边界层厚度对腔体气动声学特性影响数值模拟

为了研究来流边界层厚度对开式腔体气动声学特性的影响,基于分离涡模拟方法,计算了来流马赫数为2.0条件下,不同来流边界层厚度与腔体深度比时,长深比为5.88的腔体流动特性,得到了该腔体声压级的频谱特性.计算结果表明:随着来流边界层厚度增加,形成的剪切层稳定性增强,失稳后上下摆动幅度减少,失稳生成的大尺度涡与超声速主流的相互作用减弱,使得大尺度涡发展到腔体后缘时所具有的平动动能和转动动能降低.大尺度涡撞击腔体后缘在腔体内形成的气动噪声的声压级降低,最大减小幅度达7.5dB.同时各阶模态的频率也发生偏移,偏移值在100Hz左右.基于新的假设重新推导了Rossiter公式,明确了经验常数的物理意义,并以此解释了频率偏移现象. 

Numerical simulation on influence of boundary-layer thickness on the cavity aero-acoustic characteristics

To analyze the aero-acoustic characteristics in open cavity influenced by inflow boundary-layer thickness, the cavity with length-to-depth ratio of 5.88 was simulated under conditions of different ratios of inflow boundary-layer thickness to cavity depth and inflow Mach number of 2.0, based on detached eddy simulation method; and the features of sound pressure level spectrum were obtained. The results show that increase of inflow boundary-layer thickness leads to enhanced stability in shear layer and decreased the oscillation amplitude stemmed from shear layer instability. This weakens the interaction between large scale vortex and supersonic main flow, causing the decrease in translational kinetic energy and rotation kinetic energy of large scale vortex. Responding to the energy decrease of large scale vortex, the maximum reduction of the sound pressure level inside the cavity is 7.5dB. Meanwhile, the tone frequencies have about 100Hz shift. A new formula with clarified physical meanings of empirical constant has been deduced from the Rossiter formula based on a new hypothesis. And the frequencies shift phenomenon can be illustrated by the new formula.