Impact Of The Streamline Curvature On The Supersonic Boundary Layer

By employing Nanoparticle Planar Laser Scattering(NPLS)method,Particle Image Velocimetry(PIV)and Direct Numerical Simulation(DNS),characteristics of the supersonic boundary layer subjected to the streamline curvature are investigated.The impacts of the streamline concave curvature,the streamwise adverse pressure gradient and the streamline convex curvature are analyzed,as well as the related physical mechnisms.By employing the NPLS technique,the formation and breakup of the G(?)rtler vortices have been clearly visualized in the supersonic concave boundary layer,which has seldom been done before.Decreasing the concave curvature could delay the boundary layer transition.Different from the incompressible case,the breakup of the G(?)rtler vortices in the supersonic boundary layer is found to be mainly attributed to the varicose mode instability which presents itself in the form of hairpin vortices.Large scale vortices formed in the flat plate region are observed to break up into smaller ones immediately after they flow into the concave regions.Despite of that,the large scale motions are seemed unaffected.Attributed to the streamline concave curvature,the spanwise distance between adjacent low-speed streaks are increased in the streamwise direction.The very large scale motions(VLSMs)formed in the log-law region extend straightly downstream in the flat plate region,while in the concave region,the VLSMs meander.The extracted vortices are found to gather around the VLSMs,due to the meander in the curved region,the distributions of the vortices are found to meander too.All of the buffer layer,log-law reigon and the wake reigon are affected by the streamwise concave curvature.The height of the buffer layer is increased.The slope of the log-law region is reduced and its influential area is forced to move outerward.The wake region is strengthened.The different streamwise varying trends of the principal strain rate at different heights are noted in both PIV and DNS results.To give a physical explanation of that,the three-layer assumption is proposed and modified,which is proved to work well.Based on the distribution of the principal strain rate,a new method to define the boundary layer thickness for the flow with wall normal pressure and velocity gradient.The change of the turbulent structure is the main cause of the variations of the ensemble averaged parameters.In the concave boundary layer,the downwash events are significantly strengthened.By carrying the high-momentum fluid to move downward directly to the near wall region,the velocity gradient in the wall normal direction are promoted in the sub-layer,while the momentum mixing in the buffer layer and the log-law region is strengthed,as a consequence of which the slope of the log-law region is reduced.Since the strengthed downwash events also increase the interfacial area between the high and low momentum fluid,they are also responsible for the increased turbulent fluctuation in the log-law region.To clarify the separate impacts of the streamwise adverse pressure gradient(APG)and the concave curvature,an experiment on a APG flat boundary layer without the influence of concave curvature has been carried out,and the results are compared to that of the concave boundary layer with similar streamwise pressure distribution.The streamwise and normal turbulent intensities,Reynolds shear stress and turbulence production are all found to be increased by the adverse pressure gradient.The centrifugal instability has similar effect and its contribution is significant.To explain the turbulence amplification effect of the adverse pressure gradient,a physical interaction model between the hairpin packets and the compression wave is proposed.To clarify the isolated impacts of the pure convex curvature and the streamwise favorable pressure gradient on the turbulent boundary layer,test models are carefully designed to have a zero pressure gradient(ZPG)convex boundary layer.Measurements on the favorable pressure gradient(FPG)convex boundary layer are also conducted for comparison.The results indicate that the favorable streamwise pressure gradient and the streamline convex curvature have similar contributions in shaping the streamwise velocity profile.Both of them weaken the wake of boundary layer and reduce the principal strain rate in the near region.While both of the favorable pressure gradient and the convex curvature are found to suppress the streamwise fluctuation,their impacts on the normal fluctuation are opposite.Different to its relaxation effect on the streamwise fluctuation,the streamline curvature is observed to enhance the normal fluctuation.The contribution of the convex curvature to the normal fluctuation is much stronger than that of the pressure gradient.However,since the normal turbulent intensity is much smaller than the streamwise,the turbulent boundary layer is finally stabilized by the convex curvature.Since the retrograde vortices are related to the Q4 events,the seldom noted retrograde vortices in the FPG boundary layer suggest that the rotation of hairpin vortices could be weakened,which contributes to the decrease of turbulent intensity.