Turbulent boundary layer manipulation under a proportional-derivative closed-loop scheme

Abstract

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.

Flat plate drag reduction using plasma-generated streamwise vortices

“…Under control, the near-wall (du + /dy + ) 2 at z + = 133 and 0 in Regions R2 and R3, respectively, becomes much larger than that of the uncontrolled case. The greatly increased dissipation is consistent with the substantial DR (Qiao, Wu & Zhou 2018). The (du + /dy + ) 2 becomes smaller at z + = 183 in Region 1, where drag increases.…”

Skin-friction reduction using periodic blowing through streamwise slits

“…Following Qiao et al. (2017) and Qiao, Wu & Zhou (2018), we used four fixed wall wires, made of tungsten, to measure the streamwise velocity and approximately the WSS. The sensing element of each wall wire is 5 μm or 0.034 wall unit in diameter ( d ) and 1.2 mm or 8 wall units in length ( l ), resulting in l / d = 240.…”

“…(2017) and Qiao et al . (2018) for more details on how to determine the constants. A reasonable prediction is achieved, as demonstrated by Qiao et al .…”

“…A reasonable prediction is achieved, as demonstrated by Qiao et al . (2017, 2018). This is because the average length of streaky structures is approximately 1000 wall units in TBL (Robinson 1991), while the distance from the monitoring sensor to the leading edge of the actuators is only 47 wall units.…”

Near wall coherence in wall-bounded flows and implications for flow control

“…(coordinate denotes the wall-normal direction), between the sensor and flow structures that are targeted, in say the buffer layer (e.g. Choi et al, 1994;Qiao et al, 2018) or the outer layer (e.g. Abbassi et al, 2017).…”