Turbulence statistics and very-large-scale motions in decelerating open-channel flow

Abstract

This study experimentally examined the turbulent characteristics of decelerating open-channel flow based on particle imaging velocimetry measurement. The decelerating flow shows a similar velocity profile to that in uniform flow, but it exhibits greater turbulence intensity and Reynolds stress. Statistical evidence of the presence of very-large-scale motions (VLSMs) in decelerating open-channel flows was presented for the first time. The results indicate that VLSMs in decelerating flows can survive further away from the wall when compared to other wall-turbulence flows. The contribution rate of the VLSMs to the turbulent kinetic energy and the Reynolds stress in the decelerating open-channel flow is slightly lower than that in channels, boundary layers, and pipe flows.

Reynolds stress modeling of supercritical narrow channel flows using OpenFOAM: Secondary currents and turbulent flow characteristics

“…The secondary currents and the related turbulence modifications in narrow channels deviate the numerical and experimental profiles from the theoretical ones. Furthermore, the decelerating flow conditions in the experiments possibly have increased the u rms /u *l and w rms /u *l values (Kironoto and Graf, 1995;Song and Chiew, 2001;Zhang et al, 2019). Basically, u rms /u *l and v rms /u *l values decrease upward up to around the velocity dip-positions but increase thereafter apparently because of the free surface vortex (and velocity dips) and of the free surface damping and redistribution of w rms .…”

“…Recently, Demiral et al (2020) reported four well-developed vortices at each half of the channel width under decelerating flow conditions for Fr from 1.84 to 3.33 and a r from 0.89 to 1.91. The comparatively larger inner secondary vortex reported by Demiral et al (2020) is possibly influenced by the flow nonuniformity, which can alter the turbulent stresses (Song and Chiew, 2001;Zhang et al, 2019) and can cause surface undulations for supercritical flows (Auel et al, 2014). However, their study as well as the studies by Auel et al (2014); Jing et al (2019) were based on two-dimensional (2D) velocity measurements and the lateral velocity measurements were absent.…”

“…First three simulations investigate the effect of a r on the flow characteristics and validate the simulation results while the last two simulations (selected after running trial simulations between a r 0.9 and 1.25) solely explore the formation of intermediate vortex. for the turbulence parameters (Song and Chiew, 2001;Zhang et al, 2019). As the flow nonuniformity is higher for the case Exp_0.9, only the mean velocities and the bed shear stress are compared for the case RSM_0.9.…”