Oscillating Flow in Adverse Pressure Gradients

Separating oscillating flow in an internal adverse pressure gradient geometry is studied experimentally. Phase-locked PIV measurements and simultaneous pressure measurements reveal that during the accelerating portion of the cycle, the flow remains attached in spite of a very large adverse pressure gradient. During the decelerating portion of the cycle, the flow is more prone to separation. The duration and extent of the separation depend strongly on the oscillation displacement amplitude relative to the cross-stream dimension. In some cases, the flow separates but reattaches as the separated shear layer is accelerated temporally. The time-varying pressure measurements are used to determine the resultant minor losses for the flow in each direction. These are found to be an increasing function of displacement amplitude and independent of the Reynolds number.