Experimental and numerical investigation of the transition progress of strut-induced wakes in the supersonic flows

Strut serves as an effective and robust flameholder in a scramjet combustor, and the resultant wakes have a great impact on the fuel mixing and flame stabilization. Present work focuses on the non-reactive flow induced by strut without fuel injection, removing the effect of chemical reactions on turbulence. To investigate the transition process of turbulent wakes, both high-resolution experiments and numerical simulations are conducted. An advanced optical measurement technique is adopted to visualize the flow structures in different Mach conditions, and to disclose the compressible effect on the shear layer. Experimental results indicate that the scale of turbulent vortices decreases and the transition becomes difficult with the decline of Reynolds number in the cases with the same total temperature and pressure. Evidently, the viscous effect gets weakened for the supersonic flow with a high Reynolds number, giving rise to large scale vortex streets. On this basis, the intrinsic mechanism that governs the instability of shear layer is investigated by large eddy simulation. The strut-induced wakes can be divided into three parts, namely the recirculation zone, the transition zone and the downstream turbulent zone. A type of Kelvin-Helmholtz instability produced by the interactions of the reattachment shock and shear stress, should be responsible for the generation of the initial streamwise vortices during the transition process of shear layer. Thus, the flow instability rapidly grows as a form of nonlinear, magnifies, and changes the mean velocity profile of the shear layer, leading to its transition from laminar state to turbulence. Finally, the influence of turbulence on the temporal and spatial evolution of the vortices in the wakes is analyzed in deep. The upstream turbulent fluctuations facilitate the occurrence of transition process, and promote to generate larger scale spanwise vortex streets. Present work can help us understand the flow dynamics of turbulent wakes in a strut-stabilized scramjet combustor.