Characteristics of Large-Scale Coherent Structures on Irregularly Arranged Rough-Bed Open-Channel Flows

Abstract

Large-scale coherent structures (LSCSs) in rough-bed open-channel flow (OCF) are significant in turbulence research. A recent breakthrough is the bimodal feature of LSCSs on regular rough-bed OCF (i.e., LSCSs exhibit two typical motions: large-scale motions (LSMs) and very-large-scale motions (VLSMs)). However, the presence and characteristics of LSMs and VLSMs in irregularly arranged rough-bed OCF remain unclear. Thus, in this study, high-precision indoor flume experiments were performed under typical irregularly arranged rough-bed conditions, and time-resolved particle image velocimetry was used for velocity measurements. Statistical quantities of velocity fluctuations revealed that the friction Reynolds number and roughness exerted a certain modulation on the velocity fluctuating properties. The spectra of velocity fluctuations provided direct and statistical evidence for the presence of LSMs and VLSMs in irregularly arranged rough-bed OCF. VLSMs contributed more than 60% of the streamwise turbulent kinetic energy and 40% of the Reynolds shear stress in the outer region of the irregularly arranged rough-bed OCF, which was slightly higher than that in the smooth-bed or regular rough-bed OCF scenarios. No apparent dependence of the wavelength of VLSMs on the flow submergence (H/d50) was observed in the present irregularly arranged rough-bed OCF, which is in contrast to that reported for regular rough-bed OCF. Furthermore, the relationship between the peak wavelength of VLSMs and the aspect ratio did not strictly follow a linear increase, in contrast to that documented in the literature.