DETECTION OF FLOW STATE IN AN UNSTEADY SEPARATING FLOW

A model experiment that exhibits some of the characteristics of the leading-edge separation over unsteady airfoils was conducted, and a simple, nonintrusive technique for detection of flow reversal was developed. The unsteady separation introduced in a turbulent boundary layer, by the motion of a spanwise flap into the flow, was investigated for a range of Reynolds number and flap angular velocities. Smoke-wire flow visualization, measurements of the time-varying flow direction at various locations behind the flap, and wall pressure data were used to characterize and document the steady as well as unsteady separation. Ensemble-averaged static-pressure data measured for a representative set of parameters were used to formulate different criteria to identify a change in flow direction at different streamwise locations. These criteria were tested by playing back time-series data of the wall pressure covering a range of Reynolds numbers and flap-rise times. Phase-conditioned flow-direction measurements were used as the reference to evaluate the performance of these criteria. Flap-drop experiments representing unsteady flow reattachment, as well as combined flap motions, were also examined. Two of the criteria formulated in this study show good promise for use as flow-state identifiers in the active control of unsteady separated flows.