LAUSR

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.