Abstract:
An aerodynamic drag reduction device for use on an over-the-road cargo vehicle. The vehicle has a prismatically shaped cargo area, which includes a rear face of the cargo area substantially perpendicular to the direction of travel. The device comprises a plurality of resilient prongs arranged along a rear edge of the vehicle body, extending from a respective fixed end secured to the vehicle body rearward in a flow-wise direction beyond the rear edge of the vehicle body to a respective free end. Each prong is separated from an adjacent prong in the plurality, and each is flexible to permit deflection, under the influence of airflow over the vehicle at a predetermined speed, above and below a first plane defined by the surface of the vehicle to which the plurality of prongs is secured. Each prong is further resistant to deflecting in a direction parallel to the first plane.

Description:
BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to the field of transportation aerodynamics. More specifically, disclosed is an apparatus to improve the aerodynamic and fuel efficiency of an over-the-road cargo vehicle. 
     2. Brief Discussion of Related Art 
     The predominant mode of transportation for commercial goods throughout the United States, the developed world and elsewhere is cargo truck, among these including a tractor-trailer truck. For this mode of transportation, fuel represents the largest single cost component. Therefore, any measureable improvement in fuel efficiency of such vehicles is worthwhile. 
     In particular, in the developed world, where tractor-trailer trucks travel long distances of well-developed highways at a generally high speed, aerodynamic drag represents a major source of inefficiency. One source of such aerodynamic inefficiency is the geometry of the truck, which is essentially an elongated rectangular prism. In particular, the airflow properties over the trailing edge of the trailer create a large trailing negative pressure vortex, which greatly contributes to drag. 
     One recent technology to improve aerodynamic efficiency is colloquially called a “boat tail”. A boat tail is an attachment to the rear end of the trailer which acts as a fairing to gradually reduce the cross-sectional area of the trailer, and thus reduce the size and intensity of the trailing vortex and its associated drag. One investigation by the Platform for Aerodynamic Road Transport (PART), a research affiliate of the Delft University of Technology, Netherlands, suggests a boat tail can contribute a 4.5% improvement in fuel efficiency. 
     However, a boat tail as it is currently practiced has a practical size limit that still necessitates an abrupt geometry change at its trailing edge. Furthermore, a trailer is accessed via doors at its rear. Any sort of boat tail impedes access to such doors. For many such tractor/trailer trucks the container itself is transferable in order to be used by intermodal transportation (i.e., train, or cargo ship). In those circumstances, the aerodynamics are either substantially different (e.g. rail), or not even a concern (i.e., container ship). In such cases, the inviolable requirement is that the container keep its standardized size and shape, to enable its intermodal transfer. Permanent alterations to the shape of the trailer to improve efficiency are therefore impossible, to say nothing of the cost-effectiveness in construction of a box trailer. Even an aerodynamically effective successful boat tail should therefore be temporary, removable or interchangeable for most practical effect. 
     Furthermore, in loading or unloading, a road-use trailer is most commonly backed up to an elevated loading dock. Attempts to deal with this problem include making the boat tail inflatable, or foldable. Still, a boat tail remains an operational obstacle to loading and unloading. 
     Therefore, the present state of the art is lacking. Other solutions in place of or in addition to a boat tail may yield even better aerodynamic results and/or greater operational advantages. 
     SUMMARY 
     In order to overcome these and other weaknesses, drawbacks, and deficiencies in the known art, provided according to the present disclosure is an aerodynamic drag reduction device for use on an over-the-road cargo vehicle, the vehicle having a prismatically shaped cargo area, including a rear face of the cargo area substantially perpendicular to the direction of travel. The device includes a plurality of resilient prongs arranged along a rear edge of the vehicle body, each of the prongs extending from a respective fixed end secured to the vehicle body rearward in a flow-wise direction beyond the rear edge of the vehicle body to a respective free end. Each prong is separated from an adjacent prong in the plurality. Each prong is further flexible to permit deflection of the free end above and below a first plane defined by the surface of the vehicle to which the plurality of prongs is secured. Such deflection is caused by the properties of the airflow over the vehicle at a predetermined speed. Each prong is further resistant to deflecting in a direction parallel to the first plane. 
     Alternately or additionally, a shaft of each prong has a perpendicular cross section with an area moment of inertia that is lowest around a neutral axis of the cross section that is substantially parallel to the first plane. 
     Optionally, each prong may include a composite construction of two or more material sections, each material having a different modulus of elasticity. Each prong may optionally include a vulcanized rubber material in some embodiments. In certain embodiments, each prong has a substantially uniform cross-section. In others, each prong has a tapered cross-section, in height or width, or both. For certain embodiments of the present disclosure, each prong has radiused corners at its respective connection to the space separating it from an adjacent prong. 
     Further described according to the present disclosure, optionally the plurality of prongs are secured to the vehicle with the capability to be repositioned from a deployed position having the free ends extended beyond a rear edge of the vehicle body, to a retracted position having the free end nearer to or forward of the rear edge of the vehicle body. In some cases, the device is slideable in a flow-wise direction to reposition the prongs. 
     In other embodiments, the device is secured to a rotating frame member which is operative to be rotated between the deployed position and a retracted position. For certain rotatable deployed embodiments, the device is itself rotatable on the rotating frame member to maintain an orientation of the prongs in a rearward extending direction. Optionally, the rotating frame member may be securable in one of the deployed or the retracted positions. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       These and other embodiments of the present disclosure will become apparent from the following detailed description read in connection with the accompanying drawings, wherein 
         FIG. 1  illustrates a generally conventional tractor-trailer cargo truck, having added thereto a drag-reducing airflow baffle according to the present disclosure; 
         FIG. 2  illustrates a detailed view of the upper rear portion of the cargo truck indicated by circle  2  in  FIG. 1 ; 
         FIG. 3  illustrates a drag-reducing airflow baffle according to a first embodiment of the present disclosure; 
         FIG. 4  illustrates a cross-section view of one baffle prong taken along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  illustrates a drag-reducing airflow baffle according to a second embodiment of the present disclosure; 
         FIG. 6  illustrates a cross-section view of one baffle prong taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  illustrates on embodiment of a baffle-retracting scheme according to the present disclosure; 
         FIG. 8A  illustrates a second embodiment of a baffle-retracting scheme according to the present disclosure, having the baffle retracted; 
         FIG. 8B  illustrates the second embodiment of a baffle-retracting scheme according to the present disclosure, having the baffle in an intermediate position; and 
         FIG. 8C  illustrates the second embodiment of a baffle-retracting scheme according to the present disclosure, having the baffle deployed. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , illustrated is a tractor-trailer truck, generally  100 , the features of which are largely conventional. While a tractor-trailer  100  is described, the present disclosure will be seen as applicable to any cargo vehicle with a prismatic shape of the cargo section, for example and without limitation, a box truck, a car-pulled trailer, or the like. The tractor cab  110  includes a cabin for the operators and an engine (not shown) to power itself and pull one or more attached trailers  120 . Airflow streamlines  130 ,  140  depict the flow of air over the truck  100  at generally highway speeds, e.g., 60 miles per hour (MPH) or roughly 95 kilometers per hour (kM/h). 
     Attached to the rear of the trailer  120  is an airflow baffle  150 . Airflow baffle  150  is visible vertically in  FIG. 1 , being attached to a near side on the trailer  120 . Not visible in  FIG. 1 , is a further baffle  150  that can be mounted vertically at the rear of trailer  120  along an opposite side facing away from the viewer. Another baffle  150  may be mounted horizontally across a top of the trailer  120 , again extending rearward analogous to the baffle  150  shown in  FIG. 1 . 
     The prismatic geometry of a standard trailer  120 , in particular the abrupt change of shape at its trailing end, creates a large low pressure vortex immediately behind the trailer  120  when there is airflow over the trailer  120 , for example at highway speed. This low pressure vortex is a large contributor to aerodynamic drag. In order to minimize the drag associated with this trailing vortex it is advantageous to control or influence the flow of air into the space immediately behind the trailer. 
     With reference to  FIG. 2 , the upper rear end of the trailer  120  is depicted without any baffle  150  attached thereto to illustrate the typical airflow behavior. Experimental observation and computational fluid dynamics flow simulation indicates that, at the abrupt right-angle trailing edge of the trailer  120  the flow induced is characterized by a dynamic sinusoidal or wavelike pattern, generally indicated by streamlines  202 . This flow pattern is dynamic in the sense that the wave pattern shifts with a sinusoidal or wavelike characteristic as flow over the trailer  120  separates from the trailer  120  and mixes with fluid behind the trailer  120 . This sinusoidal or wavelike flow pattern is accompanied by mixing vortices  204 . In order to delay the separation of airflow from the trailer, and thus reduce drag formed by the separation, it would be beneficial if the surface of the trailer could be made to move with the sinusoidal or wavelike flow pattern. In this manner, the mixing of airflow over the trailer  120  into the trailing vortex would be controlled, and distributed over a greater volume as the separation is extended behind the trailer  120 . The intensity of the pressure differential behind the trailer  120  is therefore reduced, and with it the accompanying drag. 
     Referring Now to  FIG. 3 , the flow baffle  150  provides prongs  152  that are positioned to extend in the flow-wise direction, generally aligned with a longitudinal axis of the trailer  120 , which can be seen as extending in parallel to the x-axis direction as depicted in  FIG. 1 . Prongs  152  are separated from one another by spaces  154 , which spaces allow respective free ends  156  of individual prongs  152  to move independently of one another. Opposite the free end  156  of each prong  152  is a fixed end  158 . The free end  156  of each prong  152  is connected to a respective fixed end  158  by a shaft  162 . Fixed ends  158  may be secured to one another and the baffle  150  in general by a common spine  160 . The space between prongs  152  at the spine  160  may be provided with individual or blended fillets  164 , in order to avoid stress concentration. Alternately or additionally, the fixed ends  158  may be secured to the trailer  120  itself. 
     In a very particular embodiment, the prongs  152  are approximately 2 inches in width, between about 0.5 to 1 inches in thickness, and up to about 14 inches in length. Spacing  154  between the prongs  152  can be about 1 inch. However, these dimensions are offered as an example only, and should not be taken to limit the scope of the disclosure. These and other relevant dimensions are left to the particular application as determined by those skilled in the art taken in light of Applicant&#39;s present disclosure. 
     The baffle  150  is secured to the trailer  120  to permit the shaft  162  of each prong  152  to extend, in whole or in part, rearward beyond a trailing edge of the trailer  120 . Moreover, the prongs  152  are resiliently constructed to permit their flexure above or below a plane defined by a side surface of the trailer  120  to which they are secured. The degree of resiliency and flexure will be subject to adjustment according to the individual circumstances. Among the factors to be considered are the dimensions of the trailer  120 , the design operating speed at which drag is to be minimized, resultant Reynolds number for the particular flow, etc. As a first order approximation, prongs  152  constructed of vulcanized rubber display what is considered to be an adequate degree of resiliency for the present application. 
     Composite makeup may be employed as well, for example the prongs having a core of a harder material, ductile metals, resilient plastics or the like, with additional flexibility afforded by a covering of more flexible material over this core. Optionally, some or all of the baffle  150  in gross may have the same composite construction as the prongs  152 . The cross-sectional view of the prong  152  indicates a composite construction, including a core  168  having an alternate material, in particular a differing modulus of elasticity, as the material comprising the remainder of the prong  152 . The cross-sectional shape of the core  168  need not necessarily conform to that of the prong  152  as a whole. Moreover, the length of the core  168  may optionally be less than that of the prong  152 . The core  168  may have a uniform cross-section, or it may taper or otherwise change in cross-sectional area without regard to the shape of the prong  152 . 
     The precise cross-sectional dimension of the prongs  152  will also affect the flexibility of the prongs  152 . Generally speaking, it is considered desirable that the prongs have flexibility to deflect above or below the designated mounting plane, but only limited flexibility laterally within the mounting plane. To this end, the cross-sectional geometry should exhibit a greater area moment of inertia (alternately called second moment of area) around any axis extending out of the mounting plane as compared with the area moment of inertia around any axis lying in or parallel to the mounting plane. As a result, the prongs will resist flexing around any axis having a higher area moment of inertia, which can be by design an axis lying parallel to the mounting plane. 
     As an example only, and with reference to  FIG. 4 , a cross-section view of the prong  152  taken along section line  4 - 4  in  FIG. 3 , illustrates that the prong  152 , and particularly its shaft  162 , have a lowest area moment of inertia around the horizontal axis  165  passing through the center of the shaft  162 . In certain embodiments, the corners  166  of the shaft  162  may be rounded to avoid stress concentrations and improve durability in service. 
     Referring now to  FIG. 5 , illustrated is an alternate embodiment of a baffle, generally  250 . A full description of the features common with the foregoing embodiment of  FIGS. 3-4  will be apparent to those skilled in the art, and the following description will highlight the differences therewith. Baffle  250  has prongs  252  separated from one another by spaces  254 . The shaft  262  of each prong  252  is tapered in its width at it extends rearwardly in a flow-wise direction, with a tape r angle  270  defined by θ.  FIG. 6  is a cross-section view of the prong  252  taken along section line  6 - 6  in  FIG. 5 . Here again, the prong  252 , and particularly its shaft  262 , have a lowest moment of inertia around the horizontal axis  264  passing through the center of the shaft  262 . Accordingly, they will tend to flex above or below the mounting plane, and resist lateral deflection within or parallel to the mounting plane. Alternately or additionally, the prong cross-section may be tapered in height to influence the propensity of the prong to defect vertically (as viewed in  FIG. 4  or  6  only; the prepared axis of deflection will generally be laterally for baffles installed on a side surface of the trailer) rather than horizontally. 
     The cross-sectional view of the prong  252  indicates a composite construction, including a core  268  having an alternate material, in particular a differing modulus of elasticity, as the material comprising the remainder of the prong  252 . Notably, the cross-sectional shape of the core  268  need not necessarily conform to that of the prong  252  as a whole. Moreover, the length of the core  268  may optionally be less than that of the prong  252 . The core  268  may have a uniform cross-section, or it may taper or otherwise change in cross-sectional area without regard to the shape of the prong  252 . 
     A trailer  120  fitted with one or more baffles  150 ,  250 , obtains its benefit of drag reduction in transit at highway speeds. However, such a trailer  120  should preferably be compatible with the existing trucking infrastructure in other phases of operation, namely loading and unloading. Loading and unloading of the trailer  120  is most commonly accomplished by one or more doors at the rear face  122  of the trailer  120 . Moreover, for this purpose, a raised loading dock (not shown) is commonly provided level with the bottom  125  of the trailer  120 . The height of such a dock is generally standardized. In order for the trailer  120  to be backed into position adjacent to such a loading dock for loading and unloading, it is desirable that the baffles  150  or  250  be retractable such that they do not extend beyond the rear face  122  of the trailer  120 . 
     Referring now to  FIG. 7 , illustrated is a mounting arrangement where the baffle  150  is mounted to the trailer  120  in a manner that permits the baffle  150  to be shifted along a longitudinal axis of the trailer  120 . In particular, a plurality of pegs  180  is provided on the trailer  120 , which fit respectively into one or more of in spaces  154  between adjacent prongs  152 . Accordingly, the baffle  150  can slide longitudinally along the trailer  120  from a position with free ends  156  extended beyond the rear face  122  of the trailer  120 , as shown in  FIG. 7 , to a retracted position having the free ends  156  longitudinally forward of the trailer rear face  122  (not shown). Moreover, the baffle  150  may be adapted to be secured in one of several intermediate positions as well. With the baffle  150  retracted, it does not impact nor interfere with the trailer  120  backing into to a loading dock, nor access to the trailer  120  from the same. The baffle  150  may be secured in the extended, retracted, or any intermediate position by any number of conventional means known in the art. 
       FIGS. 8A-8C  illustrate an alternate mounting embodiment for baffles  150 . In this embodiment, one or more baffles  150  are mounted to a pivoting frame  310 . The pivoting frame is mounted to the trailer by a plurality of mounts  312 . A handle  314  is attached to the frame  310  to allow a user to pivot the frame  310  between retracted and deployed positions. One or more handle latches  316 ,  318  are provided to hold the handle  314 , and thereby the frame  310 , in either the retracted or extended positions, respectively. 
     Baffles  150  are carried by the frame  310  on arms  320 , such that a rotation of the frame  310  from its retracted position illustrated in  FIG. 8A , to an intermediate position illustrated in  FIG. 8B , places the baffles  150  with the fixed ends  158  in proximity to the rear face  122  of the trailer  120 . From this intermediate position of  FIG. 8B , in this particular embodiment, the baffles  150  may be mounted to the arms  320  in a pivotal manner, such that the baffles  150  are rotated into an operating position illustrated in  FIG. 8C , having the free ends  156  of the prongs  152  extending rearward beyond the rear face  122  of the trailer  120 . Thus, with the frame  310  holding the baffles  150  in their retracted position of  FIG. 8A , the baffles  150  may nonetheless be stowed with the free ends  156  of the prongs  152  generally aligned with a direction of airflow. In other embodiments, the operator of the trailer  120  might find it convenient to operate the trailer  120  with the baffles  150  in a position having the prongs  152  forward-facing. 
     The foregoing examples of baffle retraction in  FIGS. 7 ,  8  are depicted on a side surface of the trailer  120 . They will be understood to be equally applicable to the opposing side surface and/or a top surface of the trailer  120  as well. Furthermore, the baffle  150  as described herein will be seen as equally applicable to other vehicles, or portions thereon, including for example and without limitation the arrangement of a baffle  150  or  250  as described herein to the tractor cab  110 . 
     It will be appreciated that variants of the above-disclosed embodiments and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.