Extraction and verification of coherent structures in near-wall turbulence

Abstract

According to the characteristics of coherent structures in near-wall turbulence, an accurate extraction and verification method is developed based on wavelet transform (WT) and correlation analysis in this paper. At first, the fluid field of a turbulent boundary layer is measured precisely in a gravitational low-speed water tunnel. On the basis of the distribution of the coherent structures, velocity data of three test points are selected and analyzed, whose dimensionless heights are 20.8, 33.5, and 42.6. According to the frequency range of power spectrum density (PSD), coherent and incoherent structures are both extracted from the original signals using continuous and orthogonal wavelet transforms. To confirm the validity of the extracted signals, the probability density function (PDF) of each extracted signal is calculated. The result demonstrates that the incoherent structures obey the Gaussian distribution, while the coherent structures deviate from the Gaussian distribution. The PDFs of the coherent structures and the original signals are similar, which shows that the coherent structures make most contributions to the turbulence. For further verification, a correlation parameter between coherent and incoherent structures is defined, which evidently proves the validity of the extraction method in this paper.

Active control of wall-bounded turbulence for drag reduction with piezoelectric oscillators

“…[4][5][6][7][8][9][10] Active control can adapt to changes in flowing conditions and realize optimal control. [11][12][13][14][15][16][17][18][19] Modern micro-electromechanical systems (MEMS) provide powerful tools for active control. [20,21] Among various active control schemes, wall deformation is a reliable and efficient solution.…”

Extracting Coherent Structures in Near-Wall Turbulence Based on Wavelet Analysis

“…To extract the coherent structures in turbulence, the signals at the central area of turbulence should be selected. According to previous studies [20][21][22], the formation of the coherent structures in turbulence is formed in the area of 0 < y þ < 30, and the self-sustaining of the coherent structures is in the area of 20 < y þ < 60. As a result, three testing positions with the y þ 20.8, 33.5, and 42.6 were selected, whose fluctuating velocity signals are shown in Figure 6.…”

Predetermined control of turbulent boundary layer with a piezoelectric oscillator

“…Coherent structure, i.e., multi-scale quasi-order spatiotemporal structure in a self-sustaining mechanism, plays a key role in the dynamics of the TBL. [1][2][3][4][5][6] Among them, stream-wise vortices near the wall are responsible for high skin friction regions underneath the TBL, [7,8] as shown in Fig. 1.…”