Leading-edge receptivity of steady streaks in a hypersonic flat-plate boundary layer

Free-stream vortical waves can excite streamwise streaks and trigger the bypass transition of boundary layers subjected to a large magnitude of free-stream turbulence. In this paper, the harmonic linearized Navier-Stokes (HLNS) method is applied to simulate the leading-edge receptivity of streaks to free-stream vortical waves, and its reliability is validated by direct numerical simulations. For a supersonic flat-plate boundary layer in the condition of Mach number 4.8, a systematic analysis was conducted on the leading-edge receptivity process of steady streaks excited by zero-frequency vortical waves and the evolution of these steady streaks. Results suggest that the development of streaks is forced continuously by free-stream vortical waves outside the boundary layer. The amplitude of the generated streak reaches the maximum when the vertical wavenumber is zero for free-stream vortical waves with specified spanwise wavenumbers. When the vertical wavenumber is less than the critical angle, it affects the amplitudes but not the shape functions of streaks. As the local Reynolds number increases, the streaks' amplitude evolutions and the shape functions exhibit strong similarity. The peak normalized amplitude of the streaks occurs at a local non-dimensional spanwise wave number of about 0.18.