带喷流超声速后台阶流场精细结构及其运动特性研究

采用基于纳米示踪的平面激光散射技术(NPLS)对带超声速喷流的后台阶流动精细结构进行了研究. 来流马赫数为3.4, 喷流实测马赫数为2.45, 而名义马赫数为2.5. 结果清晰地揭示了激波、剪切层、混合层、Kelvin-Helmholtz涡、羊角涡及湍流大尺度结构等大量典型流场结构. 基于大量流场精细结构图像, 对典型位置处的结构进行了空间两点相关性分析, 在喷流混合层前端涡结构小于湍流充分发展的尾端, 结构角相对也小. 喷流工作时, 模型台阶下游表面由一薄层气膜覆盖. 获得了模型流向和不同高度展向平面内的流场结构, 对照纹影试验结果, 分析了流动特点及时间演化规律. 采用微型压力扫描系统测试了模型表面的压力系数分布, 靠近喷流下游处压力系数区域0.0146. 针对NPLS图像做了流动的分形维数的分析, 发现在流动初始阶段分形维数接近于1, 越靠下游分形维数越高.

Fine structures and characteristics on supersonic flow over backward facing step with tangential injection

The fine flow structure over backward facing step with supersonic injection at the free-stream Mach number of 3.4 is investigated via nano-tracer planar laser scattering (NPLS). The Mach number of injection is measured to be 2.45 actually, even though designed to be 2.5 nominally. The shock wave, shear layer, mixing layer, Kelvin-Helmholtz vortex, horn-like vortex, coherent structures, etc, are clearly revealed. Flow images with the high spatiotemporal resolution are captured involving the streamwise and spanwise flow field in planes at different heights. Based on a large number of fine images, the spatial correlation analysis is conducted to reveal the structure scale and incline angle. The results indicate that with the flow developing, the structure angle tends to be larger and the structure scale becomes smaller. While the injection is working, the downstream surface of step will be covered by a thin film layer. In addition, the schlieren technique is used to compare with NPLS results, and the surface pressure coefficients are measured. In the downstream of injection, the coefficient is 0.0146. The fractal dimensions of different zones in NPLS image are calculated, showing that in the initial stage of flow the fractional dimension is approximate to 1 and the closer to downstream, the higher the dimension is.