AU2007275575A1 - Wide area base bleed/injection apparatus for reducing aerodynamic drag of bluff body vehicles - Google Patents

Description

WO 2008/011188 PCT/US2007/016551 -1 WIDE AREA BASE BLEED/INJECTION APPARATUS FOR REDUCING AERODYNAMIC DRAG OF BLUFF BODY VEHICLES 1. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0001] The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. II. REFERENCE TO PRIOR APPLICATIONS [00021 This application claims priority in provisional application filed on July 19, 2006, entitled "Drag Reduction of Two Closely Spaced Bluff Bodies by Means of Base Bleed" serial number 60/832,041, by Jason M. Ortega et al, and incorporated by reference herein. III. BACKGROUND OF THE INVENTION A. TECHNICAL FIELD [0003] The present invention relates to drag reduction methods, and more particularly to an apparatus for bleeding, injecting, or otherwise flowing a gas from the base of a bluff body vehicle so that a wide area gas outflow profile is produced and directed in a rearward direction out to a low pressure recirculation region of a wake produced behind the base to reduce the aerodynamic drag. B. DESCRIPTION OF THE RELATED ART [0004] It is well known in the art of vehicle design that the fuel consumption of a vehicle associated with its movement is directly related to certain aerodynamic characteristics of the vehicle, such as the aerodynamic drag of the vehicle expressed as the drag coefficient, Cd. As the aerodynamic drag experienced by a vehicle increases, the fuel costs also correspondingly increase due to the greater energy required to overcome the drag. For WO 2008/011188 PCT/US2007/016551 -2 example, for a vehicle traveling 70 mph on a roadway, approximately 65% of the total fuel consumption of its engine is used to overcome aerodynamic drag. Thus, even a slight reduction in the aerodynamic drag coefficient of the vehicle can result in a significant improvement in fuel economy. [0005] Bluff bodies in particular are known to have high drag coefficients due to the presence of a sizeable recirculation zone in the wake thereof (i.e. the region of turbulence immediately to the rear of a solid body caused by fluid flow around the body), and the relatively lower pressures acting as a consequence on the rear base of the trailing end. The recirculation zone is formed due to the substantially normal orientation of the base surface with respect to the flowstream, as is commonly seen in tractor-trailer arrangements. This surface orientation creates a sharp separation of the flowstream at the edge of base surface and thereby lowers the pressure on the base surface to produce what is commonly known as "base drag." [0006] Numerous attempts have been made over the years to reduce the aerodynamic base drag of blunt-ended bluff bodies, especially land-based vehicles such as tractor-trailers and trailer vans having a flat vertical base surface behind each of the tractor and trailer. Some of the proposed concepts are passive and include such implements as boattail plates, rounding the rear corners of the vehicle near its base, and streamlining the rear of the vehicle with ogives or wedges. Other proposed concepts are active, such as plumbing systems that inject or release air near the rear corners of the vehicle or acoustic systems that actively perturb the flow coming off the rear of the vehicle. Some example prior developments are shown in U.S. Patent Nos. 4,682,808, 5,498,059, 6,286,894B 1, and U.S. Patent Publication No. US2002/0030384A1. These examples illustrate variations on reducing the aerodynamic base drag experienced by tractor-trailers having a substantially flat base surface at the trailing end. A common feature found in all of these patents is the placement of the drag reduction apparatus behind the trailing end of the trailer where access to the interior cargo space is typically provided. [0007] Moreover, for bluff bodies comprising multi-linked or otherwise gap spaced unit components with gaps between adjacent units, the aerodynamic WO 2008/011188 PCT/US2007/016551 -3 drag can be even greater when the recirculation zone formed in the gap is disrupted by a cross-flow through the gap, i.e. "gap drag." Cross-flow is characterized as a transversely directed flow of air within and/or through the gap due to a pressure difference in a transverse direction across the gap. Cross-flow is especially prevalent when side winds are present which can affect the flowstream characteristics around the bluff body. In such multi-unit or otherwise gap-spaced bluff bodies, an adjacent pair of unit components may be generally characterized as a leading portion and a trailing portion of the bluff body. Gap drag due to cross-flow is often observed with bluff bodied vehicles having towing configurations, such as tractor-trailer arrangements (e.g. having one or more trailers), auto-trailer arrangements, and locomotives, among others. Figures 1-4 illustrate gap drag in the representative case of a conventional tractor-trailer arrangement, generally indicated at reference character 100, having a tractor 101 as the leading portion, a single trailer 103 as the trailing portion hitched to and towed by the tractor 101, and a gap 106 between the tractor and the trailer enabling pivoting of one relative to the other. It is appreciated, however, that conventional tractor-trailer arrangements also include an additional trailer hitched to the first trailer (not shown). In any case, the tractor 101 has a cab portion 102 and a substantially vertical and rear-facing base surface 108. And the trailer 103 has an elongated construction with a front end 104 and a rear end 105. The front end 104 has a forward facing front surface 109 and a rear facing base surface 112, with the front surface 109 of the trailer 103 facing the base surface 108 of the tractor 101. The gap 106 is formed between the tractor 101 and the trailer 103, and in particular, between the tractor base surface 108 and the trailer front surface 109. When placed in a flowstream, such as 107 in Figure 1, i.e. when the tractor-trailer 100 is in forward motion, the airflow of the flowstream ideally separates off of the tractor 101 and completely reattaches downstream onto the trailer 101. As shown in Figures 2 and 3, however, airflow separating from the tractor 101 enters the gap 106 to form a recirculation zone defined by a vortical flow structure 110 which is similar to a vortical ring or an inverted-U shape. A stable vortical flow structure 110 (i.e. one which cannot be forced WO 2008/011188 PCT/US2007/016551 -4 out of the gap) prevents the surrounding airflow of the flowstream from further entering the gap and thus redirects the surrounding airflow to reattach with the side of the trailer. An unsteadiness in the flow field surrounding the gap, however, can produce a pressure differential in a transverse direction across the gap which can destabilize the vortical flow structure 110 and increase aerodynamic drag. Figure 4 shows an example of a cross-flow stream 111 completely traversing an empty gap 106 from one side of the tractor-trailer to the other side, through opposing first and second open ends 123 and 124. In this extreme case, the vortical structures would be eliminated altogether by the cross-flow stream 111. However, even small amounts of cross-flow present a compromise in the ability of the vortical structure to prevent airflow from further entering the gap, and can thereby increase the gap drag on the tractor-trailer 100. [0008] Various methods have been previously introduced to address this problem of recirculation zone destabilization in the gap, and the resulting gap drag. One example is shown in U.S. Pat. No. 7,216,923 which discloses a drag reduction system which introduces pressurized gases to both the gap formed between a tractor and trailer, as well as behind the base surface of the trailer. In particular, a gas discharge port is positioned in the gap and directed in a vertical direction. However, because such pressurized gases are not introduced across a large area and are directed only in a vertical direction, the prevention or otherwise reduction of cross-flow and gap drag may be limited. Another example is shown in U.S. Pat. No. 3,971,586 directed to a drag reducer for land vehicles. In particular, as shown in Figure 1 of the '586 patent, the drag reducer is a stabilizer plate 23, mounted on a forward panel 17 of a trailer 16 and extending into a gap 24 in attempting to stabilize vortices 28 and 29 formed in the gap. The stabilizer plate, however, only partially closes the gap, which is an imperfect situation since some air will be forced from one of the divided vortex regions to the other by pressure differences therebetween. By having such an opening through the gap a cross-flow is allowed to form, especially under side wind conditions, which can disturb the vortical structures to adversely impact aerodynamic drag. U.S. Pat. No.

WO 2008/011188 PCT/US2007/016551 -5 4,021,069 also shows an apparatus for reducing aerodynamic drag which is for mounting on the bluff, forward face of the trailing element of an over the road vehicle. As can be seen from Figure 1 and 2, the apparatus is a fairing element mounted at an upper region of the forward face, so as to provide deflection of an impinging air stream. As shown by Figure 1 in particular, the gap between the tractor and trailer remains substantially unblocked for preventing a cross-flow therethrough. Other examples include side extenders, gap splitter plates, and accordion-like shrouds which enclose the entire tractor trailer gap are known in the art. While side extenders in particular are widely used in the heavy vehicle industry, they are often damaged during sharp turns. The subsequent downtime for side extender maintenance and replacement can be very costly for freight companies. [0009] The need for and benefits of reducing the aerodynamic drag of bluff body vehicles, especially land-based vehicles traveling at, for example, highway speeds, are compelling and widely recognized. It would therefore be advantageous to provide a simple cost-effective apparatus and method for reducing the base drag and/or gap drag often experienced by bluff bodies to reduce the overall aerodynamic drag. IV. SUMMARY OF THE INVENTION [0010] One aspect of the present invention includes an apparatus for reducing the aerodynamic drag of a bluff body vehicle comprising: means for providing a gas flow; and a plenum enclosure having a plenum inlet connected to receive gas from said means for providing a gas flow, and an outlet wall with a wide area plenum outlet for producing a wide area gas outflow profile, said plenum enclosure capable of being positioned against a rear-facing base of the bluff body vehicle so that the outlet wall is on an opposite side from the base and gas outflow from the wide area plenum outlet is in a rearward direction out to a low pressure recirculation region adjacent the outlet wall. [0011] Another aspect of the present invention includes, in a bluff body vehicle having a base wall with a rear-facing base surface, the improvement comprising: means for providing a gas flow; and a plenum enclosure having a WO 2008/011188 PCT/US2007/016551 -6 plenum inlet connected to receive gas from said means for providing a gas flow, and a wide area plenum outlet through the base wall for producing a wide area gas outflow profile in a rearward direction out to a low pressure recirculation region adjacent the rear-facing base surface. 10012] Generally, the present invention is directed to an apparatus for reducing the aerodynamic drag of a bluff body vehicle having a rear-facing base and a low pressure recirculation region/zone produced in a wake behind and adjacent the base. The drag reduction apparatus generally operates to flow (e.g. by bleeding or ejection) a gas, such as air, engine exhaust, etc. out from or near the rear-facing base surface in a rearward direction (e.g. horizontal in the case of land based vehicles traveling on a level plane) out to the low pressure recirculation region. In addition, the drag reduction apparatus operates to a produce a wide area gas outflow profile from a wide area outlet which, in a preferred embodiment, spans substantially the entire surface area of the base. This wide area gas outflow profile directed in a rearward direction operates to displace and/or alter the low pressure recirculation region characterized by a vortex core, and thereby reduce the drag. Altering the recirculation region includes, for example, changing the 3D structural shape of the vortex core, and changing the intensity of and weakening the vortex core. 100131 The wide area outlet and wide area gas outflow profile is preferably provided by a plenum enclosure (e.g. 200 in Figures 5 and 6) having an inlet port and a wide area plenum outlet. The plenum enclosure functions to distribute gas that is received through the inlet port, out to the wide area plenum outlet for producing the wide area gas outflow profile. And the wide area plenum outlet is preferably comprised of at least two outlet ports which are grouped into at least two outlet regions having rates of gas outflow preferably controlled independently of each other to variably control and thereby shape the gas outflow profile, e.g. produce an asymmetric outflow profile. [0014] And gas flow to the plenum inlet may be provided by various mechanisms and methods known in the art, such as but not limited to, a blower WO 2008/011188 PCT/US2007/016551 -7 (not shown) e.g. a fan or bellows-like impeller; a pressure system (not shown) having a pressure vessel, a compressor for pressurizing gas in the vessel, and a valve for releasing pressurized gas from the vessel; and a flow duct (not shown) connectable to an engine exhaust line of the vehicle to use the engine exhaust ejected from the engine to provide the gas flow. In any case, the gas flow mechanism preferably actively flows gas into the plenum inlet, to bleed, eject, drive, or otherwise force gas out of the plenum outlet (and injection into recirculation region of the wake). [0015] It is appreciated that the present invention may be utilized on or as part of various types of bluff body vehicles including, but not limited to tractor trailers, automobile-trailer arrangements, trains, aircraft, etc. In the case of tractor-trailers, it is appreciated that the tractor portion alone, the trailer portion alone, or the tractor-trailer together may each be characterized as the bluff body vehicle since a rear-facing base is present in each case. As such, the drag reduction apparatus of the present invention may be especially advantages when used in conjunction with tractor-trailers and other gap divided bluff body vehicles for reducing both gap drag and base drag. V. BRIEF DESCRIPTION OF THE DRAWINGS [0016] The accompanying drawings, which are incorporated into and form a part of the disclosure, are as follows: [0017] Figure 1 is a side view of a conventional tractor-trailer type bluff body vehicle known in the prior art, and having a gap defining a recirculation zone between the tractor and the trailer when placed in a flowstream. [0018] Figure 2 is a front perspective view of the tractor trailer vehicle in Figure 1 illustrating a stable vortical flow structure produced in the recirculation zone of the gap. [00191 Figure 3 is a top view of the tractor-trailer vehicle of Figure 2 showing a stable vortical flow structure in the recirculation zone of the gap. [00201 Figure 4 is a top view of the tractor-trailer vehicle of Figure 3 showing a cross-flow through the gap due to destabilization in the recirculation zone.

WO 2008/011188 PCT/US2007/016551 -8 [0021] Figure 5 is a side view schematically showing two preferred placements of the plenum enclosure of the present invention both behind the tractor base in the gap and behind the trailer base, of a conventional tractor trailer type bluff body vehicle. [0022] Figure 6 is a side schematic view of a first exemplary embodiment of the present invention mounted on a tractor. [0023] Figure 7 is a side schematic view of a second exemplary embodiment of the present invention as a modified tractor having the plenum enclosure built in. [0024] Figure 8 is a perspective schematic view of the wide area gas outflow of the present invention from a base of a tractor. [0025] Figure 9 is a top schematic view of the wide area gas outflow profile of the present invention shown varied across the outlet wall of the plenum enclosure. 100261 Figure 10 is a perspective view of another exemplary embodiment of the plenum enclosure of the present invention having a plurality array of outlet ports arranged as a honeycomb. [00271 Figure 11 is a side cross-sectional view of the plenum enclosure of Figure 10, and shown with an optional porous material, such as a mesh or wire screen, for producing a substantially uniform gas outflow through the outlet ports. [0028] Figure 12 is a perspective view of another exemplary embodiment of the present invention having multiple plenum enclosures juxtaposed to from a combined outlet wall and each plenum enclosure connected to an independently operated blower to shape the outflow profile across the combined outlet wall. [0029] Figure 13 is a perspective schematic view of a trailing end of a trailer having a wide area gas outlet of the present invention spanning substantially the entire width of the base of the trailer. [00301 Figure 14 is a side view of the trailing end of the trailer of Figure 13 showing the low pressure recirculation region adjacent and behind the base surface prior to activating outflow through the gas outlet.

WO 2008/011188 PCT/US2007/016551 -9 [00311 Figure 15 is a side view of the trailing end of the trailer of Figure 14 showing the displacement of the low pressure recirculation region after activating outflow through the gas outlet. VI. DETAILED DESCRIPTION [0032] Turning now to the drawings, Figure 5 is a side view of a conventional tractor-trailer similar to Figure 1 in a flow stream, and schematically showing two preferred placement locations of the plenum enclosure of the present invention. In particular, a first plenum enclosure 200 is shown positioned against and behind a rear-facing tractor base surface 108 of a cab portion 102 of a tractor 101. And a second plenum enclosure 201 is shown positioned against and behind a rear-facing trailer base surface 112 at the rear end 105 of the trailer 103. As can be seen by the direction of the flow arrows from both plenum enclosures 200 and 201, the plenum outlets in both cases are located opposite the base surface, and oriented so that the wide area gas outflow profiles are directed in a rearward direction, i.e. into the gap 106 for the first plenum enclosure 200, and behind the rear end 105 of the trailer 103 for the second plenum enclosure 201. [00331 The gas flow mechanism (see Figures 6, 7) which supplies gas flow to the plenum inlet is also carried by or integrated on the tractor or trailer. It is preferably located at a suitable location accessible to a gas source, e.g. ambient air via an air intake vent, and accessible to the plenum inlet via a flow duct. For example, the gas flow mechanism may be attached to/integrated on a leading end of the bluff body experiencing stagnation pressure in a flowstream, attached to/integrated on underneath the bluff body, or in the case of a tractor-trailer, attached to/integrated on rear door/access panel of the trailer, or located in the gap between a tractor and trailer which may help to reduce cross-flow through the gap by partially occupying the gap space. It is appreciated that in the case where the gas flow mechanism is located at the front end of the trailer, the trailer itself may function as the plenum enclosure by providing an outlet port(s) through the rear end base wall of the trailer so WO 2008/011188 PCT/US2007/016551 -10 that gas that is flowed into the trailer volume is bled, injected, or otherwise flowed out of the outlet port(s). [00341 Figure 6 shows a side schematic view of a first exemplary embodiment of the drag reduction apparatus of the present invention having a modular plenum enclosure structure 200 mounted on a tractor, and a gas flow mechanism 201 providing a gas flow. The plenum enclosure 200 is shown surrounding a plenum chamber 204, and has a plenum inlet port 203 leading into the plenum chamber, and an outlet wall 202 with outlet ports 205 which together form the wide area plenum outlet. As a modular unit, the plenum enclosure 200 is capable of being positioned against the rear-facing base 108 of the tractor 101 so that the outlet wall is on an opposite side from the base 108 and gas outflow from the wide area plenum outlet is directed in a rearward direction out to a low pressure recirculation region adjacent the outlet wall 202. Positioned in this manner, it is appreciated that the outlet wall 202 effectively becomes at least an extension of the original base, and at best the new base of the tractor 101, with a low pressure recirculation region being formed next to it while the tractor is in forward motion. And the gas flow mechanism 201 is generally shown providing a gas flow to the plenum inlet port 203. The drag reduction apparatus also preferably includes a controller for controlling the rate of gas outflow from the wide area plenum outlet. As shown in Figure 6, such outflow control may be performed by controller 206 for controlling the gas flow mechanism 201, e.g. a blower speed controller, and/or controller 207 for controlling the flow area through the wide area plenum outlet, e.g. a shutter mechanism (not shown). [00351 Figure 7 shows a side schematic view of a second exemplary embodiment of the drag reduction apparatus of the present invention as a modified tractor having a plenum enclosure 300 integral with the rear-facing base 108. In particular, the plenum enclosure 300 is shown surrounding a plenum chamber 302, with a plenum inlet port 303 leading into the chamber, and base wall 108 having the wide area plenum outlet thereon for producing a wide area gas outflow profile out through the base wall in a rearward direction and out to a low pressure recirculation region adjacent the rear-facing base WO 2008/011188 PCT/US2007/016551 -11 108. Similar to Figure 6, a gas flow mechanism 201 is shown fluidically connected to the plenum inlet port 303, with the rate of gas outflow controlled by controller 206 at the gas flow mechanism 201 and/or controller 207 at the outlet ports 304. [00361 Figure 8 is a perspective schematic view of the wide area gas outflow produced from a base 108 of a tractor 101 by the drag reduction apparatus of the present invention. In particular, a wide area plenum outlet 400 is generally shown spanning substantially all of the surface area of the base 108. It is the wide area of the plenum outlet which produces the wide area gas outflow profile necessary to effectively displace and/or alter the low pressure regions which produce the drag. As used herein and in the claims, a "wide area" is considered to be a suitably large flow area relative to the base surface area, and preferably, a wide area is (a) at least 10% of the base surface area if the length/width in at least one dimension of the wide area is substantially similar to the length/width of a corresponding dimension of the base surface area, and (b) at least 25% of the base surface otherwise. Furthermore, in one embodiment, the wide area plenum outlet has a width that spans substantially all of the width of the base surface area. In another embodiment, the wide area plenum outlet also spans substantially the entire height of the base surface area, to cover substantially the entire surface area, as illustrated in Figure 8. [00371 Figure 9 is a top schematic view of a wide area gas outflow profile 402 of the drag reduction apparatus of the present invention, shown having varied intensity across the outlet wall of a plenum enclosure 401 and flowing in a rearward direction into the gap 106 between a tractor 101 and a trailer 103. In particular, the wide area gas outflow profile 402 is shown as an asymmetric profile which may be generated to impede cross-flows through the gap. As discussed in the Background, a recirculation zone is formed in the gap which can be destabilized by a cross-flow therein or therethrough. By customizing the outflow profile, the stability of the recirculation zone may be maintained to impede cross-flow therethrough, and thereby reduce the net aerodynamic drag on the bluff body.

WO 2008/011188 PCT/US2007/016551 -12 [0038] Variably controlling/customizing the wide area gas outflow profile 402 is achieved in the present invention by (1) providing at least two and preferably more outlet ports, which together form the wide area plenum outlet, and (2) providing independent gas outflow rate control through the outlet ports. Preferably, the outlet ports are grouped into outlet regions for independently controlling outflow rates per outlet region. In any case, the mechanism for controlling the gas outflow rate is preferably either a mechanism for controlling the gas flow mechanism (e.g. blower speed control), or a mechanism for controlling the outflow area through the outlet ports. In either case, various control parameters may be used for independently controlling the outflow rates, including, but not limited to relative wind velocity (i.e. flow stream velocity relative to bluff body), vehicle motion and velocity relative to ground plane, yaw angle (e.g. from cross-wind) as determined by an appropriate sensor (known in the art) on the bluff body, and cross-wind velocity. [0039] Figure 10 is a perspective view of another exemplary embodiment of the plenum enclosure of the present invention having a plurality array of outlet ports 503 arranged on outlet wall 504 as a honeycomb. The outlet ports are preferably tubular and horizontally parallel to each other to direct the outflow in a uniform horizontal direction. It is appreciated that "tubular" is not necessarily cylindrical, but can have any shaped cross-section, such as hexagonal for the densely spaced "honeycomb" shown in Figure 10. Figure 10 also illustrates the grouping of the outlet ports into at least two outlet regions. In particular the plurality of outlet ports are shown grouped into four outlet regions represented by quadrants I-IV formed by coordinate axes 501 and 502. The outflow rates through each of these outlet regions may be independently controlled by corresponding controllers (not shown). Figure 11 is a side cross-sectional view of the plenum enclosure of Figure 10 shown surrounding a plenum chamber 507. An optional porous material, such as a mesh or wire screen 505, is also shown positioned in the plenum chamber between the inlet port 506 and the outlet ports 503, for producing a substantially uniform gas outflow through the outlet ports 503.

WO 2008/011188 PCT/US2007/016551 -13 [00401 Figure 12 is a perspective view of another exemplary embodiment of the present invention, indicated at reference character 600, and having multiple plenum enclosures 601-605 which operate independently from each other. The outlet walls of the plenum enclosures are shown juxtaposed to each other to form a combined outlet wall. Gas flow to each of the plenum enclosures is supplied by a corresponding independently operated blower (only 605, 607, and 609 shown), for shaping the outflow profile across the combined outlet wall. Independent controllers 606, 608, and 610 are also shown connected to blowers 605, 607, and 609, respectively, to provide outflow rate control only to the connected plenum enclosure. It is appreciated, however, that a single controller may be used in the alternative for independently controlling all the blowers. [0041] Figures 13-14 illustrate placement of a wide area plenum outlet 700 at a trailer base, and how base the drag reduction apparatus of the present invention operates to displace and/or alter the low pressure recirculation region/zone. In particular, Figure 13 is a perspective schematic view of a trailing end 105 of a trailer 103 having a wide area gas outlet 700 of the present invention spanning substantially the entire width of the base 112 of the trailer (but not the height), and located near the lower edge of the trailer. And Figure 14 is a side view of the trailing end of the trailer of Figure 13 showing the low pressure recirculation region 701 located adjacent and behind the base surface 112 prior to activating outflow through the gas outlet. And Figure 15 is a side view of the trailing end of the trailer 103 of Figure 14 showing displacement of the low pressure recirculation region 701 after activating outflow 702 through the plenum outlet. [0042] While particular operational sequences, materials, temperatures, parameters, and particular embodiments have been described and or illustrated, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.

Claims (23)

1. An apparatus for reducing the aerodynamic drag of a bluff body vehicle comprising: means for providing a gas flow; and a plenum enclosure having a plenum inlet connected to receive gas from said means for providing a gas flow, and an outlet wall with a wide area plenum outlet for producing a wide area gas outflow profile, said plenum enclosure capable of being positioned against a rear-facing base of the bluff body vehicle so that the outlet wall is on an opposite side from the base and gas outflow from the wide area plenum outlet is in a rearward direction out to a low pressure recirculation region adjacent the outlet wall.

2. The apparatus of claim 1, further comprising a porous material in the plenum enclosure for producing a substantially uniform gas outflow through said wide area plenum outlet.

3. The apparatus of claim 1, wherein said wide area plenum outlet spans substantially the entire width of the base of the bluff body vehicle.

4. The apparatus of claim 3, wherein said wide area plenum outlet spans substantially the entire height of the base of the bluff body vehicle, to cover substantially the entire surface area of the base.

5. The apparatus of claim 1, wherein said means for providing a gas flow is selected from the group consisting of a blower; a pressure system comprising a pressure vessel, a compressor for pressurizing gas in the vessel, and valve means for releasing pressurized gas from the vessel; and a flow duct connectable WO 2008/011188 PCT/US2007/016551 - 15 to an engine exhaust line of the vehicle to use the engine exhaust for the gas flow.

6. The apparatus of claim 1, further comprising means for controlling the rate of gas outflow.

7. The apparatus of claim 6, wherein said means for controlling the rate of gas outflow includes means for controlling said means for providing a gas flow.

8. The apparatus of claim 6, wherein said means for controlling the rate of gas outflow includes means for controlling the flow area through the wide area plenum outlet.

9. The apparatus of claim 6, wherein said means for controlling the rate of gas outflow uses at least one of vehicle motion, relative wind velocity, yaw angle and cross wind velocity as control parameters.

10. The apparatus of claim 1, wherein said wide area plenum outlet comprises at least two outlet ports.

11. The apparatus of claim 10, wherein said at least two outlet ports are grouped into at least two outlet regions, and further comprising means for independently controlling the rates of gas outflow through each of the outlet regions to shape the wide area gas outflow profile.

12. The apparatus of claim 10, WO 2008/011188 PCT/US2007/016551 -16 wherein said outlet ports are tubular and horizontally oriented parallel to each other.

13. The apparatus of claim 1, further comprising at least one additional plenum enclosure, with the outlet walls of the plenum enclosures juxtaposed to form a combined outlet wall, and wherein said means for providing a gas flow includes means for independently providing a gas flow to each of the plenum enclosures.

14. The apparatus of claim 13, further comprising means for independently controlling the rates of gas outflow from each of the plenum enclosures to shape the wide area gas outflow profile across the combined outlet wall.

15. In a bluff body vehicle having a base wall with a rear-facing base surface, the improvement comprising: means for providing a gas flow; and a plenum enclosure having a plenum inlet connected to receive gas from said means for providing a gas flow, and a wide area plenum outlet through the base wall for producing a wide area gas outflow profile in a rearward direction out to a low pressure recirculation region 5djacent the rear-facing base surface.

16. The bluff body vehicle of claim 15, further comprising a porous material in the plenum enclosure for producing a substantially uniform gas outflow through said wide area plenum outlet.

17. The bluff body vehicle of claim 15, further comprising means for controlling the rate of gas outflow. WO 2008/011188 PCT/US2007/016551 -17

18. The bluff body vehicle of claim 17, wherein said means for controlling the rate of gas outflow uses at least one of vehicle motion, relative wind velocity, yaw angle, and cross wind velocity as control parameters.

19. The bluff body vehicle of claim 15, wherein said wide area plenum outlet comprises at least two outlet ports.

20. The bluff body vehicle of claim 19, wherein said at least two outlet ports are grouped into at least two outlet regions, and further comprising means for independently controlling the rates of gas outflow through each of the outlet regions to shape the wide area gas outflow profile.

21. The bluff body vehicle of claim 19, wherein said outlet ports are tubular and horizontally oriented parallel to each other.

22. The bluff body vehicle of claim 15, further comprising at least one additional plenum enclosure, with the wide area plenum outlets of the plenum enclosures juxtaposed to the base wall, and wherein said means for providing a gas flow includes means for independently providing a gas flow to each of the plenum enclosures.

23. The bluff body vehicle of claim 22, further comprising means for independently controlling the rates of gas outflow from each of the plenum enclosures to shape the wide area gas outflow profile across the base surface.