Abstract:
A device for reducing base drag on cylindrical rear truncated objects moving in a fluid, caused by the shedding of vortices at the base of the object. The device consists of ring shaped winglets attached to the rear of the object which may be sub-divided into a plurality of hinged partial winglets. The parameters of distance of winglet from cylinder shaped object, winglet circumference, angle, profile and chord length may be varied automatically for optimum drag-reducing capability.

Description:
BACKGROUND OF THE INVENTION 
     A drag force acts on an object which moves in a fluid environment such as air or water. This drag force includes several specific drag forces wherein the main one is known as a pressure drag force. The pressure drag force is caused by a net pressure force acting on the object. The rear end contribution to the pressure drag is called “base drag”. Flow separation at the base of the moving object creates a vortex system and reduces base pressure thus increasing drag. This problem exists for truncated objects, which have blunt bases, such as a box, a cylinder and the like. 
     Reference is now made to FIG. 1A which is a schematic illustration of a device for reducing drag which is known in the art (Frey, D. “ Guide Vores” Foschung Ing Wessen , 1933 and Hoemer, S. F. “ Fluid Dynamic Drag ”, 1958 p. 3-27). One of the ways known in the art for reducing the base vortex strength in two-dimensional objects such as high aspect ratio wings, is by utilizing winglets near the base of the wing or behind it. wing  10  includes four winglets  12 ,  14 ,  16  and  18 , which reduce the base drag by depressing the ascilatory vortex shedding from the base. 
     The asymmetric, ascilatory vortex shedding which greatly increases the base drag in a 2 dimensional configuration does not exist in three dimensional bodies. 
     Reference is now made to FIG. 1B which is a schematic illustration of a device, known in the art (Maull, D. J. “ Mechanisms of Two and Three Dimensional Base Drag ”, Plenum Press, 1978), which was tested for aerodynamic drag reduction. A three dimensional blunt object  20 , which in the present example is a truck, includes two rear side flow deflectors  22  and  24  and a rear top deflector  26 . This configuration has proved to be inefficient in reducing the base drag and has even shown slight increases in the drag force, as compared to the baseline configuration of a truck without such deflectors. 
     Another device aiming at base drag reduction on blunt-based trailers is described in U. S. Pat. No. 5,348,366 (Baker and Levitt, 1994). It is shown in FIG. 3 (of Baker). The amount of drag reduction achieved by deploying the device shown in FIG. 3 is 15%. The mechanism of drag reduction is similar to that in boattailing a blunt axi-symmetric object and thus increasing its base pressure, as was suggested by Mair (1965). 
     Other devices for reducing the base drag of airborne axi-symmetric bodies use air bleed through the blunt base (U.S. Pat. No. 4,807,535 by M. Schilling and M. Reuche (1989) and U.S. Pat. No. 4,554,872 by U. Schleicher (1985)). These devices require, however, modification of the internal volume to accommodate the charge used to accommodate the base bleed jet. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a device for reducing drag in a three dimensional object. 
     It is a further object of the present invention to provide a novel device for reducing drag in a three dimensional cylindrical object, which can be adapted to variable velocity in real time. 
     In accordance with the present invention there is thus provided a ringlet shaped device for reducing drag of a cylindrical rear truncated object moving in fluid, to be placed near the rear end of the object. The device includes at least one ring shaped winglet. 
     According to another aspect of the present invention, a selected one of the ring shaped winglets includes a plurality of partial winglets and winglet connectors, wherein each of the winglet connectors connects a predetermined pair of the partial winglets. Each winglet connector can be adapted to move the predetermined pair of the partial winglets connected thereto either to increase or decrease the distance between the elements. 
     Furthermore, the device may also include a ring shaped winglet, a plurality of partial winglets connected to the ring shaped winglet by a plurality of hinges, wherein the hinges enable the partial winglets to rotate along an axis tangent to the circumference of the ring which is defined by the hinges. 
     According to another aspect of the invention, the device further includes a controller for controlling at least one of the ring-shaped winglets according to predetermined parameters and a processor, for determining the values of each of the parameters, according to the speed of the cylindrical rear truncated object and the properties of the fluid, the processor providing the values to the controller. The predetermined parameters are selected from the group consisting of: 
     distance of the winglet from the cylinder shaped object; 
     winglet circumference; 
     the angle between the ring chord and the symmetry axis; 
     winglet profile; and 
     winglet chord length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
     FIG. 1A is a schematic illustration of a prior art device for reducing drag; 
     FIG. 1B is a schematic illustration of a prior art device which was tested for drag reduction; 
     FIG. 2A is a pictorial illustration of a cylinder shaped truncated object; 
     FIG. 2B is a schematic cross-sectional illustration of the vortices at the rear end of the object shown in FIG. 2A; 
     FIG. 2C is a schematic cross-sectional illustration of the vortices at the rear end of the object shown in FIGS. 2A and 2B and a device for reducing drag, constructed and operative in accordance with a preferred embodiment of the invention; 
     FIG. 2D is a pictorial illustration of the object and the device shown in FIG. 2C; 
     FIG. 2E is a schematic cross-section illustration of the device shown in FIG. 2C, on a boattailed cylindrical object; 
     FIG. 3 is a schematic illustration of a boattailed cylindrical blunted object and a device, constructed and operative in accordance with another preferred embodiment of the present invention; 
     FIG. 4A is a schematic illustration of moving object and of a drag reducing device, constructed and operative in accordance with a further preferred embodiment of the invention. 
     FIG. 4B is a rear view of the drag reducing device shown in FIG. 4A; 
     FIG. 5A is a pictorial illustration of a drag reducing device, constructed and operative in accordance with yet another preferred embodiment of the invention, in a closed state; 
     FIG. 5B is a pictorial illustration of the device shown in FIG. 5A, in an open state; and 
     FIG. 6 is a schematic illustration of a moving object and a drag reducing device, constructed and operative in accordance with a further preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIGS. 2A,  2 B,  2 C,  2 D and  2 E. FIG. 2A is a pictorial illustration of a truncated cylinder, generally referenced  200 . FIG. 2B is a schematic cross-section illustration of the vortices at the rear end of object  200 . FIG. 2C is a schematic cross-section illustration of the vortices at the base of object  200  with a device, generally referenced  202 , constructed and operative in accordance with a preferred embodiment of the invention. FIG. 2D is a pictorial illustration of object  200  with device  202 . FIG. 2E is a schematic cross-section illustration of device  202 , mounted on a boattailed cylindrical object  290 . 
     Object  200  is moving to the left in the air. Arrow  203  indicates the direction of movement of object  200 . Arrow  242  indicates the movement of air alongside and relative to object  200 . The object moving left forms a low pressure area behind its rear end  201 . The air  242  flowing adjacent to the object  200  separates from the sharp corner at the end of the cylinder and forms a free vortex ring Γ a , referenced  240 , as illustrated in FIG.  2 B. This vortex is the main cause of base drag which is a significant portion of the total drag. 
     According to the present invention, reduction of the base drag is provided by a circumferencial winglet  202  in the shape of a ring which is placed near the base of truncated object  200 , as shown in FIGS. 2C and 2D. Winglet  202  forms a vortex ring Γ b , referenced  246 , which is located away from the center of the base of object  200 . Vortex  246  causes reduction in the size and strength of the vortex  240  as can be seen by FIGS. 2B and 2C, by shifting vortex activity away from the center of the base of object  200 . 
     A device according to the invention can be adapted to any type of generally cylinder shaped objects and, for that matter, boattailed cylinder shaped objects, which in the present example is object  290  (FIG.  2 E). 
     Device  202 , constructed in accordance with a preferred embodiment of the invention, can be adapted to various objects, fluids and velocities. There are a number of parameters (shown in FIG. 2E) which determine the efficiency of the device  202  in reducing drag, among which are: 
     the distance d of the device  202  from object  290 ; 
     the height h of the front edge  204  of device  202  from rear edge  210  of object  290 ; 
     the chord length c between the device  202  leading edge  204  and the trailing edge  206 ; 
     the winglet angle, which is the angle between the ring chord and the symmetry axis, δ; and 
     the shape of the profile of device  202 . 
     Applicant has realized that fine tuning these parameters using wind tunnel experiments may result in reducing base drag greatly. 
     Reference is now made to FIG. 3 which is a schematic illustration of the base of a typical missile configuration  300  and a device, generally referenced  302 , constructed and operative in accordance with a preferred embodiment of the present invention. Device  302  is a ringlet located near the base of configuration  300 . 
     The device  302  according to the invention is also efficient in reducing drag, when added to a rear exhausting system, such as a missile  300 . The device  302  reduces drag in a mode wherein the engine of the missile is turned on, exhausting gases backwards and also, in a mode where the engine of the missile is turned off. 
     Reference is now made to FIGS. 4A and 4B. FIG. 4A is a schematic illustration of a moving object  490  and drag reducing device, generally designated  400 , constructed and operative in accordance with a further preferred embodiment of the invention. 
     FIG. 4B is a rear view of drag reducing device  400 . Device  400  includes a plurality of partial winglets, generally referenced  402 A,  402 B and  402 C. Partial winglet  402 A is connected to partial winglet  402 B via connecting unit  404 B. Partial winglet  402 A is connected to partial winglet  402 C via connecting unit  404 A. Partial winglet  402 C is connected to partial winglet  402 B via connecting unit  404 C. Each of the connecting units  404 A  404 B and  404 C, is adapted to change the distance between the two winglets connected thereto, by means of conventional electromechanical servo units. Thus, according to the present embodiment, the general diameter D of device  400  can change and thus be adapted, in real time, to a plurality of factors such as the varying velocity of object  490 , the fluid density, and the like. 
     For example, Applicant has found that a chord length c which equals 0.1 R, wherein R is the base radius, is less efficient in reducing the total drag than a chord length c which equals 0.3 R. Furthermore, a distance d of the device from base which equals 0.1 R is less efficient than, a distance d of the device from base which equals 0.05 R. 
     According to the present embodiment, device  400  is connected to a controller  412  which is operated by a processing unit  410 . Processing unit  410  receives data representing different aspects of the movement of the object. The processor  410  utilizes this data for calculating the appropriate condition of each partial winglet  402  and provides controller  412  with instructions accordingly. The controller  412  operates the connecting units  404 A,  404 B, and  404 C and instructs them to change the distance between each pair of adjacent partial winglets. 
     Reference is now made to FIGS. 5A and 5B. FIG. 5A is a schematic illustration of a drag reducing device, generally designated  500 , constructed and operative in accordance with yet another preferred embodiment of the invention, in a closed state. 
     FIG. 5B is a pictorial illustration of device  500  in an open state. 
     Device  500  is a ring shaped winglet which includes a main winglet  502  and a secondary winglet  504 . Secondary winglet  504  includes a plurality of partial winglets  506 , which are connected to the main winglet  502  by hinges  508 . The hinges  508  enable axial movement of each of the partial winglets  506 . Each of the partial winglets provides self movement and may be controlled separately. Thus the secondary winglet  504  can transform from a closed state, shown in FIG. 5A to an open state, shown in FIG.  5 B. This feature of the invention is merely an example of a winglet according to the invention, capable of dynamic shape changes. 
     Reference is now made to FIG. 6 which is a schematic illustration of moving object  690  and a drag reducing device, generally designated  600 , constructed and operative in accordance with a preferred embodiment of the invention. Device  600  includes two winglet rings  602  and  604  which are placed near the rear of object. Winglets  602  and  604  are mounted on a plurality of bars, generally referenced  608 A,  608 B, and  608 C. Bars  608 A  608 B and  608 C extend from the base of object  690 . Winglet  604  adds to the drag reduction which is initially provided by winglet  602 . According to the invention, winglet  604  can be identical to winglet  602  or be different in one or more aspects such as profile, angle, height, and the like. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow.