Abstract
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.
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