Direct Numerical Simulation Of Roughness-induced Transition In Hypersonic Boundary Layer

Boundary layer transition is one of the key aspects in the aerodynamic and aerothermaldynamic design of hypersonic vehicles. On the one hand, the heat transfer rate and skin friction of turbulent boundary layer is much higher than that of laminar one. So the state of boundary layer heavily affects the aerothermal environment and aerodynamic force of the hypersonic vehicle, thereby affects the payload to gross weight ratio. On the other hand, for the air-breathing hypersonic vehicles, such as X-43 A and X-51 A, the turbulent flow can help to stabilize the flow, eliminate or reduce the flow separation, thereby to improve the aerodynamic characteristics of the vehicles and the start performance of the inlet. The natural transition usually will not occur before the inlet of the wind tunnel or flight-test hypersonic vehicle model due to their small scales, so roughness elements are needed to trigger the boundary layer transition.In addition to the transition effect, the drag and the severe thermal environment induced by the roughness must be taken into consideration. Hypersonic boundary layer transition from laminar to turbulence induced by different roughness elements are investigated using direct numerical simulation(DNS) method, which is based on the finite volume formulation and the MDCD-WENO reconstruction scheme. The DNS results can help to design the turbulence generator for industrial applications.Firstly, the cylindrical roughness induced transition at Mach 6 with detailed unsteady experimental data is studied to validate the in-house code. The numerical results show that the flow transition is dominated by the instability of both the horse-shoe vortices and the shear layer around the cylindrical roughness element. The computational primary frequency of separation bubbles is well matched with that in experiment. At the same time, the frequency of “jet” is firstly explored. This case indicates that our methods can be applied to investigate the mechanism of roughness-induced transition.Secondly, the effects of roughness shape(ramp, diamond and cylinder) on transition are studied. The transition distance, disturbance growth rate, the wake width and the aerodynamic and aerothermodynamic performance are quantitatively analyzed. The dynamic mode decomposition(DMD) and proper orthogonal decomposition(POD) are applied to explore the relationships between the flow coherent structures and the corresponding frequencies. After the comprehensive comparisons of transition effects, drag and heat transfer rate, the ramp roughness element is thought as the most suitable transition trip.Finally, the parameters of the ramp on the transition effect are systematically studied, including the Reynolds number, ramp angle and the spacing between two ramps. The transition effect, the aerodynamic and aerothermodynamic increment are taken into consideration. When the Reynolds number decreases, the roughness-induced transition effects get worse. With increasing the ramp angle, the transition occurs more upstream due to the increment of disturbance amplitude. At the same time, the drag force and heat flux also increase. With decreasing the spacing, the interactions between wake vortices become stronger, and the transition occurs more upstream than that of the isolated roughness. The coherent structures look more uniform and developed in the spanwise direction. Generally, the ramps with medium angle and zero interval spacing are recommended.