CONTROL OF TURBULENCE

We discuss a few applications of active control of turbulent fluid flo w and their implications for the economy and the environment. We outli ne a conceptual basis for control, sketching sensors, actuators, and t he algorithm. The control of turbulence requires an understanding of t urbulent flows beyond our present capabilities, but we describe the ph ysical basis for control of the boundary layer: coherent structures an d bursts, the connection between burst frequency and friction velocity , the change of burst frequency and drag reduction possible with polym ers or active control, and other effects on burst frequency (e.g. stre amline curvature, pressure gradients, and extra rates of strain). Give n that the state of the flow must be sensed from the surface, and that this information is necessarily incomplete and aliased, sophisticated techniques may be required to interpret the signals. A control strate gy, an algorithm, is necessary, and we express the need for a model of the flow as an interpreter and a predictor. Although surface actuator s are necessary, the question of their precise effect on the fluid mus t be considered. We present surprising results of direct numerical sim ulation (DNS) of a type of actuator. Before controlling the fluid, we tried to control a model. As background, we present results of attempt s to control several different models.