Perturbation evolution in high-speed flat plate boundary layers: Non-equilibrium flow-thermodynamic interaction effects

In low-speed incompressible fluid flows, pressure acts as a Lagrange multiplier to ensure the solenoidal nature of the velocity field. On the other hand, in high-speed flows the fundamental nature of pressure changes, triggering significant flow thermodynamic interactions. In this work, we use direct numerical simulation (DNS) data to examine the effect of velocity and pressure initial conditions on the development of perturbations on a flat plate boundary layer at high speeds. DNS of temporally evolving boundary layers are performed in the Mach number (M) range of 0.12-6 for various low intensity initial perturbations. The dependence of the linear transient non-equilibrium behaviour on initial conditions is examined. The underlying flow-thermodynamic interactions in the transient regime are analysed for both first and second mode. Additionally, the velocity field is decomposed into a solenoidal and dilatational component using Helmholtz decomposition. The evolution of the solenoidal and dilatational fields is monitored and the physics underpinning the observed behaviour is explored.