BACKFLOW STRUCTURE OF STEADY AND UNSTEADY SEPARATING TURBULENT BOUNDARY LAYERS
Recent LDA measurements in unsteady separating turbulent boundary layers by the authors have revealed some new information on the phase-ensemble-averaged backflow behaviour. In the large-amplitude unsteady flows studied, very large backflow velocities exist in the detached flow at some phasees of the oscillation cycle. In some cases even grater than the freestream velocity. The Reynolds shearing stress and turbulence-energy production near the wall in the backflow region are not negligible at these phases. Under such conditions Simpson's backflow velocity profile model is not applicable and a normal turbulent boundary-layer type of structure appears to be present in the phase-ensemble-averaged near-wall backflow. These new features of backflow are discussed and a new concept of a re-entrainment velocity of the backflow into the outer region downstream flow is introduced.