Abstract:
In an aero-optical interface for an aircraft optical aperture, the separation of the upstream boundary layer from the edge of the aperture creates a region of turbulance which persists downstream over the full area of the aperture. This invention promotes an early develpment of a steady velocity profile at the upstream aperture edge which is approximately the same as a stable, self-similar shear flow velocity profile over the entire aperture. This is accomplished by thickening the boundary layer upstream of the aperture, and blowing a curtain of air across the aperture from its upstream edge at the point of separation of the boundary flow. This produces a shear flow region foot that causes the overall velocity profile to be equal to a stable self-similar free shear layer.

Description:
BACKGROUND OF THE INVENTION 
     This application is related to U.S. patent application Ser. No. 623,152 filed on Jun. 21, 1984, and entitled &#34;Apparatus and Method for Providing an Aero-Optical Interface&#34;. 
     This invention relates to structure for providing an optically clear and steady shear layer over a port in the external skin of an airborne vehicle, and a method for achieving such a shear layer. 
     In certain circumstances it is desired to provide an optically and acoustically stable shear layer over an open port in the external skin of an airborne vehicle so that acoustic resonance in the cavity beneath the port is minimized, and so that light entering or leaving the port is subject to minimal diffusion. One acoustic application is in the wheel well of an aircraft which, in some prior art aircraft, resonates violently upon opening and subjects the structure and hydraulic lines within the cavity to unnecessary vibration. In addition, the noise can actually be heard by passengers and may cause unnecessary nervousness. 
     Another more recent need for an open port over which the flow is acoustically and optically stable is in infrared scanning of the heavens from an aircraft. The infrared detectors in the scanners are extremely sensitive and the signal processing equipment extremely sophisticated, but unless the optical signal is clear and steady the value of the data is decreased. The clarity (i.e., intensity distribution and size) and steadiness of the optical image are affected by the shear layer over the open port in the airborne vehicle and vary with variations in air density within the shear layer. These density variations have heretofore been studied but little effort has been made to actually control the formation of the shear layer and to influence its properties. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a method and apparatus for promoting and controlling the growth and propagation of a shear layer that is steady and clear to allow the transmission of a steady and clear beam of light through an open port through the external skin of an airborne vehicle. 
     The objects of the invention are attained in a preferred embodiment for promoting the formation and growth of a clear and steady shear layer over a port in the external skin of an airborne vehicle, including a nozzle on the upstream side of the port for blowing a jet of air over the port and up along the shear layer. The jet of air is at the same total temperature as the free stream, and is at a mass flow rate equal to that of the incident freestream flow through an area equal to about 1-4 percent of the port area. The jet is aimed up into the shear layer at about 1-15 degrees from the plane of the external skin of the airborne vehicle with a thickness equal to about 10 percent of the port streamwise length at the upstream edge of the opening. The clear and steady transition region from the upstream edge of the port down stream to the commencement of the fully developed portion of the shear layer, where large scale turbulence commences, is at least 8-10 times the thickness of the boundary layer of the gas stream at the upstream side of the opening. The boundary layer can be thickened upstream of the port by the use of a roughness strip, such as sandpaper or a band of upstanding pins. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention, and its many attendant objects and advantages, will become better understood upon reading the description of the preferred embodiment in conjunction with the following drawings, wherein: 
     FIG. 1 is a schematic sectional elevation of an opening in the external skin of an airborne vehicle showing the structure according to one embodiment of the invention for achieving a clear and steady shear layer over the port; 
     FIG. 2 is an isometric view of a section of the external skin of the aircraft shown in FIG. 1 showing the optical port and also showing the upstream structure for thickening the boundary layer upstream of the optical port; 
     FIG. 3 is a schematic cross-sectional elevation of a variant of the embodiment shown in FIG. 1; 
     FIG. 4 is a schematic cross-sectional elevation of a second variant of the invention shown in FIG. 1; 
     FIG. 5 is a cross-sectional schematic elevation of the invention shown in FIG. 1 showing the growth of the shear layer over the optical port in the airborne vehicle and examples of measured velocity and fluctuation intensity profiles. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings, wherein like reference characters, designate identical or corresponding parts, and more particularly to FIG. 1 thereof, a cross section of a portion of an airborne vehicle is shown having an exterior skin 10 in which is formed an opening or port 12 for passage of beams of light into or out of a cavity 14 underlying the port 12. An optical instrument 16 may be placed within the cavity 14 for transmitting or receiving light beams to or from the heavens. 
     It is desired to control the nature of the shear layer over the optical port 12 so that the beam of light into or out of the port is diffused as little as possible. Diffusion of the light beam occurs by several mechanisms. First, the shear layer over the optical port can experience large scale shifting or rotating movements which tend to create unpredictable and random shifting of the light beam so that the apparent direction of the light beam continually changes in an unpredictable manner. This phenomena is commonly referred to as &#34;image wander or jitter&#34;. 
     Another mechanism for diffusion of the light beam, called &#34;unsteady defocus&#34;, is caused by large scale turbulence within the shear layer. The large scale turbulence causes the image to change shape on the focal plane in a manner which makes it difficult to determine the true center of the image. 
     The third diffusion mechanism is commonly called &#34;scattering&#34; of the light beam and is caused by small scale turbulence in the shear layer. The scattering of the light beam results in a weakening in intensity of the image and is by far the least serious of the three diffusion mechanisms since it is does not mathematically affect where the center of the weakened image occurs. Of course, a certain refraction of the light beam will occur by reason of the change in density through the shear layer from the free stream velocity to the quiescent condition of the air within the cavity, but this refraction can be easily corrected mathematically by the signal processing equipment and does not cause a problem. 
     The key to achieving a useful clear and steady image, therefore, is to produce a shear layer over the open optical port that is steady and free of large scale turbulence. 
     In my prior related application, I disclosed a structure and method for producing an undeflected, planar, two dimensional, self-similar, minimum thickness shear layer profile over an open cavity. Although this shear layer was a great improvement over the prior art, it did require a formation distance between separation from the upstream edge of the cavity and the self-similarity region equal to about 4×10 5  v/ v .sbsb.e for laminar upstream boundary layers, and about 2×10 6  v/ v .sbsb.e for turbulent upstream boundary layers, where &#34;v&#34; equals the kinematic viscosity of the air stream, and &#34;v e  ? equals the local boundary layer edge velocity. The formation distance was usually an unsteady mixing layer characterized by a series of nearly random vortices that pass on downstream in the shear flow. 
     To both introduce steadiness and accelerate formation of a self-similar shear flow velocity profile after separation of the boundary layer from the upstream edge of the port, a nozzle 20 is provided at the upstream edge of the port to blow air approximately tangentially across the opening at a shallow angle, between 0 and 15 degrees. The nozzle blows a jet of air across the entire width of the opening at a mass flow rate equal to that of the incident freestream through an area equal to about 1-4 percent of the port area. Preferably, the mass flow fraction of the jet is about 2 percent since that seems to produce the optium optical and acoustic properties at a minimal flow rate. However, the mass flow fraction can be increased above 3 percent without injury to the optical properties. The added flow appears to simply lift the shear layer higher above the cavity and position the reattachment of the shear layer on the downstream edge of the vehicle skin slightly downstream from the normal reattachment zone. The thickness of the nozzle in this embodiment is about 10 percent of the port streamwise length which allows the appropriate depth of flow of air into the shear layer at a velocity of about one order of magnitude less than the freestream velocity. As shown in FIG. 5, the effect of the lip blowing through the nozzle has the immediate effect of adding a low velocity tail on the velocity profile of the shear layer just developing downstream from the upstream edge of the cavity. This steadies the separation process and greatly accelerates creation of a similar shear layer velocity profile that retains its self-similarity as it becomes thicker in passing downstream over the cavity. Although the velocity profile is not perfectly self-similar, it is very close with the use of lip blowing. The properties of the shear layer are dominated by the profile on the high speed side of the mean velocity, and nearly ideal optical transmission properties are obtained. The increase in thickness of the shear layer does not damage the optical quality of the beam coming through the shear layer since the shear layer is steady and the large scale turbulence characteristic of a fully developed shear layer no longer dominated by the upstream boundary layer thickness has not yet occurred. 
     In some circumstances it is desirable to provide additional air from within the cavity to feed the shear layer at its downstream regions. The upstream regions of the shear layer are fully fed by the nozzle 20, but when the port is at its greatest dimension, the entrainment of the gas by the shear layer can include gas from the cavity at the extreme downstream terminal regions of the free shear layer beyond the influence at the nozzle flow, so that in these circumstances it is well to provide a source of gas to feed these downstream regions of shear layer. 
     As shown in FIG. 1, an opening is provided, communicating through channels 22 and 24 with a source of pressurized air such as aircraft ram air (not shown in FIG. 1) up into the cavity opening and the nozzle respectively. The channel 22 feeds the cavity opening through a flow smoothing honeycomb and screen structure 26 to remove any turbulence or eddies which would be created by turbulent air entering the cavity through the opening 26. Likewise a flow smoother 28 is provided in the channel leading to the nozzle 20. The flow smoother 28 includes a honeycomb structure 30 and a fine screen 32 over the top of the honeycomb 30. A flow straightener 34 composed of honeycomb alone with cell size length-to-diameter ratio on the order of 8 to 1 is preferred in the nozzle 20 exit, especially if the channel under the surface is short. 
     A reattachment ramp 36 is provided at the downstream edge of the cavity 14. The reattachment ramp is a curved structure which curves down into the cavity into a position designed to intercept the shear layer and promote smooth reattachment of the shear layer back to the external skin of the vehicle. The reattachment ramp shown in FIG. 3 is at a 45 degree angle to the plane of the exterior skin. A variant of the reattachment ramp shown in FIG. 3 is shown in FIG. 4 wherein the angle of the reattachment ramp is at 30 degrees from the plane of the external skin and the ramp does not extend as deeply into the cavity, thereby reducing the possibility of interfering with the optics within the cavity. This reattachment ramp is suitable for smaller cavities or for cavities which have a shear layer which will grow at a shallower angle than the shear layer growth shown in FIG. 3. Generally, the deeper the ramp the lower the blowing mass fraction required to optimize the shear flow. 
     As shown in FIG. 2, a boundary layer thickening structure is provided upstream of the optical port 12. The boundary layer thickening structure is a series of pins which extend vertically from the external skin of the vehicle at least 100 roughness diameters from the upstream edge of the optical port. The roughness region extends laterally beyond the optical port on both sides so that the edge of the thickened boundary layer lies beyond the sides of the optical port, that is, the edges of the thickened boundary layer do not intercept the upstream edge of the optical port which would perturb the desired steady and clear shear layer over the optical port. The length of the small scale turbulence transition region from the upstream edge of the optical port to the fully developed large scale turbulence shear layer is about 8-10 times the thickness of the boundary layer at the separation edge. By thickening the fully developed turbulent boundary layer upstream of separation, the initial steady relatively smooth sided transition region can be made as long as desired. 
     Other techniques for thickening the boundary layer include pins or bolt heads screwed into the skin of the vehicle, or a strip of grit, like sandpaper, attached to the surface. The prescription of leaving a space of at least 100 roughness diameters upstream of the separation edge is to allow the turbulent wakes from the roughness region to coalesce into a uniform layer. 
     The injection velocity through the nozzle 20 may be an order of magnitude smaller than the free stream velocity. The actual velocity varies inversely with the thickness of the lip jet. The total blowing mass flow rate in all cases is about 2 percent of the mass fraction which is defined as the incident free stream mass flow rate per unit area multiplied by the optical port open area. The optimum lip blowing jet thickness is appropriately 1/10 of the optical port streamwise length. The upper edge of the nozzle should constitute a wedge of material which has structural integrity and does not itself create eddies in the airstream off of the upstream edge of the opening. The rate of air blowing through the flow smoother 26 at the bottom of the cavity just forward of the leading edge of the ramp is about 1/5 of the lip blowing rate. 
     The evaluation of the boundary layer-shear layer velocity profile is shown in FIG. 5. Apparently, the reason that the small blowing mass fraction is able to stabilize and clarify the shear layer is that the low velocity foot of the boundary layer up to v e  /2 before separation has very little momentum to overcome. 
     In general, for a turbulent layer, the distance to v e  /2 is less than one percent of the overall boundary layer thickness from the wall because of the extremely steep (typically 7th power) velocity profile. The tangential blowing to fill in the foot of the upstream solid wall boundary layer profile reverses the profile curvature below v e  /2. The center of curvature thus goes from upstream to downstream of the foot of the velocity profile. The curvature above v e  /2 remains relatively unchanged. Measured velocity profiles near the center of the port with and without blowing are shown in FIG. 5. The major portion of the mass flow is above v e  /2 in all cases. 
     Since optical scatter varies exponentially with the square of the density fluctuation in the gas through which the optical beam is transmitted, lip blowing substantially reduces the optical density loss due to scatter since the fluctuation maximum with lip blowing is about 2/3 as large as that without lip blowing for edge Mach numbers above 0.3, which is the range in which compressibility is significant. Measured fluctuation profiles with and without flowing near the center of the port are shown in FIG. 5. 
     Thus, the invention provides a shear layer without a large scale vortex structure so that light transmitted through such a shear layer can be focused to produce much smaller images of point objects than when transmitted through a shear layer with large vortices. The invention disclosed herein therefore improves the treatment of optical transmission properties by including defocus as well as wander and jitter. Obviously, numerous modifications and variations of the preferred embodiment disclosed herein will occur to those skilled in the art. Therefore it is to be expressly understood that these modifications and variations, and the equivalents thereof, may be practiced while remaining within the spirit and scope of the invention as defined in the following claims.