Abstract: In order to understand the influence of unforced boundary layer transition on the performance of a hypersonic inlet, a numerical study is performed in this paper. The results show that the bluntness of the leading edge of the inlet influences the boundary layer transition. The boundary layer transition is delayed first when the nose of the cone is blunted, and then the transition location moves upstream once the blunt radius is large enough, but eventually the boundary layer transition is delayed again when the blunt radius becomes larger. The turbulence intensity, which induces the boundary layer to become more instable, influences the boundary layer transition greatly. Compared with the turbulence boundary layer, an inlet can obtain higher total pressure recovery and capture more mass flow rate with lower thermal load and drag when the boundary layer transition occurs at the compression surface. Compared with the turbulence boundary layer, the total pressure recovery at the throat increases by 17.3% and the drag of the inlet decreases by 17.4% when the boundary layer transition occurs at Ma=6.5. The boundary layer condition influences the distribution of heat flux greatly but has little influence on the pressure distribution along the wall. The blunt radius of the nose influences the self-starting ability of the inlet, and the self-starting Mach number becomes larger when the blunt radius increases, but the boundary layer transition almost has no influence on the self-starting ability.

Key words: transition, boundary layers, inlet, hypersonic, propulsion system