Weapons Bay Research Articles

Aircraft weapons bay acoustic resonance suppression system

A aircraft weapons bay acoustic resonance suppression system for reducing undesirable acoustic resonances within an open weapons bay of an aircraft in flight is described. The system includes a rod mounted upon and spaced a distance from an extensible aircraft spoiler. The rod has a hollow core and includes a multiplicity of apertures axially aligned into two groups, placed at the point of fluid separation. A multiplicity of synthetic jets are received within the rod, each jet in fluid communication with a corresponding aperture. The jets combine to form first and second perturbation elements. The perturbation elements are sinusoidally driven out of phase at high frequencies by an actuator. This effectively creates a high frequency forcing of the airstream which is amplified by the airflow, creating a violent shedding. Advantageously, this violent shedding dramatically reduces acoustic resonance within the weapons bay of the aircraft.

High and Low Frequency Actuation Comparison for a Weapons Bay Cavity

This paper compares the effectiveness of zero- low- and high-frequency flow control methodologies when applied to a generic weapons bay cavity. Using the High Frequency Excitation Active Flow Control for Supersonic Weapons Release (HIFEX) generic weapons bay model the actuators were tested at Mach 0.85 and 1.19. The zero-frequency actuator tested was a conventional one-delta “sawtooth” spoiler, while the low-frequency device was a pulse-blowing actuator. The sawtooth spoiler performed as expected by reducing the Overall Spectrum Level (OASL) slightly in both the subsonic and supersonic cases. The pulsed-blowing actuator reduced the OASL as much as 9.6dB for both the subsonic and supersonic cases. The high-frequency devices tested were the Powered Resonance Tube (PRT) and a Splash Jet actuator. The PRT and Splash Jet reduced the (OASL) equally well at Mach 0.85 for all the mass flow rates tested. The PRT outperformed all other actuators at Mach 1.19 except for the highest mass flow rate of 0.227 kg/s, where the Splash Jet performed equally as well the PRT.

Low Speed Wind Tunnel Buffet Testing on the F/A-22

Buffet results from the F/A-22 V-9 model test conducted in the Lockheed Martin Aeronautics Marietta Low Speed Wind Tunnel (LSWT) between 515-02 and 5-18-02 are presented. The purpose of the test was to determine the effect on stability and control parameters of CFD derived potential aerodynamic solutions for attenuating vertical tail buffet. Instrumentation was added to permit assessment of the effects of these potential configurations on buffet loads. The data from the test show buffet attenuation benefit from several of the configurations tested, with two fence configurations showing significant reductions in the buffet response up to approximately 32 degrees angle-of attack. CFD analysis predicted significant effects of Mach number, engine inlet mass flow, and scaling on buffet onset. Therefore, any consideration of incorporating such configurations as determined by wind tunnel test should be verified by model testing with these effects present and by full-scale evaluation. Significant buffet attenuation is also shown with side weapons bay doors open. (Note: The configuration showing most promise for buffet attenuation resulted in a loss in lateral stability at 32 degrees)

Comparison of Hybrid Turbulence Models for a Circular Cylinder and a Cavity

Three hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation turbulence models are applied to the flow over a three-dimensional circular cylinder and a generic weapons bay. Time-step and grid sensitivity studies are performed. The time-step study indicates that 100-200 steps per primary shedding cycle are required for temporal accuracy using a second-order implicit algorithm. The grid study indicates that these new turbulence models can achieve a reasonably grid-independent solution in simulation of unsteady turbulent flows. As the mesh resolution is increased, smaller scale turbulent structures become visible. A computational mesh with a grid resolution resulting in a ratio of the turbulent length scale to grid length scale greater than 2 produced a reasonable simulation with all three hybrid models for both cases.

Prediction of acoustic resonance phenomena for weapon bays using detached eddy simulation

AbstractIt is argued that acoustic resonance phenomena in open cavities such as weapons bays cannot be adequately predicted through numerical solution of Reynolds averaged Navier-Stokes (RANS) equations. The requirement to resolve the growth of the shear layer instability from the lip of the cavity inevitably implies that turbulence further downstream is resolved while also being modelled thus making RANS over dissipative. Large eddy simulation (LES) models only unresolved scales and a hybrid method combining RANS near walls with LES in the cavity appears a practical alternative to pure RANS. This paper compares computations of the M219 cavity configuration made with unsteady RANS and with the hybrid method known as detached eddy simulation (DES). It is shown that whilst unsteady RANS and DES give very similar predictions for the 1st and 3rd modes of the acoustic resonance the 2nd mode (which is dominant near the centre of the cavity) is absent in the RANS results but well predicted by DES. The 2nd mode is thought to arise from an interaction with vortical structures in the shear layer which are suppressed in the highly dissipative RANS method. The 4th mode, which is much weaker than the other three modes, is over-predicted by DES and under-predicted by a smaller amount in RANS.

Logic-Based Active Control of Subsonic Cavity Flow Resonance

We present the results of an experimental investigation for controlling a shallow cavity flow in the Mach number range 0.25‐0.5. The flow exhibits the characteristic staging behavior predicted by the semi-empirical Rossiter formula with multiple modes in the Mach number range 0.32‐0.38 and a single strong mode in other Mach numbers. A survey of the velocity at the exit of the zero net mass flow compression-driver actuator used for control reveals that its amplitude is frequency dependent and its behavior is little influenced by the main flow. Forcing the Mach 0.3 flow indicated that the actuator has good authority over a large range of frequencies, with reduction of spectral peaks observed at some forcing frequencies. We took advantage of this phenomenon to develop a logicbased controller that searches the frequency space and maintains the optimal forcing frequency for peak reduction at each Mach number. Optimal frequencies and the corresponding reduced resonance have been obtained for all of the flow conditions explored. The physics of optimal frequency forcing as well as some characteristics of the logic-based controller are discussed. I. Introduction A PPLICATION of closed-loop control in fluid dynamics is by its nature a challenging and fascinating problem. Although many significant results have been obtained with open-loop flow control, this technique lacks the responsiveness or the flexibility needed for application in dynamic flight environments. In contrast, closed-loop flow control, although in its infancy, appears to be the ideal technique for the successful management of flow in many applications due to its adaptability to variable conditions and to its potential for significantly reducing the power required for controlling the flow. Fo re xample, Cattafesta et al. 1 found that closed-loop control of cavity tones requires an order-of-magnitude less power than open-loop control. The results presented here are part of a larger multidisciplinary effort to develop tools and methodologies that can apply closed-loop aerodynamic flow control for manipulating the flow over maneuvering air vehicles. The first step was to select a particular flowfield relevant to Air Force applications and to utilize it in the development of various components of closed-loop flow control techniques. The case study chosen is the control of shallow cavity flow pressure fluctuations that are characterized by a strong resonance produced by a natural feedback mechanism similar to that occurring in other flows with self-sustained oscillations (e.g., impinging jet, screeching jet). In all of these cases, shear layer structures impacting a discontinuity or obstacle in the flow (e.g., the cavity trailing edge) scatter acoustic waves that propagate upstream and reach the shear layer receptivity region where they tune and enhance the development and growth of shear layer structures. In the case of flow over a weapon bay cavity, these fluctuations can lead to structural damage to the air vehicle or to the stores carried within the bay. Rossiter 2 first developed an empirical formula for predicting the cavity flow resonance frequencies, today referred to as Rossiter frequencies or modes. He also investigated the concept of a dominant mode of oscillation. Later Rockwell and Naudascher 3 ob

Suppression of Pressure Loads in Cavity Flows

The need to suppress dynamic-pressure loads in open cavities represents an important problem in many aeronautical applications. Many studies have been conducted using passive and active control techniques at the leading and trailing edges of cavities that have shown some success at reducing the dynamic-pressure levels in simulated weapons bays. In this work a leading-edge fence along with a cylindrical rod, suspended in the approaching boundary layer parallel to the leading edge of the cavity, was examined. The overall pressure levels along with the spectral distribution of the surface pressure in the cavity have been shown to he altered in a favorable manner by both of these devices. Suppressing the dynamic-pressure levels in the cavity was also found to alter the correlation between sensors along the floor cavity. Although it was found that both leading-edge devices lift the shear layer away from the cavity, the manner in which it is lifted appears to play an important role in the level of surface-pressure suppression.

Control of a Plane Jet by Fluidic Wall Pulsing

15Cole, D., Glauser, M., and Guezennec, Y., “An Application of the Stochastic Estimation to the Jet Mixing Layer,” Physics of Fluids, Vol. 4, No. 1, 1991, pp. 192–194. 16Picard, C., and Delville, J., “Pressure Velocity Coupling in a Subsonic Round Jet,” Engineering Turbulence Modeling and Experiments—4, edited by W. Rodi and D. Laurence, Elsevier, Amsterdam, 1999, pp. 443–452. 17Guezennec, Y., “Stochastic Estimation of Coherent Structures in Turbulent Boundary Layers,” Physics of Fluids A, Vol. 1, No. 6, 1989, pp. 1054–1060. 18Murray, N., and Ukeiley, L. “Estimating the Shear Layer Velocity Field Above an Open Cavity from Surface Pressure Measurements,” AIAA Paper 2002-2866, June 2002. 19Sinha,N., Hosangadi,A., and Dash, S., “TheCRAFTNS Codeand Preliminary Applications to Steady/Unsteady Reaction, Multi-Phase Jet/Plume FlowŽ eld Problems,” Chemical Propulsion Information Agency, Publ. 568, Columbia, MD, May 1991. 20Sinha, N., Arunajatesan, S., and Ukeiley, L., “High Fidelity Simulation of Weapons Bay Aeroacoustics and Active Flow Control,” AIAA Paper 2000-1968, June 2000. W. J. Devenport Associate Editor

Recent aeroacoustics research in the United States

It is expected that with the projected increase in air travel aeroacoustics research will be very critical in meeting increasingly stringent aircraft noise certification standards. In the United States aeroacoustics research has steadily progressed toward enhanced safety, noise benefits and lower costs. This report provides a brief summary of selected recent aeroacoustics activity in the US. Four topics of great interest to the aerospace industry are: (1) Advanced Subsonic Technology (AST) for future subsonic aircraft, (2) High Speed Research (HSR) for future supersonic commercial aircraft, (3) Rotorcraft noise control efforts, and (4) Weapons bay and other noise control applications for the military. Examples of good progress in the areas of jet, fan, airframe and helicopter noise as well as liner design and weapons bay noise suppression are provided herein. In the next few years we look forward to seeing major strides in noise reduction technology and our ability to predict situations of aeroacoustics i...

Aircraft weapons bay high frequency acoustic suppression appartus

An aircraft weapons bay high frequency acoustic suppression apparatus is disclosed. The apparatus includes an extendable spoiler retractably received within an aircraft weapons bay. An injector unit is received within the spoiler for injecting high frequency pulses of pressurized gas into the airstream. The injector unit includes a resonance tube in outlet fluid communication with an nozzle. The pulsating output of the resonance tube perturbs the flow of pressurized gas in the nozzle, effectively breaking it up into discrete slugs or pulses which then exit the nozzle and enter the airstream. The high frequency perturbation of the airflow across the weapons bay, created by the aircraft weapons bay high frequency acoustic suppression apparatus of the present invention effectively suppresses undesirable acoustic resonance within the open weapons bay.

Aircraft weapons bay acoustic suppression apparatus

An aircraft weapons bay acoustic suppression apparatus is disclosed. The apparatus reduces or eliminates acoustic resonance within an open cavity of an aircraft such as a weapons bay of an aircraft in flight. The apparatus includes an oscillatable spoiler plate which is extended into the airstream. At least one shaker is placed in operative engagement with the spoiler plate in order to oscillate the spoiler plate at high frequencies and high amplitudes to seed the shear layer with frequencies which directly complete with the natural frequency of the shear layer vortices. In this way, the damage arising from natural shear layer vortices can reduced or canceled within the weapons bay.

Recent Aeroacoustics Research in the United States

It is expected that with the projected increase in air travel aeroacoustics research will be very critical in meeting increasingly stringent aircraft noise certification standards. In the United States aeroacoustics research has steadily progressed toward enhanced safety, noise benefits and lower costs. This report provides a brief summary of selected recent aeroacoustics activity in the US. Four topics of great interest to the aerospace industry are: (1) Advanced Subsonic Technology (AST) for future subsonic aircraft, (2) High Speed Research (HSR) for future supersonic commercial aircraft, (3) Rotorcraft noise control efforts, and (4) Weapons bay and other noise control applications for the military. Examples of good progress in the areas of jet, fan, airframe and helicopter noise as well as liner design and weapons bay noise suppression are provided herein. In the next few years we look forward to seeing major strides in noise reduction technology and our ability to predict situations of aeroacoustics interest.

Shape memory wire improves sealing of aircraft doors

In aviation safety, providing maximum security for door seals is a crucial part of the specification. Other factors include ease of passenger entry and egress. Such doors require opening and closing only whilst on the ground. Other types of doors are required which can be opened in flight such as weapons bay doors on military aircraft not to mention doors for the undercarriage. But present generation aircraft seals have many deficiencies. To try to overcome these shortcomings Northrop Grumman has patented a new seal based on the special characteristics of a material new to the sealing industry, that of ‘shape memory alloys’.

Control of pressure oscillations in deep cavities excited by grazing flow

Grazing flow over a deep cavity can result in acoustic resonance characterized by a standing wave-vortex feedback loop. Control of cavity resonance has potential impact in stealth aircraft and aircraft weapons bay applications, where periodic stress loading can cause material fatigue failure or damage sensitive instrumentation. A previous perturbation analysis of the fundamental equations of motion for a fluid system motivates a control method based on interaction of an external source of momentum (force) with cavity velocity fluctuations. Through proper placement of stationary control screens (force addition) within the resonant cavity in laboratory experiments, the cavity rms pressure amplitude was significantly reduced without interfering with the approaching boundary layer. Reductions in cavity base spectral peaks of up to 26 dB were achieved. Both single vortex (onequarter mode shape) and double vortex (three-quarter mode shape) cases were controlled. Placement of control screens at locations of maximum velocity fluctuation yields optimum control for force addition.

Mitigation of pressure oscillations induced by supersonic flow over slender cavities

Supersonic flow over a slender cavity excited high-frequency and high-g vibrational forces inside the cavity. The excitation of the whistle is caused by vortex shedding at the upstream end of the cavity and vortex interaction with the downstream end of the cavity associated with sound generation. The sound is fed back upstream and drives the vortex-shedding process. To suppress the acoustic whistle, an experimental test program was undertaken to characterize the acoustic cavity oscillations in a deep and narrow cavity, using microphones and flow visualization, and reduce the coherence of the vortices by geometric changes of the upstream end of the cavity. The changes that included multisteps and pins extending into the supersonic approach flow were selected to break up the orderly development of coherent vortices under acoustic excitation. With these passive shearflow control devices, acoustic amplitude reductions by a maximum factor of 5 were obtained, essentially eliminating acoustic pressure oscillation in the cavity over the entire range of Mach numbers tested. In this article, the detailed geometric and flow parameter variation and its effect on the cavity pressure oscillations are described. I. Introduction A COUSTIC oscillations inside cavities, induced by flow over the cavity, were studied extensively in the last two decades.16 This phenomenon is important in subsonic and supersonic flight of airframes with cavities such as wheel wells and weapons bays, due to the excitation of structural vibrations. The oscillations are usually characterized by broadband and pure-tone components. The broadband components are primarily associated with shallow cavities, whereas deep cavities usually excite pure periodic oscillations similar to those excited by the edge-tone phenomenon. Earlier investigations1 were aimed to quantify the sound emissions caused by cavity resonance. Later investigations attempted to develop analytical predictions for the phenomenon.2'3 Plumbee et al. 2 developed a mathematical model that was able to predict the resonant modes by relating the cavity oscillation to resonance induced by turbulent noise from the shear-layer flowing over the open end. This model was particularly successful in predicting the noise produced by deep cavities. East3 showed that some of the acoustic modes can be excited even at low subsonic Mach numbers. Over the years, different investigations proposed various theories for the excitation of the cavity pressure oscillations. Basically, most works relate these oscillations to interaction between the separated shear layer that spans across the open end of the cavities and the acoustic response of the cavity. Rossiter5 studied the problem experimentally in the Mach number range of 0.4-1.2 and cavity length to depth (L/D) ratios of 1-10 and concluded that an acoustic feedback is driving the oscillations. Rossiter proposed that the vortices in the shear layer that is separated from the upstream lip of the cavity, interact with the downstream lip generating acoustic pulses. These pulses propagate upstream, inside the cavity, exciting the shear layer at the upstream lip and thus enhancing the shedding of new vortices in the shear-layer. Using these arguments he devised an empirical equation for the tone frequencies /

F-111 generic weapons bay acoustic environment

A wind tunnel test was conducted on a generic weapons bay model to define the acoustic environment in the bay and how it is affected by various missiles, suppressors, and flow conditions. Two different depth bays were tested. Three different missile designs were installed in the bay with the capability of putting one, two, or three in the bay at a time. One of the missiles was tested at three vertical positions to measure the effect it has while being deployed. Two leading-edge sawtooth suppressors were evaluated along with a slanted rear bulkhead. Overall levels as high as 163 dB were measured in the bay. Narrowband tones were strongly excited. The sawtooth spoilers were partially effective in suppressing the tones. In some cases the levels were amplified. The slanted aft bulkhead was very effective in suppressing the levels. The missiles generally reduced the acoustic levels in the bay, with the most reduction when a missile was partially out of the bay interacting with the shear layer. The measured levels were of high enough intensity to result in severe fatigue problems for sensitive missiles and components.

Analyses of Pressure Oscillations in an Open Cavity

Open cavities an aircraft exposed to high-speed flow, such as weapon bays, can give rise to intense selfinduced pressure oscillations. The amplitude of these oscillations, under certain flight conditions, can cause structural damage. Substantial experimental and analytical efforts have investigated these pressure fluctuations, resulting in some understanding of the complex interaction of the external shear layer and cavity acoustical disturbances. However, no numerical computations have been obtained for the complete governing fluid mechanical equations. The purpose of this study is to obtain numerical solutions of the Navier-Stokes equations for an open cavity in order to provide a new tool for the analysis of this phenomenon. '