Vortex-Resistance Hypothesis: Large Eddy Simulation of Turbulent Flow in Isolated Pool- Riffle Units

Abstract

By numerical simulations of turbulent flow in isolated poolriffle units with various riffle heights, four different types of vortices were found and named as follows: surface rollers (SR), corner rollers (CR), ramp rollers (CR), and axial tails (AT). Surface rollers are shaped on the flow surface due to submerged hydraulic jump or any obstacles in the forced poolriffle units. Corner rollers are shaped close to the corners near the walls at the pool head. Ramp rollers are formed at the bed of the channel on the ramp into the head of the pool. All kinds of vortices stretch in the streamwise direction as they travel to the downstream, which they are called axial tails. The simulations showed that all four types of vortices interact with each other, combine, amplify or cancel out each other as they travel downstream. The strength of vortices and how they interact result into different types of flow patterns. The surface rollers combine with corner rollers to make a jet like plunging flow near the pool bed. In other cases with lower riffle heights, ramp rollers tend to push the flow up, which in turn leads to higher turbulence near the bed and higher velocity near the flow surface (skimming flow). Moreover, if both surface rollers and ramp rollers have the similar strength (e.g., vorticity) and scale, the streamwise velocity profile has a peak around the middle of the flow, and minimum velocities near the bed and free surface. This flow pattern was named as “rifting flow.” Based on these findings, a new hypothesis is proposed called ‘vortex-resistance,’ which states that the turbulent structures, by increasing the eddy viscosity and changing the pressure domain, act as an obstacle that steers the flow. Plunging and skimming flow can thus be understood as the products of different types of turbulent structures. These findings provide new clarifications to long-standing questions related to the hydraulics of pools and riffles.

One Flood Is Not Enough: Pool‐Riffle Self‐Maintenance Under Time‐Varying Flows and Nonequilibrium Multifractional Sediment Transport

“…This observation is in agreement with previous studies (e.g. Dashtpeyma & Macvicar, 2018; de Almeida & Rodríguez, 2011). Dashtpeyma and Macvicar (2018) reported supercritical flow and strong vortex shedding in numerical simulations of pool‐riffle units with a slope of 0.001 and constant flow.…”

“…Dashtpeyma & Macvicar, 2018; de Almeida & Rodríguez, 2011). Dashtpeyma and Macvicar (2018) reported supercritical flow and strong vortex shedding in numerical simulations of pool‐riffle units with a slope of 0.001 and constant flow. Table 1 shows that our feeding methodology resulted on the same value of the relative feeding (feeding weight/hydrograph volume) of 0.04 kg/m 3 for Exps 1, 2 and 4 and a higher value of 0.06 kg/m 3 for Exp 3.…”

Amplification of Plunging Flows in Bedrock Canyons

“…Allen, 2007; Alvarez et al., 2017; Schmidt, 1990), suggesting this is a common occurrence. The velocity dip phenomenon in alluvial riffle‐pool sequences has been ascribed to the presence of large‐scale coherent flow structures that induce convective deceleration of the water surface, impeding downstream flow and pushing the CMV toward the bed (Dashtpeyma & MacVicar, 2018; MacVicar et al., 2013; MacVicar & Rennie, 2012). Our observations suggest that plunging flows may be enhanced or even caused by large‐scale coherent flow structures that decelerate flow, although the backwater‐spilling flow mechanism likely sets up the conditions necessary for plunging flows to occur and to carve deep scour pools in bedrock canyons (cf.…”

Plunging Flow and Coherent Turbulent Structures in a Straight Pool‐Riffle

“…A series of reductionist Froude‐scaled experiments in both smooth and rough channels showed that many key flow features of pool‐riffles could be reproduced in relatively simple pool‐riffle geometries (B. J. MacVicar & Best, 2013; B. J. MacVicar & Obach, 2015; B. J. MacVicar & Rennie, 2012). Preliminary work on similar geometries was completed with large eddy simulation (LES) numerical models (Dashtpeyma & MacVicar, 2018; Stoesser et al., 2010; Tokyay & Sinha, 2020) because this approach is effective for investigating geophysical flows where non‐uniform flow and turbulent mixing occurs such as in shear zones at confluences and meanders (Alvarez & Grams, 2021; Bradbrook et al., 2000; Duguay et al., 2022; Jiang et al., 2022; Kang & Sotiropoulos, 2011; Keylock et al., 2012). In the current study we further develop the LES approach for investigating pool‐riffle hydrodynamics by varying the riffle height in a series of cases designed to approach the threshold for critical flow and induce a possible regime change in the flow.…”