Mixing layer and coherent structures in compound channel flows: Effects of transverse flow, velocity ratio, and vertical confinement

Abstract

Turbulent mixing layers associated with streamwise uniform and nonuniform flows in compound channels (main channel with adjacent floodplains) are experimentally investigated. The experiments start with uniform flow conditions. The streamwise nonuniformity is then generated by imposing an imbalance in the upstream discharge distribution between main channel (MC) and floodplains (FPs), keeping the total discharge constant, which results in a transverse depth‐averaged mean flow. This study first aims at assessing the effect of a transverse flow on the mixing layer and coherent structures that form at the MC/FP interfaces. A wide range of initial velocity ratio or dimensionless shear between MC and FP is tested. The study second aims at assessing the effect of this velocity ratio on the mixing layer, for a fixed vertical confinement of flow. The total discharge was then varied to quantify the confinement effect. The results show that, far from the inlet section, Reynolds‐stresses increase with local velocity ratio for a fixed confinement and decrease with confinement for a fixed velocity ratio. It is also shown that, irrespective of confinement, the existence of quasi‐two‐dimensional coherent structures is driven by velocity ratio and the direction and magnitude of transverse flow. These structures cannot develop if velocity ratio is lower than 0.3 and if a strong transverse flow toward the MC occurs. In the latter case, the transverse flow is the predominant contribution to momentum exchange (compared with turbulent mixing and secondary currents), convex mean velocity profiles are observed, preventing the formation of quasi‐two‐dimensional structures.

The Interplay between Flow Field, Suspended Sediment Concentration, and Net Deposition in a Channel with Flexible Bank Vegetation

“…In rivers and streams, vegetated riparian zones in the streamwise direction are common. The vegetated areas induce flow resistance and influence the mean and turbulent properties of the flow, resulting in a velocity difference between the main channel and adjacent vegetation, which can result in a strong mixing layer [1,2]. The mixing layer plays an important role in redistributing momentum and mass between the main channel and vegetated areas [1,3,4].…”

“…The vegetated areas induce flow resistance and influence the mean and turbulent properties of the flow, resulting in a velocity difference between the main channel and adjacent vegetation, which can result in a strong mixing layer [1,2]. The mixing layer plays an important role in redistributing momentum and mass between the main channel and vegetated areas [1,3,4]. The mixing efficiency is expected to be affected by mean flow velocities and foliage, which regulates the suspended sediment load between the main channel and vegetated floodplain [3].…”

Lateral bed-roughness variation in shallow open-channel flow with very low submergence

“…2). According to Proust et al [23] for compound channels, when the ratio based on the bulk velocities of the two sides = (U 1 − U 2 )∕(U 1 + U 2 ) > 0.3 , largescale turbulent structures with a vertical axis of rotation develop due to Kelvin-Helmholtz shear instability. Here, taking the effective bulk velocities U e,i , = {0.16, 0.44, 0.70} for h∕k = {3, 2, 1.5} , respectively, suggesting that h∕k = 2 and 1.5 could be subject to the Kelvin-Helmholtz shear instability.…”

“…These studies concluded that the growth of the shear layer is suppressed by the bottom friction and therefore, the exchange due to coherent turbulent structures is reduced for smaller depths, higher roughness and lower velocity difference between the parallel flows. On the other hand, Proust et al [23] showed that in a compound channel, where the velocity difference is sustained by the difference in the flow depth, the coherent structures persist even for very shallow conditions. Proust et al [23] proposed that these structures are controlled only by the difference in the bulk velocities over the flood plain, U 2 , and the main channel, U 1 , expressed as a velocity ratio (also referred to as dimensionless shear), = (U 1 − U 2 )∕(U 1 + U 2 ) .…”

“…On the other hand, Proust et al [23] showed that in a compound channel, where the velocity difference is sustained by the difference in the flow depth, the coherent structures persist even for very shallow conditions. Proust et al [23] proposed that these structures are controlled only by the difference in the bulk velocities over the flood plain, U 2 , and the main channel, U 1 , expressed as a velocity ratio (also referred to as dimensionless shear), = (U 1 − U 2 )∕(U 1 + U 2 ) . For > 0.3 , large-scale turbulent structures were observed provided the presence of an inflection point in the mean longitudinal velocity profile.…”