Effects of External Disturbances on the Diamond-Induced Transition by Direct Numerical Simulation

Supersonic boundary-layer transitions triggered by a diamond-shaped roughness element were investigated by direct numerical simulation based on a finite volume formulation with a high-order minimum dispersion and controllable dissipation scheme. The simulations agree well with the experimental data. High- and low-speed streaks appear after the roughness element, which induce and set a stage for instability growth. The evolution and mode shape of the most amplified disturbance behind the roughness element show good agreement with the measurements. Both odd and even modes can be detected, and the flow transition is dominated by the even mode, about 104 kHz. After introducing different-type freestream disturbances in the velocity profile at the inlet, it was found that both the inflow disturbance level and the distribution of energy in the frequency space have significant effects on the transition location.