Abstract:
A tapered micro-plow, or a series of tapered micro-plows, are submerged in a boundary layer just upstream of a reflection point of an oblique shock. Each micro-plow develops a beneficial pair of vortices which redistribute high energy flow within the boundary layer such that flow separation is prevented or delayed. The beneficial vortex pairs rotate about an axis that is parallel to the flow of fluid, and together rotate such that they induce a velocity on one another which tends to hold them near the surface and delay vortex lift-off.

Description:
BACKGROUND 
     1. Technical Field: 
     This disclosure relates in general to passive flow control device and in particular to a passive device for delaying boundary layer flow separation within a high velocity fluid. 
     2. Description of Related Art: 
     All jet aircraft require a propulsion system which diffuses the incoming air to certain speeds before passing it through the jet engine. For supersonic aircraft, this process often involves a mixed compression inlet which initiates a series of shock waves that reflect off of the inlet surfaces. Each shock reflection causes a shock-boundary layer interaction near the point of reflection. Each interaction may include a shock-induced separation which reduces the inlet pressure recovery and degrades performance. The separation may also cause blockage, thereby reducing the effective flowpath area to a value below the critical level required for operation. This leads to an unstart, and limits the operational range of the inlet. 
     Shock induced separation can be reduced by actively bleeding (removing) the boundary layer from the flowfield, which requires porous surfaces and tubes/plumbing beneath the surface. The complexity and weight associated with porous surfaces and tubes/plumbing can degrade mission performance of the jet aircraft. 
     Vane-type vortex generators submerged in the boundary layer (i.e., micro-vanes) can modify fluid flow, but the contact surface of the micro-vanes is so small that they have a high likelihood of detaching and creating a foreign object damage (“FOD”) hazard. Ramp-type vortex generators submerged in the boundary layer (i.e., micro-ramps) can be attached more securely than micro-vanes, but their aerodynamic performance is worse than that of micro-vanes. Indeed, many studies show them to be worse than nothing at all because they introduce shock waves with an orientation which further reduces pressure recovery. Also, each micro-ramp creates a vortex pair in a position and orientation such that they induce an upward velocity (upwash) on one another which elevates them off of the surface (i.e., vortex lift-off). This upward velocity away from the surface diminishes their effectiveness at redistributing the boundary layer energy toward the floor. 
     SUMMARY OF THE INVENTION 
     A passive device, called a “tapered micro-plow,” can modify boundary layer flow over a surface by generating pairs of vortices that can keep can the boundary layer flow attached to the surface. Indeed, vortices generated by the tapered micro-plow can redistribute high energy flow within the boundary layer such that the separation is prevented or delayed. 
     In one embodiment, one or more tapered micro-plows can be affixed to the inlet surface of a mixed compression engine inlet, just upstream of a shock reflection point, such that it is submerged in the attached boundary layer adjacent to the surface. Each tapered micro-plow can generate a vortex pair which minimizes the adverse effects of upwash and shock losses which impede the performance of conventional high speed vortex generators. Other potential applications can include, but are not limited to, supersonic boundary layer control on external wings. 
     Tapered micro-plows can have a large contact area with the surface. This allows them to be securely fixed to the surface, and can enhance their resistance to breakage, thermal erosion, and ablation. Therefore, tapered micro-plows can reduce any foreign object damage hazard normally associated with fragile micro-vanes. 
     The tapered micro-plows can also reduce shock losses. The leading edge shocks emanate primarily outward into the boundary layer, rather than up into the supersonic core flow. The divergence angle can begin with a low value such that the shock is weak and attached, thereby reducing shock losses. The divergence angle increases in segments moving aft such that the local angle is effective at producing voracity. The height distribution can create a “nose-cone” effect which also reduces the shock losses. 
     The tapered micro-plows can generate vortices which beneficially delay vortex lift-off. The created vortex pair can exit the micro-device near the floor, and the direction of rotation can induce favorable downwash, which can hold the vortices down where they are most effective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective diagrammatic view of an exemplary embodiment of a micro-plow passive flow control device. 
         FIG. 2  is a top view of the micro-plow passive flow control device of  FIG. 1 . 
         FIG. 3  is a bottom view of the micro-plow passive flow control device of  FIG. 1 . 
         FIG. 4  is a schematic view of fluid flow profiles before and after flowing across the micro-plow passive flow control device of  FIG. 1 . 
         FIG. 5  is a perspective diagrammatic view of a fluid flowing across a micro-plow passive flow control device of  FIG. 1 . 
         FIG. 6  is a front view of the micro-plow passive flow control device of  FIG. 1 . 
         FIG. 7  is a perspective view of a plurality of the micro-plow passive flow control devices of  FIG. 1 . 
         FIG. 8  is a cross-sectional view of an air inlet with a plurality of the micro-plow passive flow devices of  FIG. 1 . 
         FIG. 9  is a cross-sectional view of another embodiment of an air inlet with a plurality of micro-plow passive flow devices of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specific details for purposes of illustration, one of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope and spirit of the invention. Accordingly, any exemplary embodiments of the invention described herein are set forth without any loss of generality to, and without imposing limitations thereon, the present invention. 
     Referring to  FIGS. 1-3 , micro-plow  100  is a passive device for altering fluid flow over surface  102 . Surface  102  could be, for example, a wing surface on an aircraft, a surface on a jet-engine air inlet, a surface on the hull of a ship, a surface on a propeller of a ship, or any other surface in which fluid is moving across the surface. As will be described, below, micro-plow  100  is a tapered micro-plow. Micro-plow  100  has a centerline  104 , or axis, that can be parallel to the direction of the flow of fluid over surface  102 . Nose  106  is located at the upstream end of micro-plow  100 . Nose has a point  108  that is centered on centerline  104  and extends away from surface  102 , at an apex angle, to apex  110 . The apex angle is the angle at which nose  106  rises from surface  102 . In one embodiment, apex  110  is the part of micro-plow  100  that is located furthest, vertically, from surface  102 . The sides of nose  106  diverge, laterally, from centerline  104  at nose angle  112 . 
     Glove  118  is located aft of apex  110 . Glove  118  can include a number of triangular facets. In one embodiment, glove  118  includes six triangular facets. The six facets can include three pair, identified as  120 ,  126  and  128 . Centerline  104  can divide each pair symmetrically such that each facet is a mirror image of the other in the pair. Each triangular glove facet  120  is an oblique triangle with its longest edge coinciding with outer edge  122  of micro-plow  100 . Both triangles  120  share a common edge along centerline  104  with endpoints located at apex  110  and point  124 . Outer edges  122  of glove  118  diverge from centerline  104  at glove angle  125  and can define the top edge of glove sidewall  126 . Triangular glove facets  128  can define part of the glove sidewall, and each can have one edge on the floor, in contact with or adjacent to surface  102 . 
     Glove angle  125  can be generally greater than nose angle  112 . Triangular facets  120  define the upper surface of glove  118 , and can gradually slope downward as they move laterally away from centerline  104 , or they can be generally in the same plane. Similarly, the surfaces of triangles  120  can each slope downward as they move aft from apex  110 , or they can be generally level with apex  110 . Glove sidewalls  126  can rise vertically from surface  102  to outer edges  122  or they can rise at an angle. Glove sidewall  128  can be a sidewall that is generally in the same plane as glove sidewall  126  or it can be at an angle to glove sidewall  126 . 
     Dorsal channel  130  is a depression that is concave from the top of micro-plow  100 . Dorsal channel  130  begins at point  124  between the legs of glove  118  and slopes downward and aftward from that point. When viewed from above, dorsal channel  130  has a diamond shape that is bisected by dorsal centerline  132 , which can be parallel to centerline  104 . Laterally, dorsal channel  130  slopes downward as it moves from the outer edges of micro-plow  100  toward dorsal centerline  132 . Axially, dorsal channel  130  slopes downward as it transitions aftward, to aft closure point  134  of the diamond shape. Aft closure point  134  is along centerline  104  and adjacent to surface  102 . Outer points  136  of dorsal channel  130  are located at the trailing edge of outer edge  122  of glove  118 . 
     Main plows  140  can be a pair of triangularly shaped surfaces located generally aft of outer points  136 . The top surface of each main plow  140  can slope downward from fore to aft. In one embodiment, such downward slope of main plow  140  is greater than the downward slope of dorsal channel  130 . The foremost point of main plow  140  is located at outer point  136 . From outer point  136 , inner leg  142  of each main plow slopes downward, from fore to aft, and inward toward centerline  104 , to aft closure point  134 . Outer leg  144  of each main plow  140  slopes outward, at plow angle  146 , to aft tip  148 . Plow angle  146  can be greater than glove angle  125 . Trailing edge  150  is the aft-most edge of main plow  140 , and can extend along surface  102  from aft closure point  134  to aft tip  148 . Aft tip  148  can be axially located aft of aft closure point  134 . In one embodiment, aft tip  148  is the point of micro-plow  100  located furthest, laterally, from centerline  104  and furthest, axially, from point  108  of nose  106 . The top surface of plow  140  can be generally flat or it can be slightly concave or convex. 
     Each main plow  140  can have a plow segment sidewall  154  rising from surface  102  to outer leg  144 . Sidewall  154  can be generally perpendicular to surface  102  or it can extend from surface  102  at an angle. The foremost edge of plow segment sidewall  154  meets the trailing edge of body sidewall  128 . 
     Referring to  FIG. 3 , bottom surface  160  can be used to affix micro-plow  100  to surface  102  ( FIG. 1 ). Bottom surface  160  can be generally flat, or it can have a contour. In one embodiment, the contour of bottom surface  160  is selected based on the contour of the surface  102  to which micro-plow  100  will be affixed. Indeed, the contour of bottom surface  160  can generally match the contour of surface  102  ( FIG. 1 ) to maximize the surface area in contact between bottom surface  160  and micro-plow  100 . 
     Referring to  FIGS. 1 and 3 , in one embodiment, the surface area of bottom surface  160  is greater than the combined surface area of glove sidewalls  126 , body sidewalls  128 , plow segment sidewalls  154 , and the sidewalls of nose segment  106 . This large surface area, relative to the sidewall surface area, can provide a stronger attachment between micro-plow  100  and surface  102 . 
     Micro-plow  100  can be affixed to surface  102  by a variety of techniques. It can, for example, be affixed by an adhesive, such as an epoxy, it can be welded, or it can be attached by mechanical fasteners, such as screws (not shown) that pass up through surface  102 . Bottom surface  160  can have a generally smooth surface, or it can have a texture to increase the strength of the adhesion to surface  102 . In one embodiment, micro-plow  100  is formed into surface  102 , wherein micro-plow  100  and surface  102  are in integral material. In this embodiment, it can be formed by, for example, being stamped or molded directly into surface  102 . 
     Referring to  FIG. 4 , when fluid  164  flows over surface  102 , micro plow  100  can alter the fluid flow. Fluid  164  can be, for example, air or water. In one embodiment, surface  102  can be a surface on an aircraft such as, for example, a wing or a surface within an engine air inlet duct. Alternatively, surface  102  can be a surface on a water craft or a ground vehicle. Surface  102  can move through fluid  164 , or fluid  164  can move across surface  102 . For the sake of simplicity, any relative movement between fluid  164  and surface  102  will be described as fluid  164  moving across surface  102 , regardless of whether surface  102  is moving through static fluid  164 , fluid  164  is moving across a static surface  102 , or some combination thereof. 
     Fluid  164  can include boundary layer fluid  166 . Boundary layer fluid is fluid that can have reduced velocity as a result of contact with surface  102 . Free stream fluid  168  is fluid that is not affected by surface  102 . Free stream fluid  168  can be, for example, supersonic core flow within an inlet of an aircraft. In one embodiment, the height of micro-plow  100 , measured at apex  110 , is approximately ⅓ the height of the boundary layer expected to flow past micro-plow  102 . Micro-plow  100  can also be taller or shorter. As shown by velocity profile  170 , near surface fluid  172  has a much lower velocity than upper boundary layer fluid  174 , which is boundary layer air that is a greater distance from surface  102 . The velocity of upper boundary layer fluid  174  can be roughly equal to the velocity of freestream fluid  168 . After passing over micro-plow  100 , the boundary layer velocity profile  170 ′ can be fuller, in that the near surface fluid  172 ′ has a higher velocity than near surface fluid  172 . Therefore, the boundary layer at  172 ′ is less likely to separate in an adverse pressure gradient. Furthermore, boundary layer fluid  166  can be thicker after passing over micro-plow  100 . The profile thickness is measured by the distance from surface  102  to the upper boundary layer fluid  174 . Micro-plow  100  can cause the boundary layer edge near fluid  174 ′ to be spaced further from surface  102  than the boundary layer edge near fluid  174 . In one embodiment, micro-plow  100  can energize, or accelerate, near surface fluid  172 ′ by removing energy from, or decelerating, upper boundary layer fluid  174 ′. The boundary layer, thus, can be thickened in the process. 
     Referring to  FIGS. 5 and 6 , as fluid  178  moves across surface  102 , a portion of fluid  178 , including boundary layer fluid  180 , encounters micro plow  100 . Nose  106  can act to part boundary layer fluid  180  and thus direct a portion of boundary layer fluid  180  along glove sidewall  126  and body sidewall  128 . As boundary layer fluid  180  rolls across outer edge  122  and outer leg  144 , boundary layer fluid  180  transitions to a rotational flow. As the fluid flows past a portion of dorsal channel  130  and main plows  140 , it rotates inward, toward centerline  104 , such that the rotational flow develops inwardly rotating vortices  182 . Vortices  182  exit micro-plow  100  near surface  102 , and the direction of rotation includes favorable downwash which can hold vortices  182  down where they are most effective. 
     Inwardly rotating vortices  182  generally rotate about an axis that is parallel to centerline  104 . Because the inwardly rotating vortices  182  rotate downwardly toward centerline  104 , the pair of vortices from each micro plow  100  can urge each other downward toward surface  102  as the rotating vortices extend axially rearward from micro-plow  100 . In one embodiment, the flow having rotating vortices  182  remains in close proximity to surface  102 , or attached, for a greater distance and greater period of time than it would remain attached if micro-plow  100  did not alter the flow. By emanating rearward from micro-plow  102 , vortices  182  can remain primarily in the boundary layer  174  ( FIG. 4 ), rather than drifting up into supersonic core flow  168  ( FIG. 4 ) above surface  102 . In one embodiment, nose can part the flow such that shocks remain attached, rather than emanating upward. Nose  106  and glove  118 , thus, can reduce shock losses, and dorsal channel  130  can enhance vortices  182 . 
     Referring to  FIG. 7 , in one embodiment, a plurality of micro-plows  100  may be located on a surface. They could be placed, for example, side by side and laterally spaced apart from each other. The lateral spacing x between two adjacent micro-plows  100 , as measured from outer point  136  to outer point  136 ′, can be less than the width y of a single micro-plow  100 , as measured between outer points  136 . Any number of micro-plows  100  may be used. Indeed, an array of micro-plows  100  may be spaced apart along the width of surface  102 . 
     Referring to  FIG. 8 , in one embodiment, micro-plows  188  can be located on surface  190  within duct  192 . Duct  192  can be, for example, an air inlet on an aircraft, wherein air enters duct inlet  194 , passes through duct  192 , and subsequently enters subsonic diffuser  196 . The air can pass through subsonic diffuser  196  and finally enter a jet engine (not shown). In one embodiment, duct  192  can be part of a supersonic aircraft (not shown). In this embodiment, duct  192  can be a mixed compression inlet which initiates a series of shock waves  198  that reflect off of duct  192  surfaces  190  and ultimately transitions through terminal shock  199  before entering subsonic diffuser  196 . Each shock reflection can cause a shock-boundary layer interaction near the point of reflection. 
     Micro-plows  188  can be located on surface  190  ahead of reflection location  200  wherein oblique shock waves  198  reflect from surface  190 . The streamwise, or axial, distance from micro-plow  100  to reflection location  200  can be equal to approximately 10-15 times the height of unmodified boundary layer  201 . Unmodified boundary layer  201  can be, for example, the boundary layer upstream of micro-plow  188 . Additional sets of micro-plows  204  can be located on another surface  190 ′ within duct  192 . The additional micro-plows  204  can be located 10-15 times the boundary layer  201 ′ height in front of contact location  206 , wherein another oblique shock  208  contacts surface  190 ′. In this embodiment, micro plows  188 ,  204  can be used to redistribute energy within boundary layers  202 , the modified boundary layer, such that boundary layers  202  remain attached through the adverse pressure gradient associated with a reflected oblique shock wave  198 ,  208 . In one embodiment, micro-plow  100  can be affixed to surface  190  upstream of a reflection location  200 . In this embodiment, when oblique shock  198  is reflected by surface  190 , oblique shock  198  encounters boundary layer  202  with little or no shock-induced separation. 
     Referring to  FIG. 9 , in one embodiment, micro-plows  212  can be located on surface  214  within duct  216 . Duct  216  can include cowl  218  and centerbody  220 . Duct  216  can be used to diffuse and reduce the velocity of a fluid, such as air, before the fluid enters engine  222 . In one embodiment, duct  216  can be part of a supersonic aircraft (not shown). In this embodiment, duct  216  can be a mixed compression inlet which initiates a series of shock waves  224  that reflect off of duct  216  surfaces  214 . Each shock reflection can cause a shock-boundary layer interaction near the point of reflection. 
     Micro-plows  212  can be located on surface  214  ahead of reflection location  226  wherein oblique shock waves  224  reflect from surface  214 . The streamwise, or axial, distance from micro-plow  212  to reflection location  226  can be equal to approximately 10-15 times the height of unmodified boundary layer  227 . In this embodiment, micro plows  212  can be used to redistribute energy within boundary layers  228 , the boundary layer modified by micro-plow  212 , such that boundary layers  228  remain attached through the adverse pressure gradient associated with a reflected oblique shock wave  224 . In one embodiment, micro-plow  212  can be affixed to surface  214  upstream of a reflection location  226 . In this embodiment, when oblique shock  224  is reflected by surface  214 , oblique shock  224  encounters boundary layer  228  with little or no shock-induced separation. Oblique shocks  224 , thus, transition through terminal shock  230  as the now-subsonic fluid passes through subsonic diffuser  232  and subsequently enters engine  222 .