| Hypersonic boundary layer transition has an important impact on the aerodynamic and aerothermal characteristics of hypersonic aircrafts.Accurate prediction of the boundary layer transition can improve the safety and economic performance of the aircraft significantly.Therefore,it is particularly important to study the mechanisms of hypersonic boundary layer transition.Crossflow instability is a common and important mode in the boundary layer transition process.In the current paper,nano-tracer-based plane laser scattering(NPLS),fast-response pressure transducers,and temperaturesensitive paints(TSP)are used to measure the crossflow instability.The experiments are conducted in a Mach 6 quiet wind tunnel,the experimental model includes swept flat plates and a cone with a 6° angle of attack.The characteristics of the crossflow instability over the swept flat plates are studied.Using NPLS and TSP techniques,the type of primary crossflow instability on the swept flat plate was judged.On the 45° swept flat plate,the boundary layer transition is dominated by the stationary crossflow vortices at large Reynolds number,while the transition is dominated by the traveling crossflow waves at small Reynolds number.However,in all Reynolds number conditions,the boundary layer transitions on the 60°and 75° swept flat plates are all dominated by the traveling crossflow waves.The characteristics of stationary crossflow vortices and traveling crossflow waves such as wavelength and wave angle are obtained by NPLS and TSP techniques.In addition,the influences of angle of attack and Reynolds number on the characteristics of crossflow vortex are studied.The spatial structural characteristics of the traveling crossflow waves on the 60°and 75° swept flat plates are studied by the NPLS technique.In the spanwise direction,the traveling crossflow waves are shown as large-scale structures parallel to each other.In the direction parallel to the leading edge,the traveling crossflow waves are shown as typical crossflow vortex structures with regular rolling downstream.The wavelet tool is used to analyze the NPLS results,the wavelength and amplitude distribution characteristics of the traveling crossflow waves are obtained.The characteristic frequency of the traveling crossflow instability from the NPLS image is about 12 k Hz.The characteristic frequency measured by the Kulite sensor is 9.57 k Hz,which is consistent with those measured by NPLS.A cone with a 6° angle of attack is a typical model for studying hypersonic crossflow instability.The traveling crossflow waves,stationary crossflow vortices,and high-frequency instability during the natural transition process on the sharp cone are studied in the quiet wind tunnel.The traveling crossflow waves with characteristic frequency of 10～25 k Hz and stationary crossflow vortices with circumferential wavenumber of 40～60 are detected,and the amplitude growth characteristics of these two crossflow modes are studied.In addition,it is also found that there is a highfrequency instability of 80～160 k Hz on the cone.The high-frequency instability exists at the top of the traveling crossflow waves and may be the secondary instability of the traveling crossflow waves.The nose-tip bluntness and discrete roughness elements are also used to control the crossflow induced transition.Results show that when the radius of the nose-tip is large,the bluntness will inhibit the natural transition obviously,and the affection of roughness elements on the transition will also be significantly inhibited.Three kinds of roughness elements with circumferential wavenumbers of k=35,k=50,k=70 can all advance the transition on the conical leeward surface,among which the critical roughness element(k=50)will advance the transition the most. |
