BiGlobal stability analysis of swept-wing boundary layers with forward and backward facing steps

The temporal stability characteristics of generic, swept-wing boundary-layer flows of practical engineering significance with a smooth, isolated backward facing step or a forward-facing step are investigated. A streamwise BiGlobal analysis is undertaken on previously computed steady-state, Mach 0.3 Navier-Stokes solutions that had been obtained for four spanwise invariant step heights and at two chordwise locations on an infinite swept-wing geometry. Temporal instability is detected for heights exceeding 25% of the undisturbed boundary-layer thickness, or one unit of the undisturbed displacement thickness at the step chordwise location. Forward-facing steps with the greatest height are not found to be temporally unstable in contrast to backward-facing steps. Unstable modes feature localised regions of large streamwise and spanwise perturbation velocity magnitude near the lower corner of the backward-facing step or just downstream of the forward-facing step. Wave-like modes are found to arise from the deepest backward-facing step