Experimental Study Of Flow-induced Oscillation And Noise Suppression In Rectangular Cavity

As one of the main noise sources of aircraft,weapon bay and wheel well will produce strong discrete and wide-band noise in the flow,and even cause structural vibrations.These structures can be simplified as rectangular cavity in the realm of scientific research.The effect of confinement on self-sustained oscillations and noise radiation in three-dimensional open cavities is studied experimentally,and the influence of the thickness of the incoming boundary layer on the oscillation in cavity is investigated as well.The main content and conclusions are as follows:(1)Far-field acoustic,surface pressure and high-speed PIV measurements are combined to investigate the confinement effects due to side walls on self-sustained oscillations and tonal noise generation in 3D cavities.The results show that:①When the wind speed is less than 70 m/s,it is found that there is no self-sustained oscillation corresponding to the Rossiter’s mode.There are multiple peak frequencies,which do not conform to the Strouhal number similarity criterion.However,if the velocity is greater than equal to 70 m/s,the oscillation frequency corresponding to the third Rossiter’s mode appears,and then a robust self-sustained feedback process is built and strong noise radiation is produced.②With decreasing W/L,the self-sustain oscillation and peak frequency noise corresponding to the Rossiter’s mode can be greatly weakened or even eliminated.The upstream OASPLs can be reduced by more than 3 dB when W/L decreases from 0.4 to 0.3 at a wind speed of 70 m/s.③Suppression of noise radiation due to confinement is closely related with the change of flow structures.The structures of the primary circulation,as well as the induced secondary vortices,are changed by the confinement.The primary recirculation seems play a significant role in establishing a robust feedback process,although itself is not a direct noise source.(2)New experimental models are designed to control the thickness of the incoming laminar boundary layer by connecting NACA airfoil and cavity.The velocity profile of the boundary layer is measured by hot wire anemometer,and the noise characteristics of cavity with different thickness are measured by microphone array.The results show that:①The thickness of the boundary layer reduces with the increase of the incoming wind speed to the order of 1/2,and increases linearly with the increase of the height of the NACA airfoil.②Reduction of the thickness of the incoming boundary layer can strengthen the self-sustained oscillation in cavity,and the noise intensity originated from cavity is also greater.Both of decreasing the thickness of the incoming boundary layer and increasing the incoming wind speed can reduce the value k,which is the ratio of vortex convection velocity to inflow velocity.Thus the strength of the shedding vortex increases,and then the impingement strength with the rear edge of the cavity increases too.