The Study On The Structure Of The Supersonic Streamwise Corner Boundary Layer

The streamwise corner boundary layer is common in the flow field of the wing-body junctions,inlets and nozzles.It has a significant impact on the flow structure,wall friction and total pressure loss in the corner region.Due to the strong three-dimensional characteristics caused by the sidewall,the receptivity,transition and turbulence of the streamwise corner boundary layer also change significantly.In this paper,the natural transition,bypass transition and turbulence properties of the flow are investigated to reveal the flow mechanisms and summarise the empirical relationships using a combination method of experiment,direct numerical simulation and Reynolds-averaged simulation.The natural transition in the streamwise corner boundary layer is observed for the first time with the supersonic quiet wind tunnel and high-resolution flow field visualization technology.The transition at the corner is found to be dominant by the intermittent factor analysis method.But the transition does not always originate at the corner.It is also given for the first time that the flow structure in the corner region with fully developed streamwise corner turbulent boundary layer.The instantaneous flow field is irregular turbulence structures and there is no organized corner vortex strutures.By statistical analysis,the fluctuation intensity in the corner area is weaker than that of the plate boundary layer,and the ejection events related to the formation mechanism of the secondary flow are also observed near the corner area.The bypass transition characteristics of the flow at the streamwise corner boundary layer are investigated by arranging strip roughness elements in a square duct.Based on a large number of direct numerical simulations,it was found that there are two typical forms of strip roughness induced transition in the streamwise corner boundary layer,which are called corner transition and strip transition.In the streamwise corner boundary layer,the corner transition occurs first.It originates from the corner and develops in a wedge shape along both sidewalls.As the roughness Reynolds number increases,the strip transition occurs in the subsonic cases and coexisting with corner transition.The strip transition occurs downstream of the entire strip.However,no strip transition is ever found in the supersonic case.A criterion for strip-induced transition is given by defining the roughness Reynolds number and the roughness Mach number.Further analysis reveals that the flow mechanisms of these two forms of transition are different.The typical characteristics of the strip transition suggest that it is inevitably related to the separation/re-attachment shear layer behind the strip roughness.It is found that as the roughness Reynolds number increases,the shear layer becomes progressively destabilized and eventually breaksdown to turbulence.But the mechanism is suppressed under the supersonic conditions.The corner transition is clearly related to the sidewall.A corner wake vortex can be observed behind the strip at the corner.As it progresses downstream,the corner wake vortex brings the high-momentum flow into the corner forming high-and low-speed streaks.Subsequent destabilization of the high and low velocity strips and the occurrence of turbulent burst events eventually lead to the formation of turbulence by breaking up the laminar flow structure.In order to solve the engineering problem of the streamwise corner boundary layer,it is confirmed that the Reynolds average simulation method based on Stress-ω RSM model can accurately predict the flow field parameters of flow corner boundary layer by comparing with classical turbulence model,experiment and direct numerical simulation.On this basis,the effects of dihedral angle and compressibility are investigated.The results show that the flow near the corner region will relaminarize and the self-similarity of the velocity profile will disappear for the cases of small dihedral angles.The development of corner vortex is mainly influenced by the boundary layer and dihedral angle and has little to do with the compressibility.A universal distribution law for the spatial development of the vortex core is given by multi-parameter fitting to assess the influence of the corner vortex.The general law of wall friction in the streamwise corner boundary layer is also summarised and an empirical formula for the predicted total pressure loss coefficient is given.The importance of considering turbulence anisotropy on the accuracy of the result prediction is further emphasized by comparing the k-ω SST and Stress-ωRSM turbulence models for the simulation of supersonic S-bends with square cross-section.The effect of the S-bend normal offset distance on the flow field structure is investigated.When the offset distance is small,there is no significant effect,and when the offset distance is large,a pair of streamwise counter-rotating vortices will generate,which makes the flow separation caused by the obvious droplet low-speed region in the cross-section.It leads to the degradation of the S-bend performance.By comparing with the inviscous simulation results,it is found that the baroclinic effect generated by the pressure and density gradients is the key cause of this streamwise vortex pair.