The physical mechanisms of hypersonic boundary layer transition induced by the roughness element are investigated by means of Direct Numerical Simulation (DNS) in terms of topological structure stability and boundary layer stability. Based on an analysis of the transition mechanism induced by roughness elements of different types, a new roughness element is designed. The new roughness element has the characteristics of small scale, low friction, low heat flux and easy to induce transition. The results show that all the base zones of the roughness elements of different types include a Saddle-Node-Saddle point (SNS) structure. Under the action of disturbance, the SNS structure forms into an unsteady and asymmetric oscillatory structure. In addition, comparisons are made between different types roughness (ramp and "gear" roughness), and transition mechanism of the different types roughness is analyzed, which supports design of hypersonic forced-transition device in the theory.
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