Boundary-layer receptivity and breakdown in hypersonic flow over a swept blunt wedge with three-dimensional freestream acoustic disturbances

摘要

Leading-edge receptivity and breakdown mechanisms of a hypersonic boundary layer in a Mach 6 flow over a blunt wedge of 4° half-wedge angle are investigated through direct numerical simulations (DNS) of the Navier-Stokes equations, with three-dimensional freestream fast and slow acoustic waves, in both an unswept and a swept configuration, and for two different Reynolds numbers. It is known that acoustic noise is a major contributor to freestream disturbances in hypersonic wind tunnels, and that the receptivity mechanism is strongly coupled to both the strength of the sound pressure level and the orientation of the acoustic disturbances. For this reason, the three-dimensional wave system consists of a main two-dimensional wave and two opposite angle oblique waves of lower amplitude, with multiple frequencies and spanwise wavenumbers. Moreover, the simulations are performed for two different levels of the freestream disturbance amplitude. The results show that fast acoustic waves with a high amplitude level are more efficient than slow waves in generating streaks in the nose region. These streaks grow downstream and lead to breakdown, in both the unswept and swept configurations. This appears to be related to the strong resonance mechanism at the leading edge characterising the receptivity to fast acoustic waves. Slow acoustic waves, in contrast, produce in the early nose region a wall response of lower amplitude, characterised by a more complex three-dimensional wave structure, which does not trigger transition in the unswept case even at the high amplitude level. In contrast, in the infinite swept case, both slow and fast acoustic waves lead to breakdown at both amplitude levels, with the higher amplitude leading to complete transition. Two different breakdown mechanisms are identified for the swept configuration, one for fast acoustic waves at the high amplitude level, which appears to be stronger as it leads to transition at an earlier position, through the generation and rapid growth of high-wavelength streamwise streaks, and another for all the other wave-type/amplitude combinations, which is triggered further downstream through the development of smaller-scale streamwise-oriented streaks.