Control of mixing in a nonreactive plane shear layer: I. Open-loop control. II. Feedback control.

Abstract

A control system for the enhancement and regulation of mixing in a nonreactive plane shear layer has been developed in a two-stream closed-return water facility. Mixing of a passive scalar is estimated using a thermal analog in which the two streams have uniform, steady temperatures differing by 3°C. The position of the temperature interface between the two streams is measured in the plane of its cross stream Schlieren image by an optical sensor which is placed upstream of the rollup of the primary vortices. Control is effected via an array of surface heaters flush-mounted on the flow partition and cross-stream temperature distributions are measured with a resolution of 0.03°C using an array of closely-spaced cold wire sensors. In closed-loop experiments the output from the interface position sensor is fed back to the surface heaters. A transfer function is used to predict the effect of feedback on the interface motion. The dependence of various measures of mixing on the feedback gain k and the total delay time Δ between the actuators and the sensors is studied. The feedback gain k is adaptively modified to maximize mixing at a given streamwise station. These experiments indicate that feedback control of the motion of the temperature interface can be used for controlling the nominally 2D entrainment by the primary vortices and thus enhancing mixing.