Abstract:
The present invention is a flow control actuator capable of exciting a fluid via the coupling of edge tones generated along a wedge and resonance generated within a cavity. The invention consists of a resonance cavity and ejector port separated by a wedge with fluid flow provided by a pressurized cavity and directed through a throat over the wedge. Several actuators may be arranged independently or in a coupled arrangement to generate a pulsed fluid field of desired shape.

Description:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has, a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of F33615-01-M-3137 awarded by the United States Air Force. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a flow control actuator capable of exciting a fluid so to produce a high-frequency, pulsed stream. Specifically, the invention excites a fluid via resonance produced within a cavity and edge tones generated along a wedge adjacent to the resonance cavity. 
     2. Background 
     The suppression of turbulence is a major challenge in aero-optics, acoustics, and dispensing as each relates to aircraft applications. In aero-optic applications, the integration of a directed energy device onto an aircraft platform requires the attenuation, and preferably elimination, of undesired turbulent structures that degrade both intensity and coherence of a beam projected from an uncovered bay or a covered bay with optically transmissible cover. Acoustic suppression of engine noise is critical to the safe and effective operation of both military and civilian aircraft in urban areas and desirable from a low-observable perspective in military applications. The dispensing of weapons from an open bay or cavity requires the alternation of a pressure field having a negative to-positive gradient along the bay length thereby kicking the tail of the weapon downwards and the nose upwards after release so to cause contact between aircraft and weapon. 
     The suppression of turbulence is achievable by either low-frequency or high-frequency actuation. Low-frequency actuation techniques are provided in the related arts for turbulence control within cavities. Low-frequency actuation attempts to alter or suppress the growth of spatially evolving, large-scale turbulent structures within the cavity shear layer. This approach is best described as mode competition whereby the primary objective is energy removal from targeted modes. However, low-frequency actuation often results in the redistribution of energy into other modes without overall attenuation. Consequently, low-frequency forcing often dampens one tone thereby failing to suppress other naturally occurring tones. Furthermore, the forcing frequency is quite often close to the natural modes so that when cavity modes drift one or more of the resonant modes are excited creating even higher acoustic loads. 
     In contrast, high-frequency actuation represents a strategy whereby the shear layer turbulence dissipation mechanism is directly excited by forcing near “Kolmogorov” -like frequencies thereby suppressing all modes by dissipation of the overall acoustic energy. Large scale vortical structures are replaced by fine-grain turbulence, accompanied by the attenuation of the dynamic load. In cavity applications, fine-grain turbulence attenuates the growth rate of large scale vortical structures and weakens the cavity feedback loop. Whereas in aero-optics applications, it is understood that fine grain turbulence is well with the realm of adaptive optics. 
     High frequency actuation is achievable via a pulsed airstream. Kibens et al., U.S. Pat. No. 6,375,118 issued Apr. 23, 2002, describes a Hartmann tube attached to a high-pressure (80-120 psi) chamber for attenuating engine noise and reducing turbulence in a cavity. FIG. 1 shows the device described in Kibens et al. whereby air is directly forced into a tube. Kibens&#39; tube is based on classical designs described by Hutchins et al. in  The Modulated Ultrasonic Whistle as an Acoustic Source for Modeling , J. Acoustic Soc. Am. 73(1), January 1983, and later by Kastner et al. in  Development and Characterization of Hartmann Tube Based Fluidic Actuators for High Speed Flow Control , Technical Report No. AIAA-2002-0128 presented at the 40 th  Aerospace Sciences Meeting and Exhibit, Jan 14-17, 2001. 
     The operating frequency (f) of a Hartmann tube is approximated by the equation 
     
       
           f=c /(4L) 
       
     
     thereby practically constrained by tube length (L) and sound speed (c) of the gas within the tube. The approximation assumes all waves are Mach waves and flow velocity within the tube is negligible. Operating frequencies in the range of 2500 to 5000 Hz are described by Kibens et al. 
     What is currently required is a high-frequency actuator capable of generating high-quality, high-amplitude, high-frequency tones, while minimizing mass flow rate, for the purpose of attenuating turbulence in aero-optic, acoustic suppression, and weapon dispensing applications. What is required is a device capable of producing high-quality, high amplitude, high-frequency tones exceeding those achievable with a classic Hartmann tube. Furthermore, what is required is a device capable of producing a higher-pressure, pulsed fluid than achievable with a classic Hartmann tube. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is a device capable of generating a high-quality, high-amplitude, high-frequency tone for the purpose of attenuating turbulence. 
     A further object of the present invention is a device producing a pulsed fluid stream via a resonance cavity augmented by edge tones. 
     The present invention is comprised of an inlet port, a stagnation chamber attached to the inlet port thereby providing passage of the fluid into the stagnation chamber, a wedge separating a resonance cavity and an ejector port, and a throat directing fluid from the stagnation chamber across the wedge thereby producing edge tones. The stagnation chamber is pressurized by fluid flow through the inlet port. Fluid from the stagnation chamber passes through a throat and directed across a wedge so that a portion of the fluid flows into the resonance cavity and the remainder through the ejector port exiting the actuator. In preferred embodiments, approximately one-half of the fluid is directed by the wedge into the resonance cavity and the remainder into the ejector port. 
     The cyclic pressurization and depressurization of the resonance cavity establishes an oscillating shock wave in the resonance tube. In the first half of the oscillation, the resonance cavity is pressurized and a shock is developed traveling into the cavity. In the second half of the oscillation, the shock is reversed so to travel back along the cavity purging the cavity after the shock reaches the opening into the cavity. Back flow from the resonance cavity establishes an unsteady vortex at the corner of the wedge further augmenting frequencies in the fluid expelled from the ejector tube. 
     Fluid flow over the wedge adjacent to the resonance cavity further augments frequencies within the fluid expelled from the ejector port. A vortex or swirl is created as fluid flows away from the throat in a turbulent fashion and towards the wedge thereby resulting in flow along one side of the wedge. The resulting pressure generated by this flow condition causes the redirection of one or more subsequent vortices thereby resulting in fluid flow along the opposite side of the wedge. This periodic realignment of flow from side-to-side is often referred to as flipping and produces an edge tone. Fluid flipping adjacent to the resonance cavity causes a corresponding variation in pressure which is magnified by resonance when the pressure variation has a component at the resonant frequency of the cavity. The result is a feedback loop of higher pressure, increased flipping, and greater resonance which may lock the fluid frequency to the resonance frequency of the cavity. 
     Alternate embodiments of the present invention include linear arrangements and circular arrangements of two or more actuators. For example, several actuators may be aligned in a linear fashion with separate inlet ports or a single plenum so to produce a planar flow of excited fluid. Alternately, a plurality of actuators may be arranged to form a single ring with a common plenum or separate inlet ports. Furthermore, a plurality of independently driven actuators may be arranged in a circular fashion so to impinge a column of fluid. 
     Several advantages are offered by the present invention. The invention provides higher pulse frequencies than achievable with a resonance cavity alone. The invention provides higher pressures within the excited fluid than achievable with a resonance cavity alone. The invention provides tunable control over the frequency characteristics of the excited fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a section view of a resonance device described in the related arts. 
     FIG. 2 is a section view of the present invention showing an actuator with inlet port. 
     FIG. 3 is a section view of the present invention showing an actuator with plenum. 
     FIG. 4 is an exploded view of a linearly-arranged, multi-actuator embodiment. 
     FIG. 5 is a side elevation view of a linearly-arranged, multi-actuator embodiment showing ejector ports and fluid flow. 
     FIG. 6 is a section view showing present invention adjacent to an uncovered bay in a turbulence control application. 
     FIG. 7 is a section view showing present invention adjacent to a covered bay with aero-optic cover in a turbulence control application. 
     FIG. 8 is an elevation showing a plurality of actuators arranged in a circular pattern about a nozzle. 
     FIG. 9 is a section view showing excited fluid from actuators impinging exit flow from a nozzle. 
     FIG. 10 is a top elevation view showing a plurality of actuators arranged in a circular pattern about an aero-optics cover in a turbulence control application. 
     FIG. 11 is a section view showing excited fluid from actuators impinging flow adjacent to an aero-optic cover. 
    
    
     REFERENCE NUMERALS 
       1  Hartmann tube 
       2  Inlet port 
       3  Resonance cavity 
       4  Outlet port 
       5  Pressurized chamber 
       6  Needle 
       9  Multi-actuator assembly 
       10  Actuator 
       11  Resonance cavity 
       12  Inlet port 
       13  Stagnation chamber 
       14  Wedge 
       15  Throat 
       16  Hole 
       17  Plenum 
       18  Ejector port 
       19  Aero-optic cover 
       20  Cover plate 
       21  Fastener 
       22  Duct 
       23  Bay 
       24  Structure 
       25  Nozzle 
       26  Structure 
       27  Aero-optic cover 
       28  Opening 
       30  Width 
     DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 2, the present invention, an actuator  10 , is comprised of an inlet port  12 , a stagnation chamber  13 , a throat  15 , a resonance cavity  11 , and an ejector port  18 . Ejector port  18  and resonance cavity  11  are oriented at an oblique angle so to form a wedge  14 . Preferably, stagnation chamber  13 , resonance cavity  11 , throat  15 , and ejector port  18  are machined into a planar or nearly planar element. However, actuator  10  may be cast or molded into a variety of planar and non-planar shapes. Actuator  10  materials include but not limited to metal, plastic, and ceramic. 
     Fluid flow between inlet port  12  and ejector port  18  is restricted to stagnation chamber  13 , resonance cavity  11 , throat  15 , and ejector port  18  via one or more planar or nearly-planar cover plates  20 . For example, elements comprising the actuator  10  may be machined to a depth less than the thickness of the plate thereby providing confinement along one side, as shown in FIGS. 2 and 3. A cover plate  20  is thereafter placed over stagnation chamber  13 , throat  15 , resonance cavity  11 , wedge  14 , and ejector port  18  and secured via mechanical fasteners, adhesive, or other techniques understood in the art. Alternately, an actuator  10  may require two cover plates  20  when the width  30  of stagnation chamber  13 , throat  15 , resonance cavity  11 , and ejector port  18  are equal to the plate thickness. A seal comprised of a gasket, calk, or adhesive, examples including RTV silicon and epoxy, may be applied between actuator  10  and cover plate  20  to prevent fluid leaks. 
     Fluid flow into the stagnation chamber  13  is provided by an inlet port  12  or plenum  17 . Inlet port  12  may be comprised of a tube-like element attached to the actuator  10  so that fluid freely flows into the stagnation chamber  13  from an external source, as shown in FIG. 2. A gasket, calk, or adhesive is placed along the contact surface between inlet port  12  and actuator  10  so to prevent fluid leaks. Alternately, fluid may be communicated from a single source through a plenum  17  comprised of a cavity within the stagnation chamber  13  of each actuator  10 , as shown in FIG.  4 . While various shapes are possible for the stagnation chamber  13 , the stagnation chamber  13  should be sufficiently voluminous so to facilitate required mass flow rate and pressure within the actuator  10 . 
     The throat  15  communicates fluid within the stagnation chamber  13  to resonance cavity  11 , wedge  14 , and ejector port  18 . While various throat  15  geometries are possible, a convergent-divergent arrangement to so first compress then expand fluid as it traverses the throat  15  was preferred. The throat  15  should be aligned with wedge  14  so that a portion of the fluid flows into the ejector port  18  and the remainder into the resonance cavity  11 . The partitioning of fluid flow between ejector port  18  and resonance cavity  11  is application dependent. 
     Fluid flow between throat  15  and wedge  14  should be of sufficient velocity, preferably turbulent, so to allow for the side-to-side oscillation of fluid about the wedge  14  thereby producing edge tones. The frequency of edge tones is tailored to optimize interaction with the pulsating operation of the resonance cavity  11 . Edge tone frequency (f e ) is approximated by the equation 
     
       
           f   e =( nv )/ x   
       
     
     where v is the velocity of fluid approaching the wedge  14 , x is the distance between throat  15  and wedge  14 , and n is a constant in the range 0&lt;n≦1. In preferred embodiments, resonance and edge tones are produced having nearly identical frequency characteristics. 
     Again referring to FIG. 2, the resonance cavity  11  is an elongated cavity with an opening  28  disposed between wedge  14  and throat  15  oriented in the direction of the ejector port  18 . While various cavity profiles are possible along the resonance cavity  11 , preferred embodiments had a constant height, horizontally disposed, and obliquely oriented with respect to the ejector port  18 . 
     The ejector port  18  controls the ejection of excited fluid from the actuator  10 . Ejector port  18  may be comprised of a constant height cavity or tapered so to allow expansion of the excited fluid as it traverses the length of the ejector port  18 , the latter shown in FIG.  2 . 
     The actuator  10  receives a fluid, typically a gas including but not limited to air and helium, thereafter excited producing a pulsed jet at a desired frequency, pressure, and velocity. For turbulence control applications, the gas may be preheated above the ambient temperature prior to excitation within the actuator  10 . Preferred gases have a sound speed greater than air. The dimensions of stagnation chamber  13 , resonance cavity  11 , throat  15 , and ejector port  18  are application and performance dependent. A device having a stagnation chamber  13  pressure of 50 psi and dimensions listed in TABLE 1 excited a fluid consisting of ambient temperature air to a velocity signal with a dominant frequency at 10 kHz and a pressure signal with dominant frequencies at 10 kHz and 25 kHz. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Component 
                 Dimension 
                 Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Input Port 
                 Inner Diameter (mm) 
                 5 
               
               
                   
                 Stagnation Chamber 
                 Volume (mm 3 ) 
                 200 
               
               
                   
                 Throat 
                 Height (mm) 
                 1.6 
               
               
                   
                 Resonance Cavity 
                 Opening (mm) 
                 1.7 
               
               
                   
                   
                 Length (mm) 
                 10 
               
               
                   
                   
                 Height (mm) 
                 1.6 
               
               
                   
                 Ejector Port 
                 Length (mm) 
                 15 
               
               
                   
                   
                 Entrance Height (mm) 
                 1.8 
               
               
                   
                   
                 Exit Height (mm) 
                 4.8 
               
               
                   
                   
               
             
          
         
       
     
     While single actuator  10  embodiments are described above, multi-actuator assemblies  9  are also possible. Referring now to FIG. 4, several actuators  10  similar to the design shown in FIG. 3 are arranged in a linear embodiment. Actuators  10   a - 10   h  are mechanically fastened via fasteners  22   a - 22   d , examples including but not limited to bolts and rivets, through mutually aligned holes  16   a - 16   d . A cover plate  20  is attached to the last actuator  10   h  so to confine fluid flow as described above. Contact between actuators  10  confines fluid flow within the device. A seal comprised of a gasket, calk, or adhesive, examples including RTV silicon and epoxy, may be required to prevent leakage between actuators  10 , as well as between actuator  10  and cover plate  20  terminating the linear embodiment. 
     In a typical multi-actuator assembly  9 , a duct  22  provides a single source of fluid flow that is communicated to and between the actuators  10 . FIG. 5 shows a multi-actuator assembly  9  having a single duct  22  directing fluid into a plurality of actuators  10  and thereafter expelling the excited fluid from ejector tubes  18 . The duct  22  may be mechanically fastened to the multi-actuator assembly  9  via techniques understood in the art and thereafter sealed to prevent leakage, as described above. 
     The multi-actuator assembly  9  shown in FIGS. 4 and 5 are applicable to suppressing turbulence within either an open or a covered bay. Referring now to FIG. 6, an actuator  10  is shown ejecting an excited fluid into a flow field above a cavity  23  without cover. Alternately, FIG. 7 shows an actuator  10  ejecting an excited fluid into a flow field above a cavity  23  having an aero-optic cover  19 . In both embodiments, the multi-actuator assembly  9  is fixed via mechanical means understood within the art to the structure  24  adjacent to the cavity  23  so to cause interaction between excited fluid and flow field passing over the structure  24 . An exemplary structure  24  is an aircraft fuselage. Preferably, actuators  10  are mounted so as to not physically interfere with the flow field, however positioned so to direct excited fluid directly into the flow field. 
     Referring now to FIG. 8, a plurality of actuators  10  are arranged about the exterior surface of a nozzle  25  so to inject excited fluid into the flow field entering or exiting the nozzle  25 . Actuators  10  may be separately disposed as shown in FIG. 8 or arranged in a continuous pattern so that each actuator  10  contacts two other actuators  10 . Actuators  10  are fixed via techniques understood in the art and project a plurality of excited fluid streams into the flow field exiting the nozzle  25 , as represented in FIG.  9 . Fluid flow into the actuator  10  may be provided via an inlet port  12  communicating independently with each stagnation chamber  13 , as described in FIG. 2, or a single fluid feed embodiment wherein fluid flow is directed through a plenum  17 , as described in FIG.  3 . In the latter embodiment, actuators  10  are mechanically attached and sealed as described above. Preferably, actuators  10  are mounted so as to not physically interfere with the flow field, however positioned so to direct excited fluid directly into the flow field. 
     Referring now to FIG. 10, a plurality of actuators  10  are arranged in a circularly disposed fashion about an aero-optic cover  27  so that ejector ports  18  project excited fluid onto the aero-optic cover  27 . A typical aero-optic cover  27  is comprised of an optically transmissible material as is understood in the art. Actuators  10  may be separately disposed as shown in FIG. 10 or arranged in a continuous pattern so that each actuator  10  contacts two other actuators  10 . Actuators  10  are fixed to the structure  26  via techniques understood in the art and project excited fluid into the flow field so to alter its characteristics. Fluid flow into the actuator  10  may be provided via an inlet port  12  communicating independently with each stagnation chamber  13 , as described in FIG. 2, or a single fluid feed embodiment wherein fluid flow is directed through a plenum  17 , as described in FIG.  3 . In the latter embodiment, actuators  10  are mechanically attached and sealed as described above. Preferably, actuators  10  are mounted so as to not physically interfere with the flow field, however positioned so to direct excited fluid directly into the flow field. 
     The description above indicates that a great degree of flexibility is offered in terms of the present invention. Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.