UNSTEADY BOUNDARY-LAYER-TRANSITION IN FLOW PERIODICALLY DISTURBED BY WAKES

Boundary layers on turbomachinery blades develop in a flow that is per iodically disturbed by the wakes of upstream blade cascades. These wak es have a significant effect upon laminar-turbulent boundary-layer tra nsition. In order to study these effects, detailed velocity measuremen ts using hot-wire probes were performed within the boundary-layer of a plate in flow periodically disturbed by wakes produced by bars moving transversely to the flow. The measurements were evaluated using the e nsemble-averaging technique. The results show how the wake disturbance enters the boundary-layer and leads to a turbulent patch, which grows and is carried downstream. In favorable pressure gradients, transitio n due to wake turbulence occurred much earlier than predicted by linea r stability theory. Between two wakes, laminar becalmed regions were o bserved far beyond the point at which the undisturbed boundary-layer w as already turbulent.