ACTIVE CONTROL OF SUBLAYER DISTURBANCES USING AN ARRAY OF HEATING-ELEMENTS.

Experiments have been conducted on a flat-plate model in the Moody Water Channel at Princeton University which indicate that turbulent flow instabilities can be controlled by exploiting unsteady surface heating. The effect of a single row of streamwise heating elements on the flow was evaluated by pulsing the heaters and monitoring the flow at several locations using thermocouple and hot-film probes. The heater array was successful in creating velocity disturbances which were similar to the low speed streaks found in naturally turbulent boundary layers. The induced instabilities grew as they convected downstream, subsequently evolving into turbulent spots. In the active control experiments, the front row of heating elements was used to introduce a disturbance and the back row was activated spatially and temporally out of phase to cancel the disturbance. The low-speed streaks were attenuated significantly using this technique, thus delaying the transition to turbulence and reducing the drag in the flow. A laser sheet scanning technique was developed which aided in the observation of the flow field downstream of the heater array. The technique played a major role in determining the effectiveness of active control on the boundary layer flow field. It also provided a means for quantitatively evaluating the extent of turbulent mixing occurring in the flow.