Transition Prediction of Boundary Layers in the Presence of Backward-Facing Steps

A set of available engineering models for the prediction of transition to turbulence is evaluated via comparisons with a high-quality experiment involving a backward-facing step (BFS) on a flat plate at subsonic freestream conditions. The streamwise shift in the transition location is monitored as the step height and flow speed are varied across step-height-to-local-displacement-thickness ratios of . We apply the -factor method based on the linear amplification of boundary-layer instabilities to laminar two-dimensional basic states. -factor correlations that rely on quasi-parallel linear stability theory (LST), the parabolized stability equations, and the harmonic linearized Navier–Stokes equations (HLNSE) yield good agreement with the experimental data, except if the onset of transition occurs slightly downstream of the BFS. For LST, the neglect of nonparallel flow effects results in transition locations that are further upstream compared to HLNSE. In contrast, models that use auxiliary transport equations and transition correlations based on local parameters fail to capture the physics associated with a BFS for moderate step heights. Specifically, the results obtained with the model indicate that it is unable to account for the flow history effects. Results show the amplification-factor-transport model also fails to capture the BFS effects in spite of accounting for the basic-state distortion near the step.