Unsteady effects and relaminarization in compressible channel flows

Because of their complexity and bigger parameter space, compressible wall-bounded flows are less understood than their incompressible counterpart. This is even more so when flows also experience temporal changes. In this work, we study unsteady effects in compressible turbulent channels using well-resolved Direct Numerical Simulation (DNS) for a range of Reynolds and Mach numbers. In particular, we investigate relaminarization processes and time-dependent scaling laws. Starting from a steady flow sustained by a constant body force in the streamwise direction, the forcing is cut-off and the flow is allowed to decay. As the turbulent kinetic energy dissipates, different measures of compressibility are tracked and related to quantities of practical relevance such as wall-friction and widely used compressible scaling laws for mean velocity and Reynolds stresses. Relaminarization and unsteady effects are assessed by comparing snapshots of the decaying flow with dynamically equivalent steady simulations. Preliminary results show deviations of the flow properties in the turbulent-to-laminar transition region but good agreement with both laminar solutions and turbulent incompressible approximations. Detailed studies of turbulent properties unveil the rapid shrinking of turbulent structures leading to negligible turbulent stresses and relaminarization. The possibility and evidence of hysteresis as a consequence of unsteadiness is discussed in light of the DNS data.