ACTIVE CONTROL IN THE TURBULENT WALL LAYER OF A MINIMAL FLOW UNIT

Direct simulations of flow in a channel with complex, time-dependent w all geometries facilitate an investigation of smart skin control in a turbulent wall layer (with skin friction drag reduction as the goal). The test bed is a minimal flow unit, containing one pair of coherent s tructures in the near-wall region: a high- and a low-speed streak. The controlling device consists of an actuator, Gaussian in shape and app roximately twelve wall units in height, that emerges from one of the c hannel walls. Raising the actuator underneath a low-speed streak effec ts an increase in drag, raising it underneath a high-speed streak effe cts a reduction - indicating a mechanism for control. In the high-spee d region,. fast-moving fluid is lifted by the actuator away from the w all, allowing the adjacent low-speed region to expand and thereby lowe ring the average wall shear stress. Conversely, raising an actuator un derneath a low-speed streak allows the adjacent high-speed region to e xpand, which increases skin drag.