Mechanism of transition delay by porous surface in hypersonic boundary layers is investigated. Linear stability theory (LST) is adopted to analyze characteristics of instability and comparisons with direct numerical simulation (DNS) are made. Under conditions Ma=6.0,Re=2.0×10⁴ (length scaled by displacement thickness of boundary layer),typical flow characteristics are obtained for planar boundary layers with and without distributed pores. Disturbing growth rate of smooth and porous boundary layers are computed by LST. N-factor,which is regarded as a symbol of accumulated growing of disturbance,is calculated. It shows that pores prohibit growth of Mack's mode and therefore delay transition of hypersonic boundary layer. Sequence-distributed pores stabilize boundary-layer flow more effectively than interlace-distributed pores.

For separation controlling in hypersonic turbulence flow, flat plate/roughness element interaction is studied with large eddy simulation (LES). Unsteady flow characteristics behind roughness element are analyzed. Flow stability is investigated with roughness element flow field in different height and diameter conditions. Simulation indicates that intensity inverse pressure grads are generated due to roughness element interaction in hypersonic boundary flow. In high free shear layer, inconsistent stable characteristic is influenced by height of roughness element:Short cylinder roughness element interact with flat plate boundary layer induces shock wave/boundary layer interaction and produces a free shear layer, while free shear layer is stable downstream. However, free shear layer induce by long cylinder roughness element is instable. It evolves into K-H vortex in stream-wise. On the other hand, average friction coefficient and heat-flux coefficient on flat surface interacted by roughness element are lower behind roughness element(k=1.0δ), which could be applied in anti-heat and anti-drag of aircraft inlet.

At late stages of compressible transitional boundary layers over a flat-plate, ejection and sweep flows are investigated by means of direct numerical simulation with high order accuracy. Numerical results with high resolution clearly represent complex phenomena of ejection and sweep flows in the transition process of boundary layers. Close relationships with typical vortex structures and spike structures are revealed. Details of formation mechanisms of ejection and sweep flows are analyzed. Numerical results demonstrate experimental findings. Corresponding mechanisms are analyzed.