Numerical investigation of laminar-turbulent transition in hypersonic boundary layers

Laminar-turbulent boundary-layer transition for high-speed flows is still a major unresolved topic in fluid dynamics despite some significant advances in the last decade. It is particularly important for numerous aerospace applications, especially for many that are of interest to the Department of Defense (DoD). This is because the state of the boundary layer (laminar, transitional, turbulent) has a major impact on the aerodynamic performance of high-speed flight vehicles. Transition to turbulence leads to increased drag and heat transfer. This may jeopardize the structural integrity of high-speed vehicles unless appropriate measures to guard against these increased heat loads are taken. It has been often overlooked in the past that the transitional flow regime can cover large parts of hypersonic flight vehicles, and that transitional boundary layers can be even more detrimental than fully developed turbulent flow.

In this talk, the use of Direct Numerical Simulations (DNS) to investigate and understand the fundamental flow physics of boundary-layer transition processes will be discussed. It will be shown how DNS has been successfully employed to identify the nonlinear interactions responsible for the development of streamwise streaks with locally increased heat transfer (“hot streaks”), which were also observed in experiments at the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The understanding from these simulations was used to develop a flow control method aimed at delaying transition and suppressing the formation of these hot streaks. Finally, an outlook will be provided on how changes in geometry and flow conditions can influence the dominant nonlinear mechanisms leading to laminar-turbulent boundary-layer transition, as well as the remaining open questions that need to be addressed.