低速二维后向台阶流动中的壁面压强脉动

为了能排除三维结构对涡脱和剪切层旋涡的影响，使用实验方法研究了一个小展高比（AR=0.125）的后向台阶流动.该实验台类似纯二维的Hele-Shaw Cell.流动被局限在两个平行且距离为5 mm的有机玻璃板之间.台阶高度H为40 mm，扩张比2:3.在台阶下游中央沿流动方向安装16个麦克风组成的传感器阵列采集壁面脉动压强.来流速度U₀在9~26 m/s之间连续可调.通过计算脉动压强分布、频谱，不同位置的相关性和相干性系数，发现并分析流动存在一个临界Reynolds数.流场在临界Reynolds数前后存在明显不同的流动特征.实验结果表明在低Reynolds数下依然存在剪切层的低频摆动；当Reynolds数大于临界Reynolds数时，分离后流动由涡结构传播的特性主导. 

Wall-Pressure Fluctuations for a Low-Speed Two-Dimensional Backward-Facing Step Flow

To rule out span-wise flow structures, a rig with a small aspect ratio (AR=0.125) was built. The rig was similar to the pure two-dimensional Hele-Shaw Cell. The flow was confined between two parallel plexiglas plates with a distance of 5 mm. The step height was 40 mm, and the expansion ratio was 2:3. A sensor array composed of 16 microphones was installed along the stream-wise direction in the downstream center to collect the wall pressure fluctuations. The microphones were placed between 20 mm and 320 mm from the step, and the distance between adjacent microphones was 20 mm. The income flow velocity was adjustable and coherent from 9 to 26 m/s. The distribution of pressure fluctuation, the spectrum and the correlation and coherence coefficients at different locations were obtained. The results suggest that the flapping phenomenon still exists when three dimensional flow structures are confined. In addition, the transition point of free stream Reynolds number (Re=U₀H/ν) was found, and two different characteristics were distinguished.