Predicting Laminar-Turbulent Transition Influenced by Surface-Induced Flow Distortions

Approaches to transition prediction are considered within the context of applications to aircraft, where surface-induced distortions are an integral part of the boundary-layer flow. Different routes to transition are discussed as a function of the type and level of surface irregularity perturbing the flow. Amplitude methods offer the best potential for predicting the gradual movement of transition with increasing levels of flow distortion. Global stability analysis offers the potential for capturing abrupt transition near the source of the flow distortion (typically characterized as bypass transition). A semi-empirical amplitude method, based on a combination of stability theory and experiments, is considered for a-priori predictions to capture gradual changes to transition for realistic surface irregularities. This predictive framework is then used to characterize the different mechanisms and flow regimes responsible for boundary-layer transition in the presence of surface steps and gaps.