震荡小球在不可压缩流体介质中产生扰动场的理论分析

A THEORETICAL ANALYSIS OF THE FLOW DISTURBANCE INDUCED BY A SPHERE OSCILLATING IN INCOMPRESSIBLE FLUID

为了改进基于不可压缩流场的声类比法的气动声数值预测方法,首先要明确扰动在可压缩和不可压缩流体媒介中的传播特性.推导了震荡小球在不可压缩流体中产生的小扰动的理论解,分析其速度场与压力场的特点,并与可压缩情况的解进行比较.结果显示,速度场中包含传播速度为无穷大和有限值的分量:而压力场只有传播速度为无穷大的分量.当流体黏性趋于零或小球震荡频率趋于无穷大时,其流场与经典声学中震荡小球声辐射问题的近场声一致,这表明震荡小球产生的近场扰动为不可压缩流场,即伪声.

In order to improve the prediction method of the aerodynamic noise using the acoustic analogy theory based on incompressible flow fields, the characteristics of the disturbance propagation in the incompressible and compressible fluids should be studied firstly. In this paper, the analytical solution for the flow disturbance generated by a sphere oscillating in an incompressible fluid is obtained. The velocity and pressure fields are analyzed, and are compared to the results in a compressible fluid, namely, the acoustic field. It is shown that with respect to the velocity fields the disturbance has components with infinite and finite propagating speeds, while with respect to the pressure only disturbance with infinite propagating speed exists. When the fluid viscosity vanishes or the oscillating frequency becomes infinite, the velocity is identical to the near-field sound generated by the sphere oscillating in the compressible fluid; thus the acoustic near-field disturbance is incompressible, i.e., the pseudo-sound.