Effects of boundary-layer thickness on aero-acoustics characteristics of subsonic shallow cavity flow

High-speed flows in cavities such as grooves, wheel wells, and weapon bays, are often encountered in aerospace and aeronautical vehicles. In most cases the flow field is complex and unsteady, featured by shear-layer instability, flow separation and formation of vortex structures, shock wave/boundary-layer interactions at transonic and supersonic speeds. Complex unsteady flow characteristics occur therein, such as fluctuating pressure and velocity. Some aerodynamic noise inside the cavity can reach 170dB, which can damage some installed apparatus inside the cavity and structural components of the cavity. Free-stream boundary layer has significant influence on aero-acoustic characteristics inside cavities. This paper presents that sound pressure level (SPL) distributions and sound pressure frequency spectrum (SPFS) characteristics at different measurement positions on cavity floor centerline utilizing two terms of experimental results under experimental conditions. Effects of different free-stream boundary-layer thickness to cavity depth ratios on shallow cavities (transitional and closed cavity flow) were discussed. The data presented herein was obtained about the cavities with length to depth ratios of 12 and 15 over a Mach number of 0.6 at a Reynolds number of 1.23×10⁷ per meter. The results show that decrease in the boundary-layer thickness to the cavity depth ratios (δ/D) causes SPL amplification and noise enhancement at different measurement positions inside the cavities, especially the front and middle. Moreover, decrease in δ/D has a significant influence on SPFS characteristics and leads to increase in SPL in the frequency range of 0Hz<f<1000Hz at subsonic speeds (Ma=0.6). According to that above, decrease in the free-steam boundary layer has important effect on the aerodynamic noise, and it cause SPL amplification and noise enhancement, which has some bad effect on structural security of something in the cavities.