Flow imaging of flat-plate zero-pressure-gradient hypersonic boundary layer at Mach 8 in free-piston shock tunnel is presented using NPLS technique and analyzed using intermittency function and fractal theory. The flow structures of hypersonic boundary layer are very similar to those of supersonic in the streamwise-wall-normal plane, but in streamwise-spanwise plane the structures have some differences from supersonic boundary layer, especially in the transition process. The intermittency function of hypersonic boundary layer is almost the same as that of incompressible flow.
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