绕水翼空化非定常动力特性的时频分析

为了解绕水翼附着空化的流体动力特性，用实验方法研究了绕水翼的空化流动现象。实验在空化水洞中完成，采用高速摄像技术观测了不同空化阶段的空穴形态，并测量了水翼所受的升阻力。利用时频分析中的Choi-Williams变换和小波变换对水翼升力信号进行了分析处理，得到了水翼升力信号在不同空化阶段的时频特征。结果表明：在不同的空化阶段，对应于各种不同的空化形态，其流体动力的频谱特性也表现了不同的特点。在没有空化产生时，水翼升力的时频无规则，升力随时间的变化不存在周期性；在片状空化阶段，升力信号变化的频率随着空化数的减小而减小；在云状空化阶段，空穴波动增强，升力信号的周期性更加显著，而此时频率基本不随时间和空化数变化。

Unsteady dynamics of cavitating flows around a hydrofoil

The dynamics of cavitating flow around a hydrofoil were investigated experimentally. The experiments were carried out in a rectangular test section of a cavitation tunnel. A high-speed video camera was used to visualize the unsteady cavitating flows. The lift and drag forces around the hydrofoil have been measured. The time-frequency characteristics ofs lift signals at different cavitating regimes were obtained by using Choi- Williams transform and wavelet analysis technique. The results show that there is no remarkable frequency in time-frequency space for the forces when the cavitation is not generated. For sheet cavitation, the frequency of the lift oscillation decreases with cavitation number decreasing. At cloud cavitation stage, the frequency is relatively insensitive to changes in cavitation number. The joint time-frequency analysis can describe the characteristic of signals changing with time and frequency, which has been applied successfully to different aspects of cavitation.