The effects of adverse pressure gradient and streamline curvature on oblique-wave transition and turbulence

Direct numerical simulations have been performed to study the evolution of oblique waves in flat plate boundary layers and concave wall boundary layers under adverse pressure gradient and zero pressure gradient conditions, at a freestream Mach number of M_∞=3. The investigation aims to understand the effects of adverse pressure gradient and streamline curvature on the development of oblique waves and turbulence. The findings reveal that the adverse pressure gradient can result in rapid growth and saturation of spanwise vortex modes, while initial oblique waves suppress spanwise vortex modes. As the initial amplitude of oblique waves increases downstream, a three-dimensional chessboard-like structure forms in the flow field. This structure evolves into staggered Λ vortices, with longer legs in the Λ vortices observed in zero pressure gradient boundary layers. Under adverse pressure gradient conditions, Λ vortical structures are more prone to rapid deformation due to strong shear layers and baroclinic effects, ultimately breaking down within two periods and transitioning the flow into turbulence. A detailed analysis of the turbulent structures reveals that the normal position of the point of maximum anisotropy and the maximum turbulent kinetic energy moves closer to the wall in the presence of an adverse pressure gradient.