Abstract:
Cross-flow instability is the main transition mechanism in three-dimensional hypersonic boundary layers. However, up to now, there are few numerical investigations on the breakdown process to turbulence for hypersonic boundary layers induced by cross-flow instability. In this paper, the boundary-layer transition is investigated for a blunted flat plate with Mach number 6 and sweep angle of 45 deg. Direct numerical simulations are performed for the linear and nonlinear evolution of stationary cross-flow vortex, as well as the development of the second instability waves on the saturated mean flow, and the transition to turbulence. The results show that the modification of the mean flow caused by the nonlinear effect of the stationary vortices can lead to the rise of the wall friction coefficient curve. However, without introducing the secondary instability waves, the wall friction coefficient stay constant and cannot rise to the value of turbulence level. The introduced high frequency secondary instability waves grow dramatically, and the low frequency disturbances, including stationary ones, which are generated due to the nonlinear interaction, amplify significantly as well. As a result, the mean flow is modified rapidly, leading to a steep rise in the wall friction coefficient. Following that, the saturated cross-flow vortex structure breaks down, and the flow transitions to turbulence.