Study on the morphological characteristics of large-scale structures in open-channel flows based on tomographic particle image velocimetry

The investigation of the three-dimensional morphology of large-scale structures is crucial for advancing the understanding of the physical mechanisms underlying turbulent transport. Tomographic particle image velocimetry measurements were performed to investigate the three-dimensional morphological characteristics of large-scale turbulent structures in open-channel flows. Taylor's frozen turbulence hypothesis and the two-point correlation method were employed to directly infer the three-dimensional morphology of large-scale turbulent structures. The main attention is paid to the wall-normal evolution of the structural inclination angle and spatial length scale. The Reynolds number invariance of the structural morphological characteristics was also investigated, which provided useful information for speculating on the three-dimensional shapes of large-scale structures in open-channel flows. The results indicated that the large-scale structures of open-channel flows exhibited an inclination angle ranging from 10° to 30°. The free surface in open-channel flows did not exert a decisive influence on the variation of the structural inclination angle; however, it influenced the distribution of the wall-normal length scale of large-scale structures in the near-surface region. A piecewise linear increase in the spanwise width scale was observed with increasing water depth, with a slope of 0.1 in the logarithmic region and a slope of 0.3 for y > 0.4H. The variation of the streamwise length scale within the logarithmic region aligned closely with the findings observed in the atmospheric boundary layer. The streamwise and wall-normal length scales decreased as the Reynolds number increased. The spanwise width scale and inclination angle of large-scale structures in open-channel flows all exhibited Reynolds number invariance. This study will facilitate settling the severe debate on the existence of large-scale structures and lay the foundation for the wall-bounded turbulence theory.