Effects of the boundary-layer thickness at the cavity entrance on rarefied hypersonic flows over a rectangular cavity

A comprehensive numerical study is performed to investigate rarefied hypersonic flows past a flat plate with a two- (2D) or three-dimensional (3D) rectangular cavity using the direct simulation Monte Carlo method. In this work, we obtain different boundary-layer thicknesses at the cavity entrance by varying the length of upstream flat plate. Then, the effects of boundary-layer thickness at the cavity entrance on flow characteristics and aerodynamic surface quantities are analyzed in depth. The concept of total-enthalpy boundary layer successfully identifies the boundary-layer edge while the traditional boundary layer based on velocity fails to captures the shock-wave front due to the presence of the weak shock wave in the rarefied hypersonic flow past a flat plate. The boundary-layer thickness at the cavity entrance has strong effects on the flow past the 2D cavity. As the boundary layer at the cavity entrance becomes thicker, the external stream can penetrate deeper into the cavity and transfer more momentum and energy to the cavity, so the cavity flow develops gradually from an open flow to a transitional one and ultimately to a closed one. In addition, the 3D effect plays an important role for the cases of extremely thick boundary layers at the cavity entrance, while it can almost be neglected when boundary layers at the cavity entrance are exceedingly thin.