Global Mode Analysis in the L/T Transition of a Hypersonic Boundary Layer Forced by Wall Injection

The laminar-turbulent transition of a Mach 4.6 flat-plate boundary layer forced by wall injection is investigated using Implicit Large Eddy Simulation. The boundary layer edge conditions match those of a 1:3 scale hypersonic vehicle forebody at Mach number 6 in the T-313 blow-down wind tunnel of ITAM - Russian Academy of Science. Two different injection pressures are considered. The breakdown to turbulence is observed through instantaneous Q-criterion visualizations and wall pressure fluctuations. Global instability analysis is performed by the computation of Koopman modes from 2D pressure and density snapshots of the ILES flow field. Some coherence is found in the spatial structures and temporal frequencies of global modes computed on two different grids, indicating a weak dependency on grid resolution. Physical insight is gained, confirming different conjectured transition mechanisms for the different injection pressures.