Open-channel flows through emergent rigid vegetation: Effects of bed roughness and shallowness on the flow structure and surface waves - Archive ouverte HAL Access content directly
Journal Articles Physics of Fluids Year : 2021

Open-channel flows through emergent rigid vegetation: Effects of bed roughness and shallowness on the flow structure and surface waves

Écoulements à surface libre au travers de végétation rigide émergée: effets de la rugosité de fond et du confinement vertical sur la structure des écoulements et sur les ondes de surface

Abstract

Free-surface flows through a staggered cylinder array were investigated in an open-channel flume. The cylinders simulated rigid emergent vegetation. Specifically, we studied four flow cases with a two-factor design comprising flow rate (7 and 13 l s−1) and bed-surface state (hydraulically rough and smooth). We have primarily assessed the effects of bed roughness and shallowness on the time-averaged flow structure and the transverse fluctuating flow motion in the cylinder wake. Secondarily, the effects of the former on the vortex-shedding-induced surface waves were quantified. To gain further insight into the bed roughness effect on flow structure, we conducted transient flow simulations using a hybrid Reynolds-Averaged Navier–Stokes/Large Eddy Simulation turbulence model. For all cases, downstream of a cylinder, an upward flow occurs and two counterrotating secondary current cells develop. The two cells bring high-momentum fluid from the high-speed region into the cylinder wake, resulting in a near-bed streamwise velocity-bulge. The measured upward flow and velocity-bulge are greater for the rough-bed cases than for the smooth-bed cases. The simulated upward flow and velocity-bulge increase with an increasing roughness height, while secondary currents decay faster in the longitudinal direction. For the rough-bed cases, in the cylinder wake, the transverse fluctuating flow motion is hindered by the rough-bed induced turbulence over the whole water column, irrespective of the shallowness level. Coupled with the fluctuating flow motion, we have observed for three flow cases noticeable surface oscillations (termed “seiche waves”), whose amplitude decreases with decreasing flow depth. Under the combined effects of strong shallowness and a rough bed, seiche waves vanished.
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Dates and versions

hal-03369211 , version 1 (07-10-2021)

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M. Chatelain, S. Proust. Open-channel flows through emergent rigid vegetation: Effects of bed roughness and shallowness on the flow structure and surface waves. Physics of Fluids, 2021, 33 (10), pp.106602. ⟨10.1063/5.0063288⟩. ⟨hal-03369211⟩
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