CONTROL OF TURBULENCE

We discuss a few applications of active control of turbulent fluid flow and their implications for the economy and the environment. We outline a conceptual basis for control, sketching sensors, actuators, and the algorithm. The control of turbulence requires an understanding of turbulent flows beyond our present capabilities, but we describe the physical basis for control of the boundary layer: coherent structures and bursts, the connection between burst frequency and friction velocity, the change of burst frequency and drag reduction possible with polymers or active control, and other effects on burst frequency (e.g. streamline curvature, pressure gradients, and extra rates of strain). Given 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 strategy, an algorithm, is necessary, and we express the need for a model of the flow as an interpretor and a predictor. Although surface actuators are necessary, the question of their precise effect on the fluid must be considered. We present surprising results of direct numerical simulation (DNS) of a type of actuator. Before controlling the fluid, we tried to control a model. As background, we present results of attempts to control several different models.