Numerical simulation of wavy surface effect on the stability of a hypersonic boundary layer

Generally the boundary layer transition is tripped by the surface roughness, while at certain circumstances, the surface roughness can delay the boundary layer transition. In this paper, by introducing the two-dimensional wall blow-suction upstream, high-order numerical method is used to simulate the Ma = 6 hypersonic boundary layer stability with a typical wavy-wall model. First, the mechanism of transition delay by the wavy wall is analyzed by comparison of the disturbance development in the flat-plate and wavy-wall boundary layer. Then the effects of the wavy-wall wave number, depth and streamwise location on the boundary layer stability are investigated. The results show that the disturbance development is closely related to the separation region. Compared with the flat plate, the wavy wall can suppress the second-mode instability via multiple separation regions, thus delaying the boundary layer transition. Concerning the wave number, the wavy wall with a single wave number has only a single short separation region, so the second-mode instability grows rapidly downstream; the wavy wall with multiple wave numbers means multiple separation regions and suppresses the second-mode instability sharply. As for the depth, the separation region grows with the increase of the wavy-wall depth and the second-mode instability is suppressed more evidently. Also as the location of the wavy wall moves downstream, the suppression on the second-mode instability is more obvious.