Studies On Aeroacoustic Characteristics And Noise Suppressions For Internal Weapon Bays

It is a development trend for internal weapon bays to be selected for advanced aircrafts. However, supersonic-speed flow field over the bay features complex flow phenomena, such as flow separation, shear-layer instabilities, vortex generation and shedding, and expansion waves/shock waves, shock waves/shock waves, shock waves/boundary-layer interactions. Pressure fluctuation, flow self-oscillation and intense noise induced by the flow phenomena can damage aircraft components'and internal weapons'structures. Severe noise environment can represent a potential hazard to the sensitivities of apparatuses in the bay. In extreme cases, noise can also disturb the safety separation of stored weapons, which could even lead to uncontrolled weapon movement and air crash. Therefore, it is very significant to investigate sound generations induced by self-sustained oscillations, internal influencing correlation of flow field and aerodynamic noise, influencing factor for aeroacoustic characteristics, and some feasible methods for noise suppression in the bay to be of practical reference value for the optimized design of aircraft weapons bay as well as the solution of related acoustic problems.The results indicate that it is a key correlation for aeroacoustic characteristics influenced by flow field in the bay. The noise is induced by the shear-layer instabilities, vortex generation and shedding induced by flow separation, interactions in the shear-layer and flow, and impingement of the shear-layer and rear-face in the bay. Expansion and shock waves appear at supersonic speed, which leads to intenser noise in the bay than at subsonic speed. For open weapons bay, the shear-layer has little effect on flow inside the bay, and a cycle of sound wave feedback mechanism could occur within the weapons bay. Self-sustained oscillation and some sound pressure peak at weapons bay tone come into being. However, the shear-layer impinges the floors of transitional and closed weapons bay. The impingement results in one or two shock waves in the bay. The shock waves partition the weapons bay to two or three flow regions. The intense aerodynamic noise can not radiate from the rear region to the front region of the bay. Therefore, there are few oscillation frequencies at peak sound pressure and sound feedback mechanism inside transitional and closed weapons bay.Increase in free-stream Mach number (M) leads to an intense impingement of the shear-layer and flow inside the bays or bay rear-face, and expansion or shock waves occur at supersonic speed, which results in less uniform distribution of sound pressure level (SPL) fore and aft of the weapons bay, increase in SPL at same discrete frequencies and intensified noise. Decrease in free-stream boundary layer thickness-to-cavity depth ratios (δ/D) results in intensified interactions between shear-layer and flow inside weapons bay and severer pressure fluctuation. Decrease in cavity length-to-cavity depth (L/D) leads to a transition in flow pattern from closed to transitional or open flow, increase in SPL at most measurement positions, increased number of self-sustained oscillation frequencies, and intensified noise. Decrease in cavity width-to-cavity depth (W/D) leads to slight increase in SPL at most measurement positions without change in flow pattern and oscillation frequencies at peak sound pressure.The prediction methods for flow-sustained oscillation modes and oscillation frequencies inside open weapons bay are key technique to study some noise suppression methods. An analysis of flow-sustained oscillation modes inside open weapons bay and selected method for constant value in the Rossiter's and Heller's semi-experienced equation are validated to be reasonable and feasible by wind-tunnel experimental results.Increase in cavity side-wall angle (?s) has little influence on flow pattern aeroacoustic characteristics, but it is advantageous for noise suppression. Increase in cavity rear-wall angle (?r) leads to decrease in SPL at most measurement positions (more apparent in the region near the rear end of the bay). The effect of some passive control methods on noise suppression inside the weapons bay was investigated. Research results indicate that the methods are feasible under certain conditions. Effect of geometrical modificated arc of the rear-face on noise suppression is similar with the increase in rear-face angle. Perpendicularity sawtooth and aclinic sawtooth installed on the front-face or rear-face of the weapons bay have some impact on free-stream boundary-layer characteristics, which leads to more exquisite pressure fluctuation and intensified noise. Some pipes installed on the bay floor can effectively suppress aerodynamic noise, and it is advantage for noise suppression that the pipes are bended and plugged up, but disadvantage when the pipes are shorten.Jet at the fore edge of the weapons bay has good effect on noise suppression. The low-frequency piston Zero-Net-Mass-Flux Jet is effective for noise suppression at subsonic speeds, but it yielded poor results at supersonic speeds. The control methods that the Zero-Net-Mass-Flux Jet exit direction is vertical with weapons bay front-face or free-stream flow direction have some effect on noise suppression, however, bad effect when the Zero-Net-Mass-Flux Jet exit direction is vertical with weapons bay rear-face. The control method that the Zero-Net-Mass-Flux Jet exit shape is narrow slot has more influence on noise suppression than cycle hole method. The change of vibration amplitude and vibration frequencies of low-frequency piston Zero-Net-Mass-Flux Jet has little effect on noise suppression. Increase in M and L/D results in bad effect of the Zero-Net-Mass-Flux Jet on noise suppression. High-frequency sound actuator Zero-Net-Mass-Flux Jet has suppression impact on aerodynamic noise at subsonic speeds, but no at supersonic speeds. When its jet frequency is close to oscillation frequencies at first or second flow-sustained oscillation mode, under some conditions, the High-frequency sound actuator Zero-Net-Mass-Flux Jet has obvious effect on noise suppression in the weapons bay. For some L/D, the change in he High-frequency sound actuator Zero-Net-Mass-Flux Jet frequency has little effect on SPL distribution, but obvious effect on SPFS characteristics.