This work explores experimentally the control of a turbulent boundary layer over a flat plate based on wall perturbation generated by piezo-ceramic actuators. Different schemes are investigated, including the feed-forward, the feedback, and the combined feed-forward and feedback strategies, with a view to suppressing the near-wall high-speed events and hence reducing skin friction drag. While the strategies may achieve a local maximum drag reduction slightly less than their counterpart of the open-loop control, the corresponding duty cycles are substantially reduced when compared with that of the open-loop control. The results suggest a good potential to cut down the input energy under these control strategies. The fluctuating velocity, spectra, Taylor microscale and mean energy dissipation are measured across the boundary layer with and without control and, based on the measurements, the flow mechanism behind the control is proposed.

This work aims to experimentally investigate the manipulation of a turbulent boundary layer over a flat plate using a proportional-derivative (PD) controller. The control action is generated by an array of two flush-mounted piezo-ceramic actuators. Two different schemes are examined, i.e., feed-forward and feedback PD controls, with a view to suppressing the viscous-scaled near-wall cycle of high-speed events in the near-wall region and hence reducing skin friction drag. It has been found that the use of the feed-forward PD scheme may reduce the local maximum drag reduction by up to 33% at 14 wall units downstream of the actuator array, exceeding the open-loop control result (30%) as well as our previously reported combined feed-forward and feedback scheme (28%) [Z. X. Qiao, Y. Zhou, and Z. Wu, “Turbulent boundary layer under the control of different schemes,” Proc. R. Soc. A 473, 20170038 (2017)], and furthermore, this significantly cuts down the required input energy by 27%, compared to the open-loop control. On the other hand, the feedback PD scheme achieves the same control performance as the open-loop control, that is, producing a local maximum drag reduction of 30% without any saving in the input energy. The underlying control mechanism behind these control schemes is proposed based on the analyses of the hot-wire data measured with and without control.

Abstract

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