer 3. For an expected trailer speed of about 85 kph, the radius of curvature "R" of each curved edge portion 5 and 6 in the plane at right angles to the associated trailer wall, is chosen to be greater than 200 mm. Preferably, the radius of curvature "R" is greater than 250 mm and for maximum benefits the radius "R" preferably exceeds 300 mm. Selection of a radius "R" having a value as indicated enables the flow to remain attached on the leeward side even under high cross-wind conditions. If maximum drag reduction is being sought - e.g., for a trailer velocity of 85 kph - the. Reynolds number may • exceed 450,000. Each curved edge portion 5 and 6 preferably extends for ardly from the trailer face 2 for a distance such that it subtends a substantial angle pt (Figures 7 and 8) . Preferably, the angle o is greater than 30° and test results have shown that an angle greater than 45° is most satisfactory. In the particular fairing 1 shown in Figures 6 to 9, the surface 4 includes a section 9 of convex curvature extending inwardly of the fairing 1 from each side edge portion 6. Each section 9 has a continuous curvature and may merge with the other, but in the arrangement * shown a substantially flat central region 10 intervenes between the two sections 9. Preferably, as shown, each edge portion 6 is a circular sector subtending, for " example, an angle cC of roughly 60° and merging into a section 9 of a part elliptical contour. Such a profile can be generated by an approximation technique and each elliptical section 9 may be derived from an ellipse having its minor axis 11 in the direction of travel of the trailer 3 and its major axis 12 parallel to but spaced rearwardly of the rear side 13 of the fairing 1 (Figure 8) . The top edge portion 5 is preferably in the general ra s ape o a q according to the criteria described above to eliminate or reduce flow, separation, and the central maximum forward extent .of the surface 4 of the f iring 1 is preferably along a straight line as formed by the region 10. * The arrangement as shown in Figures 6 to 9 is suitable for use with a trailer 3 being drawn by a prime mover 14 having a relatively low cab 15, and the relatively high trailer front face 2 is the portion of the trailer 3 which generates a high drag problem. Figures 10 to 13 show another arrangement of the fairing 1 which is suitable for prime movers 14 having a relatively high cab 15. The top edge portion 5 may be generally quadrant shaped as before having a radius chosen according to the criteria described above. After reaching the maximum forward extent however, the surface 4 may curve rearwardly as shown with a small radius of curvature "r" (Figure 11) so as to then merge with a substantially horizontal lower surface 16 which extends to the trailer front face 2. When viewed in top plan or horizontal section, the central region 10 of the surface 4 is located at the maximum forward extent and may follow a straight line as shown in Figure 12. Once again, the side edge portions 6 merge with part elliptical sections 9 which in turn merge with the straight line region 10. In any of the fairings described, it is possible to provide lower side flow deflectors 17 as shown, for example, in Figures 6 and 10. ' Each such deflector 17 has a curved surface 18 which forms a substantial continuation of a respective side edge portion 6. That is, each -surface 18 is a convex surface arranged to merge with a respective side wall 8 of the trailer 3 below the main body of the fairing 1. The deflector surfaces pre er view) which are chosen according to the same criteria as the radius "R" of the side edge portion 6 so as to create a Reynolds number value for the predetermined speed which is above the critical value. Each deflector surface 18 may be generally in the form of a quadrant of a circle in horizontal section and may extend for any desired vertical extent for the particular trailer 3 on which the fairing 1 is to be used. In order to facilitate merging of the fairing 1 with the trailer walls 7 and 8, the fairing 1 may be arranged to closely fit around the periphery of the front face 2. That is, the rearmost edges of the fairing curved surface portions 5 and 6, may closely overlie the forward end portions of the trailer walls 7 and 8. The rearmost edge of each curved surface 5 and 6 may be tapered to a point in sectional view so as to promote an airflow behaviour in which there is no or minimal separation of the flow. It will be apparent from the foregoing description that a fairing made in accordance with the invention provides a substantial advantage over prior fairings in that it substantially reduces the drag on an associated vehicle, both under direct and cross-wind conditions. A fairing according to the invention can be moulded or otherwise formed from a plastics material. Various alterations, modifications and/or additions may be made to the construction and arrangement of parts as herei described without departing from the spirit or scope of the present invention as defined by the appended claims.

Claims: 1. A fairing for attachment to the front face of a vehicle, including a leading surface which faces generally in the intended direction of travel of the vehicle, said leading surface having at least one continuously curved edge portion arranged to substantially merge with a plane extending rearwardl from an edge of said front face, and said curved edge portion has a radius of curvature which is selected so that the Reynolds number for air flow around the curved edge portion at a particular vehicle speed exceeds the critical value of the Reynolds number for that speed. 2. A fairing according to claim 1, wherein the or each said curved edge portion subtends an angle greater than 30°. 3. A fairing according to claim 1 or 2, wherein said fairing is substantially rectangular in front elevation and has three said curved edge portions, one of which forms a top edge portion and the other two form respective opposite side edge portions. 4. A fairing according to claim 3, wherein said leading surface has a convex curvature merging with and extending inwardly from each said side edge portion. 5. A fairing according to claim 4, wherein said convex curvature forms part of an ellipse having its minor axis extending generally in the intended direction of travel of said vehicle and its major axis located rearwardly of the terminal edge of each said side edge portion. 6. A fairing according to claim 4 or 5, wherein each said convex curvature merges with a central region which follows a substantially straight line from one said convex curvature to the other. 7. A fairing according to any one of claims 3 to 6, wherein each said side edge portion subtends an angle of approximately 60°. 8. A fairing according to any one of claims 3 to 7, wherein said leading surface includes a rearwardly curved bottom edge portion. 9. A fairing according to claim 8, wherein said leading surface has a convex curvature extending between and merging with said top and bottom edge portions. 10. A fairing according to claim 8, wherein said leading surface extends along a substantially straight line between said top and bottom edge portions. 11. A fairing according to claims 8 to 10, wherein said bottom edge portion has a radius of curvature, substantially less than that of said top edge portion. 12- A fairing according to any one of claims 3 to 11 , wherein a side flow deflector extends downwardly from each said side edge portion, and each said deflector has a curved surface which is a substantial continuation of the curved surface of the respective adjacent side edge portion. 13. A vehicle including a fairing according to any preceding claim, said fairing being attached to a front face of said vehicle so that said edge portions of the fairing do not project substantially beyond planes extending rearwardly from upper and side edges of that front face and merge smoothly with those planes. 14. A vehicle according to claim 13, wherein said vehicle is a trailer adapted to be coupled beyind a prime mover or another trailer. 15. A fairing for attachment to a vehicle substantially OM shown in the accompanying drawings. 16. A vehicle including a fairing substantially as herein particularly described with reference to what is shown in the accompanying drawings.

Inventors: HOFFMANN PETER HAROLD, SAUNDERS JEFFREY WILLIAM, WATKINS SIMON, HOFFMANN, PETER, HAROLD, SAUNDERS, JEFFREY, WILLIAM, WATKINS, SIMON
Applicants: MELBOURNE INST TECH, ROYAL MELBOURNE INSTITUTE OF TECHNOLOGY LIMITED
Application year: 1984
Main classification: B62D35
IPC codes: B62D35