Experimental Study On Hypersonic Boundary Layer Transition

With the development of hypersonic vehicle,the problem of boundary layer transition becomes more and more prominent.On the one hand,because the drag and heat flux of objects in laminar flow are very different from those in turbulent flow,the boundary layer transition in hypersonic flow will lead to the sharp increase of vehicle drag and surface temperature.If the boundary layer on the surface of hypersonic vehicle maintains a complete laminar flow state,the ratio of payload to total weight will be doubled compared with which in the turbulent flow.Therefore,the study of boundary layer transition plays an important role in the design of hypersonic vehicle.In this paper,the natural transition of hypersonic boundary layer and the bypass transition induced by roughness element are investigated.For the natural transition,the characteristics of natural transition on flat plate boundary layer under different Mach number and unit Reynolds number are studied,and the transition prediction is made by combining the transition model modified by compressibility.For the bypass transition induced by roughness element,the effects of Mach number,different configuration,and height of roughness element on the roughness element wake and transition are studied.For the natural transition of the hypersonic plate boundary layer,the results predicted by the wind tunnel experiment are basically consistent with those predicted by the compressibility modified transition model.It is found that the transition Reynolds number increases with the increase of unit Reynolds number.In addition,the compressibility has the effect of stabilizing the boundary layer.At the same unit Reynolds number,as Mach number increases,so does the transition Reynolds number.For the boundary layer bypass transition induced by roughness element,it is found that the wake region induced by roughness element can be divided into three regions: the first is the separation region of upstream and downstream of roughness element,which corresponds to the formation of "horseshoe vortex" and flow direction vortex structure;the second is the development region of counter-rotating streamwise vortex pair,in which the width of roughness element wake is kept constant and the heat flux on both sides of wake center line is high.The third is the transition wedge region,where the wake width begins to increase and the flow has developed into turbulence.Generally speaking,the higher the height of roughness element,the better the effect of induced transition.For the roughness element configuration,diamond shaped roughness element is the best and square shaped rough element is the worst in promoting transition.With the increase of Mach number,the boundary layer is more stable and the effect of transition induced by roughness element is weak.