Active control of multiscale features in wall-bounded turbulence

Abstract

This study experimentally investigates the impact of a single piezoelectric (PZT) actuator on a turbulent boundary layer from a statistical viewpoint. The working conditions of the actuator include a range of frequencies and amplitudes. The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer. The mean velocity profiles and other basic parameters are reported. Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region. The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy. The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales. Moreover, under the impact of the PZT actuator, the symmetry of vibration scales' velocity signals is promoted and the structural composition appears in an orderly manner. Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency. Based on the flatness factor, the bursting process is also detected. The vibrations reduce the relative intensities of the burst events, indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control.

Effect of high- or low-speed fluctuations on the small-scale bursting events in an active control experiment*

“…The results confirm that with f p kept constant, the drag reduction effect is positively correlated with A p . [21,23] As shown in Fig. 2, the velocity curve of the uncontrolled case fits well with linear law and log law, indicating a full developed TBL in the measuring location.…”

“…About the local intermittent bursting event analysis, two principal indicators are employed to extract the bursting signals in current study, which have been confirmed by former works. [6,16,21] Flatness factor is defined as follows: [42] FF(r) = w(r,t)…”

“…Then, we make some definitions for clarity. Based on the previous study, [6,21] the PZT perturbed frequency f p = 160 Hz is between the characteristic frequencies of scales r = 8 and r = 9. Thus, the perturbed-scale mode is estimated by u P = w(8) + w (9), which represents the periodical forcing input by the actuator.…”

“…7(a)-7(c), the ratios of u brms,S and u rms,S in different cases are calculated to describe the relative importance of the bursting process. [21] Here, u brms,S represents the mean rms values of the small-scale bursting signals, and u rms,S represents the mean rms values of the small-scale fluctuations. As can be seen, the perturbation apparently weakens the small-scale bursting process in the near-wall region (about y + < 30) in all controlled cases, and case A has the best performance.…”

“…[6,[18][19][20] As mentioned above, quasi streamwise vortices (QSV) are the typical coherent structures, which contain ejection and sweep processes. [21][22][23][24] The ejection process occurs on the updraft side of QSVs, wherein the low-speed fluids in the inner layer are lifted away from the wall to the outer layer. The sweep process occurs on the downdraft side of QSVs, wherein the high-speed fluids move downward to the wall.…”