Contributions of different scales of turbulent motions to the mean wall-shear stress in open channel flows at low-to-moderate Reynolds numbers

Smooth-walled open channel flow datasets, covering both the direct numerical simulation and experimental measurements with a friction Reynolds number Re-tau at a low-to-moderate level of 550 similar to 2400, are adopted to investigate the contributions of different scale motions to the mean wall-shear stress in open channel flows (OCFs). The FIK identity decomposition method by Fukagata et al. (Phys. Fluids, vol. 14, 2002, L73) combined with a scale decomposition is chosen for this research. To see whether/how the contributions in OCFs differ with those in closed channel flows (CCFs), comparisons between the two flows are also made. The scale-decomposed 'turbulent' contribution results of present OCFs exhibit two dominant contribution modes (i.e. large-scale motions (LSMs) and very-large-scale motions (VLSMs)) at a streamwise wavelength lambda(x) = 1 similar to 2h and O(10h), where h is the water depth. The large scales with lambda(x) > 3h and lambda(x) > 10h are demonstrated to contribute to over 40% and 20% of the mean wall-shear stress, respectively. Compared with CCFs, slightly higher and lower contributions in the lambda(x) > O(10h) and lambda(x) < O(10h) wavelength ranges are observed in OCFs, revealing the important free-surface effects in OCFs. Possible mechanisms are discussed to lend support for the observed differences between the two flows.

Automated summary

This study investigates the contributions of different scale motions to the mean wall-shear stress in smooth-walled open channel flows, revealing the significant impact of large-scale and very-large-scale motions on the wall-shear stress. Comparisons with closed channel flows show slightly higher and lower contributions in specific wavelength ranges in open channel flows, indicating the important role of free-surface effects in open channel flows. Potential mechanisms are discussed to support the observed differences between the two types of flows.