Linear Modal Instability Analyses of High-Speed Laminar Separated Flow Over the ROTEX-T Vehicle

We perform local and global stability analyses and direct numerical simulation to study the unresolved questions on laminar-turbulent transition mechanisms that condition performance of ROTEX-T flight vehicle at hypersonic flow conditions. Specifically, we identify conditions at which the Mack mode is stable on the conical forebody preceding the compression region, so as to be able to solve the BiGlobal eigenvalue problem in these regions under physically admissible homogeneous Dirichlet inflow boundary conditions for the perturbations. The global eigenmodes responsible for unsteadiness on the flare of the two vehicles are then identified, alongside several stationary and traveling members of the global eigenspectra. Axisymmetric direct numerical simulations provide evidence of linear destabilization of the flow on the ROTEX-T vehicle as the Reynolds number is systematically increased. Three-dimensional direct numerical simulations on a 90◦ section and the full ROTEX-T vehicle produce identical results which point at linear instability mechanisms originating at the reattachment zone of the laminar separation bubble formed at the flare.