Abstract:
A wake stabilization device for reducing the aerodynamic drag of ground vehicles. An improved device for the reduction of aerodynamic drag and for improved performance of bluff base vehicles by increasing the pressure on the bluff base of the vehicle by controlling the wake flow and the interaction of the wake flow with the vehicle bluff base region. The device generates a reduction in drag force on the bluff base of a body moving through a fluid. The apparatus consists of two opposing panels attached to the bluff base and aligned approximately parallel to the side edge of the bluff base and lying in a plane that is parallel to the vehicle centerline. The drag force reduction results from controlling the flow entering the bluff base trailing wake from the left side surface, right side surface, bottom surface, and top surfaces of the vehicle. The objects and advantages also extend to other applications in which an object, body, or vehicle is moving through either a gas or a fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. Ser. No. 11/519,493, filed Sep. 11, 2006, titled, “Wake stabilization device and method for reducing the aerodynamic drag of ground vehicles,” which claims benefit of priority from U.S. Ser. No. 60/717,017, filed Sep. 15, 2005, both of which are hereby incorporated by reference. 
     
    
     ORIGIN OF THE INVENTION 
       [0002]    The invention described herein was made by employees of the United States Government, and may be manufactured and used by or for the Government without payment of any royalties thereon or therefor. 
     
    
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
       [0003]    Not applicable. 
       BACKGROUND 
       [0004]    1. Field of Invention 
         [0005]    The invention relates to the reduction of aerodynamic drag for moving ground vehicles; specifically to an improved method and device for the reduction of aerodynamic drag and for improved performance of ground vehicles by increasing the pressure on the base area of a vehicle or vehicle component by controlling the flow in wake of the vehicle or vehicle component. 
         [0006]    2. Background 
         [0007]    In the prior art there have been attempts to reduce the aerodynamic drag associated with the bluff base of the trailer of a tractor-trailer truck system. The wake flow emanating from the bluff base trailer is characterized as unsteady and dynamic. The unsteady nature of the wake flow is a result of asymmetric and oscillatory vortex shedding of the side surface and top surface flow at the trailing edge of the top and side surfaces of the vehicle. The boundary-layer flow passing along the top and side surfaces of the vehicle is at a low energy state and is unable to expand around the corner defined by the intersection of the side or top surfaces with the base surface. The boundary-layer flow separates at the trailing edge of the top and side surfaces and forms rotational-flow structures that comprise the bluff-base wake flow. The low energy flow separating at the trailing edges of the side surfaces and top surface of the trailer is unable to energize and stabilize the low energy bluff-base wake flow. The large rotational-flow structures comprising the wake interact with each other imparting an unsteady pressure loading on the vehicle base. The resulting flow interaction in the vehicle wake and unsteady pressure loading on the vehicle base contributes to the low pressures acting on the vehicle base and therefore high drag force. The resulting bluff-base wake-flow structure emanating from the base area of the vehicle is comprised of the vortex structures that are shed from trailing edges of the side surfaces and top surface of the vehicle. Contributing to the low-energy bluff-base wake is the low-energy turbulent flow that exits from the vehicle undercarriage at the base of the vehicle. The unsteady wake flow imparts a low pressure onto the aft facing surface of the trailer base that results in significant aerodynamic drag. Prior art has addressed the bluff base flow phenomena by adding to or installing various devices to the bluff base region. Examples of these devices are: a contoured three-dimensional aerodynamic surface referred to as a boat-tail; bluff base extensions/flaps/fairings/panels/plates which extend rearward from the side, top and/or lower surfaces that create a cavity; three or four surface panels/plates that extend rearward from the bluff base and are aligned approximately parallel to the side, top and/or lower surfaces of the vehicle and are designed to trap vortices shed from the trailing edges. All of these previous devices have geometrically-simple trailing edges and have an upstream edge that abuts the bluff base surface. Prior art also show the forcing the side surface and top surface flow into the base region through the use of turning vanes or jets of air. 
         [0008]    Prior art has used the aerodynamic boat-tail fairings applied to the trailer base in order to eliminate flow separation and associated drag, see U.S. Pat. Nos. 4,458,936, 4,601,508, 4,006,932, 4,451,074, 6,092,861, 4,741,569, 4,257,641, 4,508,380, 4,978,162 and 2,737,411. These representative aerodynamic boat-tail fairing devices, while successful in eliminating flow separation, are complex devices that are typically comprised of moving parts that require maintenance and add weight to the vehicle. These devices take a variety of form and may be active, passive, rigid, flexible and/or inflatable. These attributes have a negative impact on operational performance and interfere with normal operations of the vehicle. 
         [0009]    Other concepts as documented in U.S. Pat. Nos. 5,348,366, 4,682,808 and 421,478 consist of three or four plates/panels that are attached to the base of a trailer or extend from support mechanisms that are attached to the base of a trailer. These devices operate by trapping the separated flow in a preferred position in order to create an effective aerodynamic boat-tail shape. These representative trailer base devices, while successful in reducing the drag due to base flow are complex devices that are typically comprised of moving parts that require maintenance and add weight to the vehicle. All of these devices add significant weight to the vehicle. These attributes have a negative impact on operational performance and interfere with normal operations of the vehicle. 
         [0010]    U.S. Pat. Nos. 3,010,754, 5,280,990, 2,569,983 and 3,999,797 apply a flow turning vane to the outer perimeter of the trailer base on the sides and top to direct the flow passing over the sides and top of the trailer into the wake in order to minimize the drag penalty of the trailer base flow. These devices provide a drag reduction benefit but they require maintenance and interfere with normal operations of the trailers fitted with swinging doors. These devices also add weight to the vehicle that would have a negative impact on operational performance of the vehicle. 
         [0011]    Several concepts employ pneumatic concepts to reduce the aerodynamic drag of tractor-trailer truck systems. U.S. Pat. No. 5,908,217 adds a plurality of nozzles to the outer perimeter of the trailer base to control the flow turning from the sides and top of trailer and into the base region. U.S. Pat. No. 6,286,892 adds a porous surface to the trailer base and to the sides and top regions of the trailer abutting the trailer base. These porous surfaces cover a minimum depth plenum that is shared by the sides, top and base regions of the trailer. These two patents provide a drag reduction benefit but as with the other devices discussed previously these devices are complex devices, comprised of moving parts, interfere with normal operations of the truck and add weight to the vehicle. These characteristics of the devices result in a negative impact on the vehicle operational performance. 
       SUMMARY OF THE INVENTION 
       [0012]    The object of the invention is to prohibit the interaction of naturally occurring large-scale bluff-body wake flow structures by creating a three-dimensional, self-optimizing, fluidic, aerodynamic surface, that is of minimum stream wise extent and consists of two panels attached to the base of a bluff base vehicle. The self-optimizing fluidic surface is created through the selected use of three flow control concepts; vortex trapping, flow venting, and trailing edge energizing. The technologies employed to accomplish the flow control objectives are; two opposing panels aligned approximately parallel to the side edge of the vehicle and lying in a plane that is parallel to the vehicle centerline to trap vortex structures, gap between the bluff base and the plate leading edge to allow flow venting, and complex geometric shaping of the plate trailing edge and/or the use of micro vortex strakes to energize and promote turning of the panel trailing edge flow. The use of each of these three technologies in the design of the wake stabilization device for a vehicle is dependent upon the vehicle geometry, operational requirements, and maintenance requirements. Trapping of the two vertically orientated and symmetrically positioned vortex structures and energizing and turning of the trapped-vortex external flow field is accomplished with; two opposing panels aligned approximately parallel to the side edge of the vehicle and lying in a plane that is parallel to the vehicle centerline that extend aft from the bluff base a distance that is between 15 and 25 percent of the vehicle width and are configured with either a complex, planar, geometrically shaped trailing edge, micro vortex strakes and a vent slot at the plate leading edge. The panel aft extension distance is related to the panel inset distance from the vehicle side edge. Panel inset distance should preferably be less than 20% of the panel width. This ratio ensures that the trapped vortex is located on the panel and that the external flow field impacts the panel immediately forward of the panel trailing edge. The panel inset distance is related to the boundary layer displacement thickness at the trailing side edge of the vehicle. The boundary layer displacement thickness is a function of the vehicle length, hence the proportion provided. To ensure maximum drag reduction the panel inset distance should preferably be greater than twice the boundary layer displacement thickness. For vehicle lengths in operation on the road, the range of boundary layer displacement thicknesses at a vehicle trailing edge varies from about 0.25 inches to 1.25 inches. As a conservative estimate, the preferred panel inset distance shall be no more than four times the boundary layer displacement thickness. This defines that the maximum value of inset distance for the longest vehicle in operation to be about 5.0 inches. The preferred non-dimensional inset distance is thus 5% of the vehicle width. Taking into account the ratio of inset distance to panel aft extension length defines the maximum panel aft extension distance as 25% of the vehicle width. To ensure that a vortex is trapped along the full vertical extent of each panel the uppermost and lowest most points of the panels are open to allow the trapped vortex flow to exit from the top and bottom terminations of each panel. Each of the trapped vortex structures on each panel forms a fluidic surface that allows the flow exiting the trailing edge of the side and top exterior surfaces of the trailer to expand into the base region and provide drag reduction, increased fuel economy and improved operational performance. Each of the two trapped vortex fluidic surfaces generated by the device are a cylindrical column of rotating air that extends along the full vertical extent of each panel. The vortex flow that resides on the outward facing surface of each panel is continually exiting the open uppermost and lowest most extents of the panel edge as this out flow is continually replaced by the boundary layer flow moving along each side of the vehicle surface. The trailing edge geometric shaping promotes turning of the trapped-vortex external flow field thereby increasing the drag reduction benefit of the trapped vortex technology. The vent slot located at the plate leading edge allows flow to pass from bluff base region inboard of the plate to the outboard region of the plate thereby stabilizing the trapped vortex and improving the flow turning into the base and resulting in increased drag reduction. Aerodynamic drag reduction is created by increasing the average pressure loading on the bluff-base aft-facing surface of the vehicle or vehicle component such as the trailer of a tractor-trailer truck. The invention relates to flow in the base region behind a bluff-base vehicle or vehicle component. The flow in the base region behind a bluff-base vehicle or vehicle component is a function of vehicle geometry, vehicle speed and the free stream flow direction. 
         [0013]    The device provides improved performance for both the no crosswind condition, in which the ambient air is still, as well as the condition when crosswind flow is present. For all moving vehicles that operate on the ground, a crosswind flow is always present due to a combination of atmospheric and environmental factors and the interaction of the naturally occurring wind with stationary geological and manmade structures adjacent to the vehicle path as well as interfering flows from adjacent moving vehicles. The device is designed to reduce aerodynamic drag for all cross wind conditions for single and multiple-component bluff-base vehicles. The subject device uses vortex flows to allow the flow passing along the exterior top and side surfaces of a bluff-base ground vehicle to smoothly exit the vehicle at the trailing edge and pass into the wake. The device provides reduced aerodynamic drag for all of bluff-base ground vehicles. 
         [0014]    The present invention is a simple device comprised of two thin, slender and rigid panels that attach to the exterior surface of the bluff base of a ground vehicle or vehicle component. The spacing and orientation of the two panels, comprising the device, are dependent upon the vehicle geometry and vehicle operating conditions. 
         [0015]    The present invention pioneers a novel device that is comprised of two opposing panels aligned approximately parallel to the side edge of the vehicle and lying in a plane or surface that is parallel to the vehicle centerline that are attached to the bluff base exterior surfaces of a bluff-base vehicle or vehicle component. The two panels are located on the base area on the vehicle. The two panels are symmetrically positioned about the vehicles vertical plane of symmetry. To maximize the ability of each panel to trap a vortex structure over the full vertical extent of each panel the panels are aligned approximately parallel to the side edge of the vehicle base and lie in a plane that is parallel to the centerline of the vehicle Each of the two panels extends rearward an equal distance from the exterior rear surface of the bluff-base vehicle. Each of the two panels extends rearward between 15 and 25 percent of the vehicle width from the exterior rear surface of the bluff-base vehicle. Each of the two panels is inset from the respective side edge a distance that is between 0.0 and 5.0 percent of the vehicle width. The intersection of the leading edge of each of the two panels and the base of the vehicle form a rearward facing step. The rearward facing step is comprised of the base surface area outboard of each panel and the outward facing surface of each panel. The rearward facing step extends along the full vertical extent of each panel. The upper most and lowest most extent of the two rearward facing steps formed by each panel are not capped but are open. The two panels are applied symmetrically to the vehicle, about a vertical plane passing through the centerline of the vehicle. Each of the two panels may be comprised of multiple elements or segments and/or may be contain local gaps, holes, cutouts, and/or bumps in order to accommodate specific vehicle or vehicle component geometry. 
         [0016]    For ground vehicles such as tractor-trailer trucks, which have a cross-section shape that is predominately rectangular or square, the two panels will be predominately planar, except as required to accommodate vehicle specific geometric features. The flow passing over this class of vehicle is parallel to the vehicle centerline and moving aft along the vehicle surface. Each of the two panels may be comprised of various segments that may vary in number, shape, width and orientation that is determined by vehicle geometry. The preferred embodiment of the invention is to have each panel, comprising the invention, located near the outboard side edge of the vehicle base and extending vertically from the lower edge of the base surface to the upper edge of the base surface. The trailing edge shape of each panel pair shall be the same and is a function of vehicle geometry and operating conditions. Panel trailing edge shape may be either linear and parallel to the vehicle base surface or may be defined as a complex geometric shape such as serrated, notched, curved, or sawtooth shape to stabilize the wake shed from the panel trailing edge. The leading edge of each panel may be inset from the vehicle base surface to create a vent slot that will allow the high pressure air located inboard of the panel to flow outward to the low pressure region outboard of the panel. The leading edge of each panel may be notched or inset from the vehicle base surface to provide clearance of existing vehicle structures and mechanisms. The intersection of the panel leading edge with the vehicle base forms an aft facing step. The aft facing step is located between the aft facing base surface of the vehicle that is located outboard of the panel and the outward facing surface of the panel. The aft facing step extends along the full vertical extent of each panel. The top and bottom edge of each panel are the termination points of the device. The upper most and lowest most extents of each aft facing step are not covered and are left open to allow the trapped vortex to exit the device. 
         [0017]    The present invention is designed to be a maintenance free device that does not interfere with typical operational procedures or add additional operational procedures to ensure successful operation of the device. For ground vehicles, such as tractor-trailer trucks, which have a single roll-up door or two swing doors located on the vehicle base, each panel of the subject invention is attached to the base by means of an actuation/attachment system that may consist of a spring-hinge or the combination of a simple hinge with either a pneumatic spring or mechanical support system. For vehicles with two swing doors on the base a single panel would be attached to each vehicle door or attached to the swing door hardware. The actuation/attachment system is designed to prohibit the panel from rotating outward but allows the panel to rotate inwards 90° towards the vehicle centerline. Each panel is attached with either a spring hinge or a simple hinge with pneumatic spring that allows the panel to rotate 90° to its stowed position when the swing door is opened and rotated about its hinges an angle greater than 180°. When the door is rotated past 180° the panel trailing edge contacts the vehicle side exterior surface. As the door is rotated to 270° the spring-hinge system allows the panel to rotate 90° to the stowed position. To ensure that there is no damage to the vehicle side surface the trailing edge of the panels incorporate either a rolling device(s), low friction wear pad(s), or the trailing edge of each panel comprising the invention is composed of a material that would not damage the vehicle side surface. The side surface of the vehicle that would make contact with each panel comprising the invention is configured to eliminate damage that may result from the impact of the panel on the vehicle side surface. The spring coefficient for the spring-hinge or pneumatic spring is designed to; position each panel in the deployed position when the doors are closed and to minimize the force required to rotate the vehicle door 270° to its open and stowed position. 
         [0018]    For vehicles with a rollup door on the base a single panel is installed on the vehicle by means of a partial swing door system. The partial swing door system would attach to the vehicle roll-up door frame and operate as a typical swing door. The partial swing door system would be capable of rotating 270° outward when the door is to be opened and the panel is stowed. The lateral positioning requirement of the subject panels defines the inboard extent of the partial swing door system. Each panel of the subject invention is attached to the partial swing door system by means of a spring-hinge system or other suitable means that allows for the required operation of the panel. The spring-hinge panel attachment system is designed to prohibit the panel from rotating outward but allows the panel to rotate inwards 90° towards the vehicle centerline, from the deployed position of perpendicular to the vehicle base. The 90° rotation of the panel to its stowed position is automatically accomplished when the partial swing door is opened and rotated about its hinges an angle greater than 180° at which point the panel trailing edge contacts the vehicle side exterior surface. The spring-hinge system allows the panel to rotate 90° to the stowed position when the vehicle partial swing door is opened to its stowed position of 270°. To ensure that there is no damage to the vehicle side surface the trailing edge of the panels incorporate either a rolling device(s), low friction wear pad(s), or the trailing edge of each panel comprising the invention is composed of a material that would not damage the vehicle side surface. The side surface of the vehicle that would make contact with each panel comprising the invention is configured to eliminate damage that may result from the impact of the panel on the vehicle side surface. The spring coefficient for the spring-hinge or pneumatic spring is designed to; position each panel in the deployed position when the partial swing doors are closed and to minimize the force required to rotate the vehicle door 270° to its open and stowed position. 
         [0019]    The reduction of aerodynamic drag, improved operational performance and improved stability of multiple component vehicles is obtained by increasing the pressure loading on the bluff base of the vehicle or vehicle component. The pressure loading on the bluff base is increased by eliminating the interaction of the side edge separated flow structures and promoting the side and top surface flow to smoothly transition from the vehicle trailing edges and turn into the wake region. The flow control is accomplished by trapping a vortex structure over the full vertical extent of the outward facing surface of each panel, increasing circulation at the panel trailing edge by venting the inboard high pressure outboard, and energizing the panel trailing edge flow to promote inboard turning of the flow. The invention creates a virtual extension of the vehicle side surfaces thereby allowing the flow to exit the vehicle side surface and flow into the wake. More specifically, this invention relates to a device and method for reducing aerodynamic drag utilizing two opposing, symmetrically positioned, and minimum length and width panels that are specifically shaped, sized, and orientated to inhibit the formation and prohibit the interaction of large separated flow structures in the wake. The invention creates a virtual surface that acts as an extension of the vehicle side surfaces resulting in a stabilization of the vehicle wake, reduced unsteady flow separation, increased pressures acting on the bluff base area and reduced vehicle aerodynamic drag. The structure and segmentation of the two panels, the lateral positioning of the panels, the streamwise extent of the panels, the vertical extent of the panels, the vent gap size, and the trailing edge shape of the panels are the primary design variables that are used to determine the drag reduction capability of the device. 
         [0020]    The invention may be used to reduce the drag of all existing and future ground vehicles (i.e., cars with trailers, tractor-trailer trucks, trains, etc.). 
       OBJECTS AND ADVANTAGES 
       [0021]    Several objects and advantages of the present invention are:
       (a) to provide a novel process to reduce the aerodynamic drag of vehicles;   (b) to provide a means to use vortex flow to reduce aerodynamic drag;   (c) to provide a means to use flow venting to reduce aerodynamic drag;   (d) to provide a means to reduce the aerodynamic drag and improve the operational efficiency of vehicles;   (e) to provide a means to reduce the aerodynamic drag and improve the fuel efficiency of vehicles;   (f) to provide a means to conserve energy and improve the operational efficiency of vehicles;   (g) to provide a means to reduce the aerodynamic drag without a significant geometric modification to existing vehicles;   (h) to provide an aerodynamic drag reduction device that uses a minimum number of panels;   (i) to allow the geometric details of each pair of panels to be variable to meet the specific needs of the application;   (j) to allow the spacing, location, and orientation of each pair of panels to be variable to meet the specific needs of the application;   (k) to create a high pressure and low aerodynamic drag forces on the bluff base of a vehicle to reduce the aerodynamic drag of the subject vehicle;   (l) to allow the device to be fabricated as a number of independent segments and parts that may be applied to an existing vehicle;   (m) to allow the device to be fabricated as a single independent unit that may be applied to an existing vehicle;   (n) to allow the device to be fabricated as an integral part of a vehicle;   (o) to allow for optimal positioning of each panel on the vehicle base surface;   (p) to have minimum weight and require minimum volume within the vehicle;   (q) to have the ability to automatically fold to a stowed position and deploy to a operational position with normal operational of the door system;   (r) to have minimum maintenance requirements;   (s) to have no impact on operational requirements.       
 
         [0041]    Further objects and advantages are to provide a device that can be easily and conveniently used to minimize aerodynamic drag on any ground vehicle for the purposes of improving the operational performance of the vehicle. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  is a rear perspective view of the aft most portion of a trailer of a tractor-trailer truck system with the subject invention installed on the rear surface and side surface of the trailer. 
           [0043]      FIG. 2   a  to  2   b  are cross section views, in planes horizontal to the ground and perpendicular to the ground, of the wake flow conditions for a tractor-trailer system without the subject invention installed. 
           [0044]      FIG. 3   a  to  3   e  are perspective views of drawings of various prior art concepts that use more than 2 panels mounted to the base of a trailer of a tractor-trailer truck system. 
           [0045]      FIG. 4   a  is a rear perspective view of a trailer of a tractor-trailer truck system with a typical three and four panel rear mounted drag reduction device showing the preferred dimensions and positions of each panel that produces the lowest drag. 
           [0046]      FIGS. 4   b  and  4   c  are a side view and two cross section views, in planes horizontal to the ground, of the vortex flow structures located on the panels and the wake flow structures aft of the rear of a trailer of a tractor-trailer truck system for the case of a three or four panel system. 
           [0047]      FIGS. 5   a  and  5   b  are a side view and a cross section view, in planes horizontal to the ground, of the vortex flow structures located on each side panel of the subject invention and the wake flow structures aft of rear of a trailer of a tractor-trailer truck system with the subject invention installed. 
           [0048]      FIG. 5   c  is a top perspective view of the aft most portion of a trailer of a tractor-trailer truck system with the subject invention installed showing the preferred dimensions of each panel that produces the lowest drag. Also in  FIG. 5   c  is a plot of experimental data showing percent drag reduction against panel inset position for the subject invention installed on the base of a trailer of a tractor-trailer truck system. 
           [0049]      FIG. 6   a  to  6   d  are a side view, top view, top oblique view and rear view of the flow features on the side surface of a trailer of a tractor-trailer truck system and of the flow on a panel of the subject invention installed on a trailer of a tractor-trailer truck system. 
           [0050]      FIG. 7   a  to  7   d  are side and top views of various ground vehicles with and without the present invention installed. 
           [0051]      FIGS. 8   a  to  8   h  are views of the various panel leading edge and trailing edge treatment embodiments. 
           [0052]      FIG. 9  is a perspective view and cross section views of a vented panel concept installed on the base of a tractor trailer. 
           [0053]      FIG. 10   a  to  10   c  is a perspective view and cross section views of various attachment and actuation concepts for the invention installed on the base of a tractor trailer with swing doors. 
           [0054]      FIG. 11  is a perspective view and cross section views of the invention installed at the base side edge of a tractor trailer with swing doors. 
           [0055]      FIG. 12  is a perspective view and cross section views of the attachment and actuation concept for the invention installed on the base of a tractor trailer with a rollup door. 
           [0056]      FIG. 13   a  to  13   d  are side views of alternate embodiments of the subject invention installed on a tractor-trailer truck. 
           [0057]      FIG. 14   a  to  14   d  are side views of alternate embodiments of the subject invention installed on various ground vehicles. 
           [0058]      FIG. 15  is a side rear perspective view of the aft portion of a trailer showing alternate embodiments of the subject invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0059]    The following descriptions are of exemplary embodiments of the invention only, and are not intended to limit the scope, applicability or configuration of the invention in any way. Rather the following description is intended to provide a convenient illustration for implementing various embodiments of the invention. As will become apparent, various changes may be made in the function and arrangement of the elements described herein without departing from the spirit and scope of the invention. For example, though not specifically described, many shapes, widths, leading edge shapes, spacing and orientation of the forward extended plurality of panels, candidate vehicles that can benefit from the device, fabrication means and material, attachments means and material should be understood to fall within the scope of the present invention. 
         [0060]    Referring now in detail to the drawings, like numerals herein designate like numbered parts in the figures. 
         [0061]      FIG. 1  is a rear perspective view of the aft portion of a typical trailer  30  with sides  32  and  33 , upper surface  34 , and rear surface  35 . During operation of the trailer there is flow  100  passing over the trailer  30 .  FIG. 1  show the trailer  30  with the present invention installed on the rear surface  35  of a trailer  30 . An embodiment of the present invention  40  is comprised of two panels  41   a  and  41   b , attachment/actuation hardware  71 , roller or low friction pad to assist in stowage of panel  72 , and low friction landing strip  73  that is mounted to the sides  32 ,  33  of the vehicle  30 . The subject invention is comprised of two aft-extended panels  41   a  and  41   b  that are attached to vehicle base surface  35 . The two panels  41   a  and  41   b  are symmetrically positioned about the vehicle vertical plane of symmetry. Each panel has a width W and a length L and is inset from the vehicle side edge a distance Y, where Y may be any between 0.0 and 10.0 percent of the vehicle width. The self-stowage feature of the invention  40  is accomplished when the panels  41   a  and  41   b , or roller  72 , contacts the vehicle side surface  32 ,  33 . To facilitate the folding process and to minimize damage to the vehicle  30  a low friction landing strip  73  may be mounted to the side  32 ,  33  of the vehicle  30 . The location of the landing strips  73  is defined by a leading edge position Xb and a trailing edge position Xa. 
         [0062]    The length L of each panel  41   a  and  41   b  of the invention  40  is equivalent to the full vertical height of the vehicle  30 . The width W of each panel  41   a  and  41   b  of the invention  40  is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30  and is in the range of 15 to 25 percent of the vehicle width. The type, size and structure of the attachment/actuation hardware  71  of the invention and the type, size and structure roller/low friction hardware  72 ,  73  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0063]    The present invention  40 , comprised of components  41 ,  71 ,  72 ,  73 , provides aerodynamic drag reduction for all free stream flow  100  conditions including crosswind conditions. Aerodynamic drag reduction occurs when flow  100  that separates at the vehicle side surface  32 ,  33  trailing edge and is turned into the base wake region with the use of vortex trapping, flow venting and trailing edge energizing flow control technologies. The flow  100  leaving the side surface  32 ,  33  trailing edge forms a vortex that reside on the outward facing surface of panels  41   a  and  41   b . The trapped vortex located on panels  41   a  and  41   b  acts as a fluidic surface to the external flow that is turned into the wake region. The trapped vortices promote the turning of the external flow into the base wake region which results in a stable bluff-base wake flow and a high pressure that acts on the base surface  35  of the trailer  30 . The strength of the trapped vortices located on panels  41   a  and  41   b  will provide increasing aerodynamic drag reduction with increasing velocity of the flow  100 . The effectiveness of the subject invention to reduce drag and thereby increase fuel economy of a vehicle is determined by panel  41  width W, length L, position Y, trailing edge shape, micro vortex strake placement and vent gap as shown in  FIG. 4 . 
         [0064]      FIG. 2   a  and  FIG. 2   b  show flow patterns in the wake of a bluff-base tractor-trailer truck without the present invention installed. In  FIG. 2   a  and  FIG. 2   b  the airflow about the vehicle and in the base region is represented by arrow tipped lines and swirl structures  100 ,  110 ,  120 , and  130 . The shaded swirl structures represent rotational wake flow  110 . The small swirl structures represent turbulent flow structures  120  in the base area and from the vehicle undercarriage. The spiral structures represent trapped vortices  130 . 
         [0065]      FIG. 2   a  shows a cross section view, in a plane horizontal to the ground, of the aft portion of a trailer  30  and the bluff-base wake flow, without the subject invention installed. For this condition, a surface flow  100  develops on the trailer that separates at the trailing edge of the side surfaces  32  and  33 , and forms rotational-flow structures  110  that comprise the bluff-base wake flow. The rotational-flow structures  110  are shed asymmetrically from the opposing side surfaces  32  and  33 . These rotational-flow structures  110  continue to move downstream in a random pattern. The asymmetric shedding of the rotational-flow structures  110  produce low pressures that act on the base surface  35  of the trailer. These low pressures result in a high aerodynamic drag force. The low energy flow  100  separating at the trailing edges of the side surfaces  32  and  33  of the trailer  30  is unable to energize and stabilize the low energy-bluff-base wake flow. The resulting bluff-base wake-flow structure emanating from the base area of the vehicle is comprised of the vortex structures  110  that are shed from trailing edges of the side surfaces  32  and  33  of the trailer  30 . Contributing to the low-energy bluff-base wake is the low-energy turbulent flow  120  that exits from the vehicle undercarriage at the base of the vehicle. 
         [0066]      FIG. 2   b  shows a centerline cross section view of the aft portion of a trailer  30  and the bluff-base wake flow, without the subject invention installed. For this condition, a surface flow  100  develops on the trailer that separates at the trailing edge of the top surface  34  and forms rotational-flow structures  110  that comprise the bluff-base wake flow. The rotational-flow structures  110  that are shed from the trailing edge of the top surface  34  are asymmetrically located in the wake. These rotational-flow structures  110  continue to move downstream in a random pattern. The unsteady shedding of the rotational-flow structures  110  produce low pressures that act on the base surface  35  of the trailer. These low pressures result in a high aerodynamic drag force. The low energy flow  100  separating at the trailing edges of the top surface  34  of the trailer  30  is unable to energize and stabilize the low energy bluff-base wake flow. Contributing to the low-energy bluff-base wake is the low-energy turbulent flow  120  that exits from the vehicle undercarriage at the trailing edge of the vehicle. The resulting bluff-base wake-flow structure emanating from the base area of the vehicle is comprised of the vortex structures  110  that are shed from trailing edges of the side surfaces  32  and  33  and the top surface  34  of the vehicle. The low-energy turbulent flow  120  that exists from the vehicle undercarriage also enters into the bluff-base wake flow. The unsteady wake flow imparts a low pressure onto the aft facing surface  35  of the trailer base that results in significant aerodynamic drag. 
         [0067]      FIG. 3   a  through  FIG. 3   e  show rear oblique views of various rear mounted drag reduction devices with inset panels. The five multi-panel devices shown in  FIG. 3   a  through  FIG. 3   e  have three or more inset panels in combination with a non-inset lower panel.  FIG. 3   a  shows a multi panel drag reduction device for a trailer of a tractor-trailer truck system that consists of a lower panel  932 , an upper inset panel  934 , and two side inset panels  930  and  928 . The upper panel  934  and the two side panels  928 ,  930  are inset from the base surface upper edge and the two side panels  928 ,  930  are inset from their respective base surface side edges. The lower edge for each of the two side panels  928 ,  930  intersect the lower panel  932  and each intersection forms an inside corner. The upper edge of each side panel  928 ,  930  do not intersect another panel and therefore are free edges. The upper panel  934  intersects both side panels  928 ,  930  creating two inside corners. 
         [0068]      FIG. 3   b  shows a four panel  982 ,  984 ,  986 ,  988  system with four inside corners and two free edges.  FIG. 3   c  shows a four panel  992 ,  994 ,  996 ,  998  system with four inside corners and two free edges.  FIG. 3   d  shows a four panel  9112 ,  9114 ,  9116 ,  9118  system with two inside corners and two outside corners.  FIG. 3   e  shows a six panel  9112 ,  9114 ,  9116 ,  9118 ,  9122 ,  9124  system with six inside corners. The drag reduction capability of multi-panel drag reduction devices with inset panels is related to the ability of the inset panels to capture and control the low energy flow exiting the trailing edges of the upstream surfaces of the vehicle. The inset panels form an aft facing step with the rear surface of the vehicle. The low energy flow exiting the vehicles trailing edge is trapped in this aft facing step region. To maintain a stable trapped vortex flow condition during operation the trapped vortex flow must flow out of the aft facing step area to make allowance for the oncoming flow along the upstream surfaces to enter the aft facing step area. The presence of inside corners on multiple panel systems blocks the natural outflow venting of the trapped vortex thereby limiting the effectiveness of the trapped vortex flow to control the flow exiting the side and top surfaces of the trailer of a tractor-trailer truck system. The close proximity of a free edge to an inside corner will also result in a restricted outflow from the free edge. The restricted outflow limits the span-wise extent of the trapped vortex condition that is required to reduce the drag. 
         [0069]      FIG. 4   a  shows a graphic of the rear portion of a trailer of a tractor-trailer truck system with a multi panel device with more than two inset panels. The graphic shows the critical panel parameters XL, XD, XH and inset parameters XD, XG that are needed to maximize drag reduction for an inset panel device with more than two panels. Experimental data show that the panel width XL shall be between 25% and 100% of the vehicle width. Experimental data show that panel inset XD, XG shall be between 10% and 20% of the vehicle width. A graph showing the percent drag reduction values resulting from the optimum settings of panel inset XD, XG are shown at the bottom of  FIG. 4   a . The experimental data show a 10% drag reduction is possible. The experimental data also show a loss in effectiveness with panel inset values XD, XG less than 10% of the vehicle width. 
         [0070]      FIG. 4   b  and  FIG. 4   c  show details of the flow passing over the sides of a trailer of a tractor-trailer truck system and the flow  944  on the inset panel  928  of the multi panel device shown in  FIG. 3   a . Also shown in  FIG. 4   b  are the trapped vortex outflow structures  950  and  951  from the upper free edge and inside corner, respectively. The trapped vortex outflow structures are asymmetric and are shown to migrate vertically toward the center of the panel. Shown in  FIG. 4   c  are two cross section views, in planes horizontal to the ground, of the vortex flow structures located on the panels and the wake flow structures aft of the rear of a trailer of a tractor-trailer truck system for the case of a three or four inset panel system. Section cut X-X shows the flow conditions on panels  928 ,  930  for a stable vortex trap situation. Section cut X-X is taken at approximately half the distance between the lower and upper edges of the panel  928 . The image shows the aft flowing boundary layer flow  129  that is present on the vehicle surface upstream of the side surface trailing edge. This boundary layer flow  129  separates at the side surface trailing edge and forms the trapped vortex  944 . Also shown is the resultant stable vortex structure  110  that comprises the downstream wake. The section cut X-X shows the symmetric and stable flow features on the inset panels  928 ,  930  and in the wake region of the vehicle for the condition when the trapped vortex is not contaminated by the redirected outflow resulting from the inside corners. Section cut Y-Y shows the flow conditions on inset panels  928 ,  930  for conditions where the trapped vortex  944  is unstable due to interference from the outflow  951 ,  953 . Section cut Y-Y is taken through the inside corner flow conditions present at the lower portion of the inset panels  928 ,  930 . The image shows the aft flowing boundary layer flow  129  that exits the vehicle surface is unable to form the trapped vortex  944  due to the unstable and asymmetric outflow  951 ,  953  caused by the inside corner. Also shown are the resultant asymmetric and unstable vortex structures  110  that comprises the downstream wake 
         [0071]      FIG. 5   a  and  FIG. 5   b  show the stable and symmetric flow conditions on the aft portion of a trailer of a tractor-trailer truck system with the present invention installed.  FIG. 5   a  is a side view of the aft portion of a trailer of a tractor-trailer truck system with the subject invention installed. Also shown in the figure are the flow streamlines  100  on the side of the trailer, the trapped vortex  130  on the inset panel  41   a  and the upper and lower trapped vortex outflow vortices  131 ,  132 . The panel upper edge  413  and panel lower edge  414  of panel  41   a  of the present invention  40  are free edges that are not compromised by their proximity to inside corners or outside corners. The present invention  40  allows the trapped vortex flow  130  to efficiently flow out of the aft facing step area by flowing vertically upward and downward.  FIG. 5   b  shows a single cross section view that represents the cross section at all points along the vertical extent of the inset panels  41   a ,  41   b  comprising the subject invention  40 . The cross section cuts A-A, B-B, C-C, D-D are in planes horizontal to the ground. The section cuts show the trapped vortex flow structure  130   s  located on panels  41   a ,  41   b  and the wake flow structures  110  aft of the rear of a trailer of a tractor-trailer truck system for the case of a three or four panel system. The graphic shows that the flow on the panels  41   a ,  41   b  and in the vehicle trailing wake are stable and symmetric. The sketch of  FIG. 5   b  also shows that the trapped vortex  130  extends along the full vertical extent of each panel  41   a ,  41   b . The characteristics of the trapped vortex  130  for the invention  40  are dramatically different than that observed for the three and four panel conventional devices discussed in  FIGS. 3   a - e  and  FIGS. 4   a - c . Compared to these conventional efforts, the trapped vortex  130  of the present invention is more stable and extends over the full length of the panel which allows the panel to extend a shorter distance aft and to have a smaller inset distance allowing the present invention to weigh less and minimize the impact on vehicle operations while providing high levels of performance. 
         [0072]      FIG. 5   c  shows a top view of the aft portion of a trailer of a tractor-trailer truck system with the two panels  41   a ,  41   b  of subject invention installed. Also shown in  FIG. 5   c  are the critical parameters and the associated values that define a preferred embodiment of the present invention. The critical parameters are the panel width W, and panel inset Y. The panel inset Y and panel width W are expressed as a fraction or percent of the vehicle width VW. Experimental testing has shown a preferred embodiment of the present invention  40  is for inset Y values that are less than 5% of the vehicle width VW and for panel width W values that are less than 25% of the vehicle width VW. 
         [0073]    Shown in the lower portion of  FIG. 5   c  are experimental data for the present invention  40  in use with a body having a bluff base. The data presented are the percent drag reduction versus panel inset Y for a panel width W that is 25% of the vehicle width VW. These data also reflect the results obtained for panel widths W between 10% and 25% of the vehicle width VW. The data show optimum performance for panel insets Y less than 5% of the vehicle width VW. The data also show that increasing panel inset Y above 5% of vehicle width results in a dramatic loss in effectiveness for bodies having a bluff base. The loss in effectiveness results from an unstable trapped vortex due to a mismatch in the size of the aft facing step compared to the volume of the boundary layer flow that forms the trapped vortex. The oversized aft facing step allows the trapped vortex to migrate aft and become unstable as it interacts with the trailing edge of the inset panel  41   a  this interaction disrupts the desired two dimensional trapped vortex flow that provides optimum flow control. 
         [0074]      FIG. 6   a  through  FIG. 6   d  show additional details of the trapped vortex  130  and the trapped vortex outflow  131  and  132  that occurs on the inset panels  41   a ,  41   b  of the subject invention  40 .  FIG. 6   a  is a side view of the aft portion of a trailer of a tractor-trailer truck system with the present invention  40  installed. Shown in  FIG. 6   a  are three additional view angles of the aft portion of a trailer of a tractor-trailer truck system. Shown in  FIG. 6   b  is view angle A-A, Shown in  FIG. 6   c  is view angle B-B. Shown in  FIG. 6   d  is view angle C-C. View angles A-A, B-B, C-C all show that the trapped vortex  130  extends along the full height of the panel  41   a  and that the trapped vortex outflow vorticess  131 ,  132  exit the open ends of the aft facing step and maintain their vortex structure as they flow aft. 
         [0075]      FIG. 7   a  through  FIG. 7   d  are side and top views of example ground vehicles with and without the subject invention installed.  FIG. 7   a  shows a typical tractor-trailer truck system  1 , comprised of a powered tractor  10  that pulls a trailer  30 . The trailer has a top surface  34  and two side surface  32  and  33 , and a rear surface  35  or bluff base. The tractor  10  is comprised of a cab  11  and an aerodynamic fairing system  20  that may be an integral part of the tractor  10 .  FIG. 7   b  shows the same tractor-trailer truck system  1  as that of  FIG. 7   a  with the present invention  40  installed on rear surface  35  of the trailer  30 . The present invention consists of two aft extending panels  41   a  and  41   b . The two aft extended panels  41   a  and  41   b  that comprise the invention  40  are symmetrically positioned about the centerline of the trailer  30 .  FIG. 7   c  and  FIG. 7   d  show an automobile  50  pulling a trailer  60  with and without the present invention  40  installed on both the automobile rear surface  55  and the trailer rear surface  66 . The automobile has a top surface  54  and two side surfaces  52  and  53 , and a rear surface  55 . The trailer has a top surface  64  and two side surfaces  62  and  63 , and a rear surface  65 .  FIG. 7   d  shows the same automobile  50  pulling a trailer  60  as that of  FIG. 7   c  with the present invention  40  installed on rear surface  35  of the trailer  30 . The present invention consists of two aft extending panels  41   a  and  41   b . The two aft extended panels  41   a  and  41   b  that comprise the invention  40  are symmetrically positioned about the centerline of the trailer  30 . The various vehicles depicted in  FIG. 7  shows a powered vehicle towing/pulling an un-powered towed vehicle. Additionally, other multiple component vehicles may be considered than those depicted. 
         [0076]      FIG. 8   a  through  FIG. 8   g  are three views of eight different panel geometry types; baseline panel  41   a , notched panel  42   a , sawtooth panel  43   a , curved panel  44   a , stall strip panel  45   a , gurney panel  46   a , vented panel  47   a  and panel with micro vortex strakes installed  48   a . Additional panel types may be defined by combining the notched panel  42 , sawtooth panel  43 , curved panel  44 , stall strip panel  45 , gurney panel  46 , the vented panel  47 , and the micro vortex strake panel  48 . 
         [0077]      FIG. 8   a  is a three view depiction of the baseline panel  41 . Shown in the figure is the right side panel  41   a . The baseline panel  41  is a rectangular shaped panel with width W, length L, and thickness t. The baseline panel  41  has a linear trailing edge  411  and linear leading edge  412  that abuts the rear surface  35  of a vehicle. The width W of the baseline panel  41  is between 15 and 25 percent of the vehicle width. The length L of the baseline panel  41  is equal to or less than the height of the vehicle base area. 
         [0078]      FIG. 8   b  is a three view of the notched panel  42 . Shown in the figure is the right side panel  42   a . The notched panel  42  is a rectangular shaped panel with width W, length L, and thickness t. The notched panel  42  has a notched trailing edge  421  and linear leading edge  422  that abuts the rear surface  35  of a vehicle  30 . The width W of the notched panel  42  is between 15 and 25 percent of the vehicle width. The length L of the notched panel  42  is equal to or less than the height of the vehicle base area. The notch geometry is defined by a notch depth Nd, notch length Nl, notch spacing Ns, and notch inset Ni. The definition of the notch geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0079]      FIG. 8   c  is a three view of the sawtooth panel  43 . Shown in the figure is the right side panel  43   a . The sawtooth panel  43  is a rectangular shaped panel with width W, length L, and thickness t. The sawtooth panel  43  has a sawtooth trailing edge  431  and linear leading edge  432  that abuts the rear surface  35  of a vehicle  30 . The width W of the sawtooth panel  43  between 15 and 25 percent of the vehicle width. The length L of the sawtooth panel  43  is equal to or less than the height of the vehicle base area. The sawtooth geometry is defined by a sawtooth depth Sd, sawtooth length Sl, and sawtooth inset Si. The definition of the sawtooth geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0080]      FIG. 8   d  is a three view of the curved panel  44 . Shown in the figure is the right side panel  44   a . The curved panel  44  is a rectangular shaped panel with width W, length L, and thickness t. The curved panel  44  has a curved trailing edge  441  and linear leading edge  442  that abuts the rear surface  35  of a vehicle  30 . The width W of the curved panel  44  is between 15 and 25 percent of the vehicle width. The length L of the curved panel  44  is equal to or less than the height of the vehicle base area. The curved geometry is defined by a curve function Cf which is based upon a curve depth Cd, curve length Cl, and curve inset Ci. The definition of the curved geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0081]      FIG. 8   e  is a three view of the stall strip panel  45 . Shown in the figure is the right side panel  45   a . The stall strip panel  45  is a rectangular shaped panel with width W, length L, and thickness t. The stall strip panel  45  has a stall strip trailing edge  451  and linear leading edge  452  that abuts the rear surface  35  of a vehicle  30 . The width W of the stall strip panel  45  is between 15 and 25 percent of the vehicle width. The length L of the stall strip panel  45  is equal to or less than the height of the vehicle base area. The stall strip geometry is defined by a stall strip thickness SSt and stall strip width SSw. The definition of the stall strip geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0082]      FIG. 8   f  is a three view of the gurney panel  46 . Shown in the figure is the right side panel  46   a . The gurney panel  46  is a rectangular shaped panel with width W, length L, and thickness t. The gurney panel  46  has a gurney trailing edge  461  and linear leading edge  462  that abuts the rear surface  35  of a vehicle  30 . The width W of the gurney panel  46  is between 15 and 25 percent of the vehicle width. The length L of the gurney panel  46  is equal to or less than the height of the vehicle base area. The gurney geometry is defined by a gurney thickness Gt and gurney width Gw. The definition of the gurney geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0083]      FIG. 8   g  is a three view of the vented panel  47 . Shown in the figure is the right side panel  47   a . The vented panel  47  is a rectangular shaped panel with width W, length L, and thickness t. The vented panel  46  has a linear or treated trailing edge  471  and vented leading edge  472  that does not abut the rear surface  35  of a vehicle  30 . The width W of the vented panel  47  is between 15 and 25 percent of the vehicle width. The length L of the vented panel  47  is equal to or less than the height of the vehicle base area. The vent geometry is defined by a vent width Vw. The definition of the vent geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0084]      FIG. 8   h  is a three view of the micro vortex strake panel  48 . Shown in the figure is the right side panel  48   a . The micro vortex strake panel  48  is a rectangular shaped panel with width W, length L, and thickness t. The micro vortex strake panel  48  has a linear trailing edge  481  and a linear leading edge  482  that abuts the rear surface  35  of a vehicle  30 . The micro vortex strake panel has micro vortex strakes installed on the outward facing surface of the panel near the trailing edge. The width W of the micro vortex strake panel  48  is between 15 and 25 percent of the vehicle width. The length L of the micro vortex strake panel  48  is equal to or less than the height of the vehicle base area. The micro vortex strakes are defined by the micro vortex strake width W VG , micro vortex strake length L VG , micro vortex strake spacing S VG , and micro vortex strake angle α VG . The micro vortex strakes geometry is a function of the vehicle geometry, vehicle operational requirements, and vehicle maintenance requirements. 
         [0085]      FIG. 9  shows a rear perspective view of the aft portion of a typical trailer  30  with a top surface  34  and side surfaces  32  and  33  of a tractor-trailer truck with the subject invention  40  with vented panels  47   a  and  47   b  installed on the rear surface  35  of a trailer  30 . Also shown is a horizontal section cut through the trailer  30  and subject invention  40  with vented panels  47   a  and  47   b  and a side view of the trailer  30  and subject invention  40  with vented panels  47   a  and  47   b . The invention  40  is comprised of two vented panels  47   a  and  47   b , attachment/actuation hardware  71 , roller or low friction pad to assist in stowage of panel  72 , and low friction landing strip  73  that is mounted to the sides  32 ,  33  of the vehicle  30 . 
         [0086]    The length L of each vented panel  47   a  and  47   b  of the invention  40  is equivalent to the full vertical height of the vehicle  30 . The width W of each panel  41  of the invention  40  is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . The type, size and structure of the attachment/actuation hardware  71  of the invention and the type, size and structure roller/low friction hardware  72 ,  73  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0087]    The present invention  40  provides aerodynamic drag reduction for all free stream flow  100  conditions including crosswind conditions. Aerodynamic drag reduction occurs when flow  100  that separates at the vehicle side surface  32 ,  33  trailing edge and is turned into the base wake region with the use of vortex trapping and flow venting flow control technologies. The flow  100  leaving the side surface  32 ,  33  trailing edge forms a vortex that reside on the outward facing surface of panel  47 . The trapped vortex located on panels  47   a  and  47   b  acts as a fluidic surface to the external flow that is turned into the wake region. The trapped vortices promote the turning of the external flow into the base wake region which results in a stable bluff-base wake flow and a high pressure that acts on the base surface  35  of the trailer  30 . The strength of the trapped vortices on panels  47   a  and  47   b  will provide increasing aerodynamic drag reduction with increasing velocity of the flow  100 . The effectiveness of the subject invention to reduce drag and thereby increase fuel economy of a vehicle is determined by panel  47  width W, length L, lateral position Y, trailing edge vent gap Vw. 
         [0088]      FIG. 10   a  shows a rear perspective view of the aft portion of a typical swing door trailer  30  with top surface  34  and side surfaces  32  and  33  of a tractor-trailer truck with an embodiment of the present invention  40 , configured with a spring hinge  71  attachment/actuation mechanism, installed on the rear swing doors  36  and  38  of a trailer  30 . Also shown in  FIG. 10   a  are horizontal section cuts through the trailer  30  and present invention  40  detailing the stowage of the present invention  40 . This embodiment of the invention  40  is comprised of two panels  41   a  and  41   b , spring hinge attachment/actuation hardware  71 , roller or low friction pad  72  to assist in stowage of panels  41   a  and  41   b , and low friction landing strip  73  that is mounted to the sides  32 ,  33  of the vehicle  30 . The self-stowage feature of the invention  40  is accomplished when the panels  41   a  and  41   b , or roller  72 , contacts the vehicle side surface  32 ,  33 . To facilitate the folding process and to minimize damage to the vehicle  30  a low friction landing strip  73  may be mounted to the side  32 , 33  of the vehicle  30 . 
         [0089]    The type, size and structure of the spring hinge attachment/actuation hardware  71  of the invention and the type, size and structure roller/low friction hardware  72 ,  73  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0090]      FIG. 10   b  shows a rear perspective view of the aft portion of a typical swing door trailer  30  with top surface  34  and side surfaces  32  and  33  of a tractor-trailer truck with an embodiment of the present invention  40 , configured with a pneumatic spring  77  actuation mechanism and simple hinge  75  attachment hardware installed on the swing doors  36  and  38  of a trailer  30 . Also shown in  FIG. 10   b  are horizontal section cuts through the trailer  30  and present invention  40  detailing the stowage of the present invention  40 . The invention  40  is comprised of two panels  41   a  and  41   b , pneumatic spring actuation hardware  71 , simple hinge  75  attachment hardware, roller or low friction pad  72  to assist in stowage of panels  41   a  and  41   b , and low friction landing strip  73  that is mounted to the sides  32 ,  33  of the vehicle  30 . The self-stowage feature of the invention  40  is accomplished when the panels  41   a  and  41   b , or roller  72 , contacts the vehicle side surface  32 ,  33 . To facilitate the folding process and to minimize damage to the vehicle  30  a low friction landing strip  73  may be mounted to the side  32 ,  33  of the vehicle  30 . 
         [0091]    The type, size and structure of the pneumatic spring attachment/actuation hardware  71 , simple hinge  75  and roller/low friction hardware  72 ,  73  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0092]      FIG. 10   c  shows a rear perspective view of the aft-portion of a typical swing door trailer  30  with top surface  34  and side surfaces  32  and  33  of a tractor-trailer truck with an embodiment of the present invention  40 , configured with a mechanical support  74  and simple hinge  75  attachment hardware installed on the rear surface  35  of a trailer  30 . Also shown in  FIG. 10   c  are horizontal section cuts through the trailer  30  and present invention  40  detailing the stowage of the subject invention  40 . The invention  40  is comprised of two panels  41   a  and  41   b , mechanical support  74 , simple hinge  75  attachment hardware, roller or low friction pad  72  to assist in stowage of panels  41   a  and  41   b , and low friction landing strip  73  that is mounted to the sides  32 ,  33  of the vehicle  30 . The self-stowage feature of the invention  40  is accomplished when the panels  41   a  and  41   b , or roller  72 , contacts the vehicle side surface  32 ,  33 . To facilitate the folding process and to minimize damage to the vehicle  30  a low friction landing strip  73  may be mounted to the side  32 ,  33  of the vehicle  30 . 
         [0093]    The type, size and structure of the mechanical support  74 , simple hinge  75  and roller/low friction hardware  72 ,  73  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0094]      FIG. 11  shows an alternate embodiment of the subject invention.  FIG. 11  show a rear perspective view of the aft portion of a typical swing door trailer  30  with top surface  34  and side surfaces  32  and  33  of a tractor-trailer truck with the an embodiment of the present invention  40  installed on the rear doors  36  and  38  of a trailer  30  at the side edge of the trailer  30 , without a lateral inset Y. Also shown in  FIG. 11  are horizontal section cuts through the trailer  30  and present invention  40  detailing the stowage of the present invention  40  from the outboard mounted position, Y=0.0. The invention  40  is comprised of two panels  41   a  and  41   b  and spring hinge  71 . The self-stowage feature of the invention  40  is accomplished when the panels  41   a  and  41   b  contacts the vehicle side surface  32 ,  33 . 
         [0095]    The type, size and structure of the spring hinge  71  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0096]      FIG. 12  shows an alternate embodiment of the subject invention.  FIG. 12  show a rear perspective view of the aft portion of a typical rollup door  89  trailer  30  with top surface  34  and two side surfaces  32  and  33  of a tractor-trailer truck with the present invention  40  installed on rear support panels  87  and  88  of a trailer  30 . Also shown in  FIG. 12  are horizontal section cuts through the trailer  30  and present invention  40  detailing the stowage of the present invention  40 . The invention  40  is comprised of two panels  41   a  and  41   b  and spring hinge  71 . The self-stowage feature of the invention  40  is accomplished when the panels  41   a  and  41   b  contacts the vehicle side surface  32 ,  33 . 
         [0097]    The type, size and structure of the spring hinge  71  and rear support panels  87  and  88  of the invention is determined by the geometric characteristics of the vehicle  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . 
         [0098]      FIG. 13   a  to  13   d  are side views of various embodiments of the present invention  40  installed on a tractor-trailer truck  1 .  FIG. 13   a  is a side view of a tractor-trailer truck  1  with the invention  40 , comprised of a linear trailing edge panel  41  installed on the rear surface  35  of vehicle  30 .  FIG. 13   b  is a side view of a tractor-trailer truck  1  with the subject invention  40 , comprised of a notched trailing edge panel  42  installed on the rear surface  35  of vehicle  30 .  FIG. 13   c  is a side view of a tractor-trailer truck  1  with the present invention  40 , comprised of a sawtooth trailing edge panel  43  installed on the rear surface  35  of vehicle  30 .  FIG. 13   d  is a side view of a tractor-trailer truck  1  with the present invention  40 , comprised of a curved trailing edge panel  44  installed on the rear surface  35  of vehicle  30 . 
         [0099]      FIG. 14   a  to  14   d  are side views of various embodiments of the subject invention  40  installed on various ground vehicles.  FIG. 14   a  is a side view of a panel truck  140  with the present invention  40 , comprised of a sawtooth trailing edge panel  43  installed on the rear surface of vehicle  140 .  FIG. 14   b  is a side view of a pick-up truck  150  with the subject invention  40 , comprised of a sawtooth trailing edge panel  43  installed on the rear surface of cab and bed of the vehicle  150 .  FIG. 14   c  is a side view of a van  160  with the present invention  40 , comprised of a sawtooth trailing edge panel  43  installed on the rear surface of vehicle  160 .  FIG. 14   d  is a side view of a bus  170  with the subject invention  40 , comprised of a sawtooth trailing edge panel  43  installed on the rear surface of vehicle  170 . 
         [0100]      FIG. 15  show and alternate embodiment of the invention.  FIG. 15  is a rear perspective view of the aft portion of a typical trailer  30  of a tractor-trailer truck showing an alternate embodiment of the present invention  40  installed on the rear surface  35  of a trailer  30 . The shape, size, and position of the subject invention  40  are a function of the geometry of the trailer  30 , the operational requirements of the vehicle  30  and the maintenance requirements of the vehicle  30 . The present invention  40  is comprised of a two opposing aft extended panels  43  that are symmetrically positioned about the vehicle  30  vertical plane of symmetry. The leading edge  432  of each panel  43  may abut the rear surface  35  of vehicle  30  or may be inset from the rear surface  35  of vehicle  30  a distance Vw. A panel that is inset from the rear surface  35  a distance Vw is a vented panel  47 . A vented panel  47  promotes the movement of air from the high pressure region on the rear surface  35 , inboard of the subject invention  40 , to a low pressure region on the rear surface  35 , outboard of the subject invention  40 . The trailing edge  431  of each panel  43  may be linear (panel  41   a ) or of a complex shape (representative panels  42   a ,  43   a ,  44   a ,  45   a ,  46   a ,  47   a ,  48   a ). The complex shaped trailing edges  431  are designed to energize the external side flow  100  and promote flow turning into the base wake region. 
         [0101]    From the description provided above, a number of advantages of the wake stabilization plates become evident:
       (a) The invention provides a novel process to reduce the drag of a bluff-base body.   (b) The invention provides a means to use vortices located on the base surface of a bluff-base body to reduce drag.   (c) The invention provides a means to reduce the aerodynamic drag and improve the operational efficiency of bluff-base vehicles.   (d) The invention provides a means to reduce the aerodynamic drag and improve the fuel efficiency of bluff-base vehicles.   (e) The invention provides a means to conserve energy and improve the operational efficiency of bluff-base vehicles.   (f) The invention provides a means to reduce the aerodynamic drag without a significant geometric modification to existing bluff-base vehicles.   (g) The invention may be easily applied to any existing bluff-base vehicle or designed into any new bluff-base vehicle.   (h) The invention allows for the efficient operation of the invention with a limited number of panels.   (i) The invention allows for the matching of complex surface shapes by the shaping and placement of the panels.   (j) Large reductions in drag force can be achieved by the trapping of two vortices.   (k) The structure, placement, and shape of each aft extended panel may be adapted to meet specific performance or vehicle integration requirements.   (l) The trailing edge shape of each aft extended panel may be linear or complex to meet specific performance or vehicle integration requirements.   (m) The ability to optimally position each aft extended panel on the vehicle rear surface.   (n) The ability to minimize weight and volume requirements within the vehicle.   (o) The ability to minimize maintenance requirements.   (p) The ability to minimize the impact on operational and use characteristics of the vehicle door system.   (q) The ability to maximize the safety of vehicle operation.       
 
       CONCLUSION, RAMIFICATIONS, AND SCOPE 
       [0119]    Accordingly, one may see that the wake stabilization device can be used to easily and conveniently reduce aerodynamic drag on any ground vehicle for the purposes of improving the operational performance of the vehicle. Furthermore, the two panels comprising the wake stabilization device has the additional advantages in that:
       it provides a aerodynamic drag reduction force over the base of the vehicle;   it allows the contour of the host surface to be easily matched;   it allows easy application to any existing vehicle or designed into any existing vehicle;   it allows the device to be fabricated as an independent unit that may be applied to an existing surface;   it allows for optimal positioning of each aft extended panel on the vehicle base surface;   it allows the design of a system with minimum weight and to require minimum volume within the vehicle;   it allows minimum maintenance requirements;   it allows minimum impact on door operation and use.   it allows for the maximum safety of vehicle operation;       
 
         [0129]    Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, the rearward extending panels can be composed of various planar shapes such as ellipsoid, quadratic, etc.; the thickness and width can vary along the length; the material can be any light-weight and structurally sound material such as wood, plastic, metal, composites, etc.; the substrate can be any metal, wood, plastic, composite, rubber, ceramic, etc.; the application surface can be that of a metal, wood, plastic, composite, rubber, ceramic, etc. The attachment and actuation hardware can be either conventional off the shelf or designed specifically for the subject invention. 
         [0130]    The invention has been described relative to specific embodiments thereof and relative to specific vehicles, it is not so limited. The invention is considered applicable to any road vehicle including automobiles, trucks, buses, trains, recreational vehicles and campers. The invention is also considered applicable to non-road vehicles such as hovercraft, watercraft, aircraft and components of these vehicles. It is to be understood that various modifications and variation of the specific embodiments described herein will be readily apparent to those skilled in the art in light of the above teachings without departing from the spirit and scope. 
         [0131]    Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.