Flow Separation in Bends in a 2-D Closed Channel

Experiments were conducted in a turbulent 2-D channel flow leading to a range of curved bends that result in an inner-radius flow separation.  The ratio of the bend centerline radius to channel height was 1.125. The bend angle was adjustable and ranged from 0 to 150 degrees. The approaching channel flow to the bends was approximately fully developed, and did not vary with the bend angle. The flow conditions were adjusted to maintain a constant centerline velocity to account for the different pressure losses associated with the different bend angles. The flow field within the bend was documented using smoke wire flow visualization, a traversable Pitot probe, and particle image velocimetry over a range of channel Reynolds numbers. The objective was to identify characteristics of flow separation and reattachment, as well as approaches to flow separation control. Flow separation control by both passive and active means are investigated. Active control involves a diaphragm-driven unsteady tangential wall jet, and a pulsed-DC plasma actuator. An optimum frequency was found in each case to minimize the reattachment length. The scaling of the optimum frequency to the separated flow characteristics is presented.