Transition of swept-wing boundary-layer flow induced by a forward-facing step

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This work investigates the laminar–turbulent transition of an incompressible swept-wing boundary-layer flow interacting with a forward-facing step using direct numerical simulations and two-dimensional linear secondary stability analysis. The focus is placed on the most critical transition scenario, where the step abruptly shifts the transition front upstream. A single-wavelength stationary crossflow perturbation is artificially triggered and allowed to develop upstream of the step. The effect of the amplitude of the incoming perturbation on the salient mechanisms of instability at the step are assessed for a fixed step height. The regime of abrupt laminar–turbulent transition at the step develops only in the case of the largest incoming perturbation amplitude considered, implying that the transition regime due to the step cannot be categorised exclusively based on local Reynolds number and instability wavelength. Step-induced transition is attributed to the stationary deformation of the shear layer immediately downstream of the step, thereby promoting rapid spatial amplification of high-frequency unsteady instabilities. The mechanisms supporting this shear-layer deformation, a mixture of linear and nonlinear growth effects, are examined by deriving energy-balance equations for the stationary spanwise-harmonic perturbation modes.