Hydrodynamic behavior in the outer shear layer of partly obstructed open channels

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Turbulence measurement of a dense jet perpendicularly issued into a crossflow is investigated experimentally. The flow-velocity components were extensively measured with a high frequency Acoustic Doppler Velocimeter (ADV) system, whereas, a Micro Scale Conductivity Temperature instrument was used to measure the jet salinity. Special attention is given to understand the jet flow-structures in the flow symmetry plane. The flow velocity-fields, the jet trajectory, the turbulence intensities, the turbulent kinetic energy, the turbulent length scales, and the dispersion coefficients have been analyzed. The flow velocity-fields show that the dense jet is characterized by two distinct regions: an ascending region, of jet-like mixing, and a descending region of plume-like mixing. In this study, a new scaling approach of the jet trajectories, based on the jet characteristic length scales, is proposed, leading to an empirical closed-form expression to predict the jet trajectory. The turbulence analysis shows that the jet is accompanied by high levels of flow-turbulence intensities and large kinetic energy production. The results of the turbulent length scales indicate that the ambient flow-field, without jet effect, is an isotropic process. However, in the jet flow-field, a significant spatial-variation of the turbulent length scales was observed, indicating an anisotropic process. The trends of the dispersion coefficients follow those of the turbulent length scales. In comparison with the ambient flow, the jet flow-field shows a decrease of the longitudinal dispersion coefficient and an increase of the vertical one, leading to the increase of the jet width.

Section: Experimental Methodsmentioning

confidence: 99%

Turbulence Measurement of Vertical Dense Jets in Crossflow

Meftah

Mossa

2018

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“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Box

Västilä

Järvelä

2019

This paper investigates the interplay between the flow, suspended sediment concentration (SSC), and net deposition at the lateral interface between a main channel and riverbank/floodplain vegetation consisting of emergent flexible woody plants with understory grasses. In a new set of flume experiments, data were collected concurrently on the flow field, SSC, and net deposition using acoustic Doppler velocimeters, optical turbidity sensors, and weight-based sampling. Vegetation largely affected the vertical SSC distributions, both within and near the vegetated areas. The seasonal variation of vegetation properties was important, as the foliage strongly increased lateral mixing of suspended sediments between the unvegetated and vegetated parts of the channel. Foliage increased the reach-scale net deposition and enhanced deposition in the understory grasses at the main channel–vegetation interface. To estimate the seasonal differences caused by foliation, we introduced a new drag ratio approach for describing the SSC difference between the vegetated and unvegetated channel parts. Findings in this study suggest that future research and engineering applications will benefit from a more realistic description of natural plant features, including the reconfiguration of plants and drag by the foliage, to complement and replace existing rigid cylinder approaches.

“…Among the many available models, the simple model created by Nepf [25] for the case of a dense array of emergent rigid cylinders provides reasonable and effective results [46,47]. Therefore, this approach is used in the present study for the estimation of C D in the uniform flow experiments.…”

Section: The Theoretical Modelmentioning

confidence: 99%

Wave Height Attenuation and Flow Resistance Due to Emergent or Near-Emergent Vegetation

Peruzzo

Serio

Defina

et al. 2018

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Vegetation plays a pivotal role in fluvial and coastal flows, affecting their structure and turbulence, thus having a strong impact on the processes of transport and diffusion of nutrients and sediments, as well as on ecosystems and habitats. In the present experimental study, the attenuation of regular waves propagating in a channel through flexible vegetation is investigated. Specifically, artificial plants mimicking Spartina maritima are considered. Different plant densities and arrangements are tested, as well as different submergence ratios. Measurements of wave characteristics by six wave gauges, distributed all along the vegetated stretch, allow us to estimate the wave energy dissipation. The flow resistance opposed by vegetation is inferred by considering that drag and dissipation coefficients are strictly related. The submergence ratio and the stem density, rather than the wave characteristics, affect the drag coefficient the most. A comparison with the results obtained in the case when the same vegetation is placed in a uniform flow is also shown. It confirms that the drag coefficient for the canopy is lower than for an isolated cylinder, even if the reduction is not affected by the stem density, underlining that flow unsteadiness might be crucial in the process of dissipation.