Modeling streamwise velocity and boundary shear stress of vegetation-covered flow
TL;DR: In this article, a two-power law expression was adopted to predict the vertical profile of streamwise velocity and the influence of roughness of the floating vegetation patches and channel bed was also analyzed.
About: This article is published in Ecological Indicators.The article was published on 2017-04-16. It has received 15 citations till now. The article focuses on the topics: Shear velocity & Vegetation (pathology).
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TL;DR: In this paper, a 2D particle tracking velocimetry was used to explore aspects of the flow through and beneath suspended canopies constructed from rigid cylinders, and the experimental data showed that the penetration of the shear layer into the canopy is limited by the distance between the canopy and bottom boundary layer.
Abstract: Aquatic suspended canopies are porous obstacles that extend down from the free-surface but have a gap between the canopy and bed. Examples of suspended canopies include those formed by aquaculture structures or floating vegetation. The major difference between suspended canopies and the more common submerged canopies, which are located on the bottom boundary, is the influence of the bottom boundary layer beneath the suspended canopy. Data from laboratory experiments are presented which explore aspects of the flow through and beneath suspended canopies constructed from rigid cylinders. The experiments, using both acoustic Doppler and two-dimensional (2D) particle tracking velocimetry, give details of the flow structure that may be divided vertically into a bottom boundary layer, a canopy shear layer, and an internal canopy layer. The experimental data show that the penetration of the shear layer into the canopy is limited by the distance between the canopy and bottom boundary layer. Peaks in velocity spectra indicate an interaction between the bottom boundary and canopy shear layer. An analytical model is also developed that can be used to calculate a drag coefficient that includes the effect of both canopy drag and bed friction. This drag coefficient is suitable for use in 2D (depth-averaged) hydrodynamic modeling. The model also allows the average velocity within and beneath the canopy to be calculated, and is used to investigate the effect of canopy density and thickness on both total drag and bottom friction.
63 citations
TL;DR: In this paper, a two-domain model is proposed to divide the flow region into an upper part characterizing the flow through suspended vegetation and an inner part describing the vegetation-free zone.
Abstract: Floating treatment wetlands (FTWs) are efficient at wastewater treatment; however, data and physical models describing water flow through them remain limited. A two-domain model is proposed dividing the flow region into an upper part characterizing the flow through suspended vegetation and an inner part describing the vegetation-free zone. The suspended vegetation domain is represented as a porous medium characterized by constant permeability thereby allowing Biot's Law to be used to describe the mean velocity and stress profiles. The flow in the inner part is bounded by asymmetric stresses arising from interactions with the suspended vegetated (porous) base and solid channel bed. An asymmetric eddy viscosity model is employed to derive an integral expression for the shear stress and the mean velocity profiles in this inner layer. The solution features an asymmetric shear stress index that reflects two different roughness conditions over the vegetation-induced auxiliary bed and the physical channel bed. A phenomenological model is then presented to explain this index. An expression for the penetration depth into the porous medium defined by 10% of the maximum shear stress is also derived. The predicted shear stress profile, local mean velocity profile, and bulk velocity agree with the limited experiments published in the literature.
20 citations
16 citations
TL;DR: In this paper, the mean flow and turbulence structure of an open channel with suspended vegetation through theoretical analysis and laboratory experiments was investigated, and the measured data showed that the vertical profile of streamwise velocity obeys a two-power law and that the maximum velocity at the middle depth is close to the smooth boundary under the combined action of vegetation cover and channel bed.
Abstract: In combination with a channel bed, suspended vegetationin an open channel can alter flow structure and generate vertically asymmetric flow. This study investigated the mean flow and turbulence structure of an open channel with suspended vegetation through theoretical analysis and laboratory experiments. Three patterns of bionic leaves with different roughness were adopted to imitate suspended vegetation, and three-dimensional velocity field was measured by using an acoustic Doppler velocimeter. The measured data showed that the vertical profile of streamwise velocity obeys a two-power law and that the maximum velocity at the middle depth is close to the smooth boundary (i.e., the channel bed in the experiment) under the combined action of vegetation cover and channel bed. Shear stress is linearly distributed along the vertical axis, and the vertical profile of turbulence intensity obeys an exponential law. Then, a two-power law expression was adopted to predict the vertical profile of streamwise velocity. Theoretical models for the vertical distribution of shear stress and turbulence intensity were also established. The predicted results validated by measurements showed that the different magnitudes of vegetation cover and channel bed boundary roughness exert an obvious impact on flow structure.
6 citations
TL;DR: In this article, the vertical velocity, suspended sediment, and phosphorus concentration were measured, based on the convection-diffusion equation of sediment, the analytical solution of sediment vertical distribution given, and the coefficient of determination (R2) between the measured data and predicted data ranging from 0.9519 to 0.9777 and the root mean square error (RMSE).
Abstract: Ice cover is a common channel phenomenon in some cold regions. But most of the researchers focused on the velocity distribution; the suspended sediment and phosphorus distribution in an ice-covered channel need more attention. Six different discharge treatments were carried out in this paper; and vertical velocity, suspended sediment, and phosphorus concentration were measured. The results showed, based on the convection–diffusion equation of sediment, the analytical solution of sediment vertical distribution given, and the coefficient of determination (R2) between the measured data and predicted data ranging from 0.9519 to 0.9777 and the root mean square error (RMSE) between the measured data and predicted data ranging from 0.2959 to 0.5892, that the analytical solution has a good simulation result. Particulate phosphorus concentration (Cp) is smaller in the top layer and larger in the bottom layer, and the dissolved phosphorus concentration (Cd) is larger in the middle and small in both sides; partition coefficient (Kd) is uniform in the channel which is assumed as a constant in previous studies. This result is helpful to know the retention law of suspended sediment and phosphorus in an ice-covered channel.
6 citations
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TL;DR: In this article, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation, which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds' numbers.
Abstract: Aquatic plants convert mean kinetic energy into turbulent kinetic energy at the scale of the plant stems and branches. This energy transfer, linked to wake generation, affects vegetative drag and turbulence intensity. Drawing on this physical link, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds' numbers. The model is supported by laboratory and field observations. In addition, this work extends the cylinder-based model for vegetative resistance by including the dependence of the drag coefficient, CD, on the stem population density, and introduces the importance of mechanical diffusion in vegetated flows.
1,199 citations
TL;DR: In this article, the velocity at the edge of the viscous sublayer is used as a detector signal for bursts and sweeps, and the scaling of the mean time interval between bursts with outer flow variables is confirmed.
Abstract: Additional experimental studies of the structure of Reynolds stress which supplement our previous work (Willmarth & Lu 1971) are reported. The velocity at the edge of the viscous sublayer is again used as a detector signal for bursts and sweeps. The signal uv obtained from an X-wire probe at various locations is conditionally sampled and sorted into four quadrants of the u, v plane. Using this method it is found that, when the velocity uw at the edge of the viscous sublayer becomes low and decreasing, a burst occurs. On the other hand, a sweep occurs when uw becomes large and increasing. The convection speeds of the bursts and the sweeps are found to be equal and are about 0·8 times the local mean velocity and 0·425 times the free-stream velocity at a distance y ≈ 0·15δ* from the wall (δ* is the displacement thickness). Throughout the turbulent boundary layer, the bursts are the largest contributors to from different events. Both mean time intervals are approximately equal and constant for most of the turbulent boundary layer. The scaling of the mean time interval between bursts with outer flow variables is confirmed. It is suggested that many of the features of the fluctuating flow revealed by the measurements may be explained by convection past the measuring station of an evolving deterministic flow pattern such as the hairpin vorticity model of Willmarth & Tu (1967).
1,050 citations
TL;DR: In this article, the influence of roughness caused by aquatic vegetation (av), in particular submerged macrophytes, on the overall flow field was analyzed, where the authors focused on the definition of a characteristic hydraulic roughness parameter to quantify the resistance of av.
246 citations
TL;DR: In this paper, the authors present the results of a detailed experimental examination of fully developed asymmetric flow between parallel planes, which was introduced by roughening one of the planes while the other was left smooth; the ratio of the shear stresses at the two surfaces was typically about 4:1.
Abstract: The paper presents the results of a detailed experimental examination of fully developed asymmetric flow between parallel planes. The asymmetry was introduced by roughening one of the planes while the other was left smooth; the ratio of the shear stresses at the two surfaces was typically about 4:1.The main emphasis of the research has been on establishing the turbulence structure, particularly in the central region of the channel where the two dissimilar wall flows (generated by the smooth and rough surfaces) interact. Measurements have included profiles of all non-zero double and triple velocity correlations; spectra of the same correlations at several positions in the channel; skewness and flatness factors; and lateral two-point space correlations of the streamwise velocity fluctuation.The region of greatest interaction is characterized by strong diffusional transport of turbulent shear stress and kinetic energy from the rough towards the smooth wall region, giving rise, inter alia, to an appreciable separation between the planes of zero shear stress and maximum mean velocity. The profiles of length scales of the larger-scale motion are, in contrast to the turbulent velocity field, nearly symmetric. Moreover, it appears that at high Reynolds numbers the small-scale motion may in many respects be treated as isotropic.
226 citations
01 Jan 2004
TL;DR: In this paper, the authors show that the dispersive flux terms represent a contribution to momentum transfer arising from spatial correlations of the time-averaged velocity components within a hori- zontal plane embedded in the canopy sublayer.
Abstract: Dispersive flux terms are formed when the time-averaged mean momentum equation is spatially averaged within the canopy volume. These fluxes represent a contribution to momentum transfer arising from spatial correlations of the time-averaged velocity components within a hori- zontal plane embedded in the canopy sublayer (CSL). Their relative importance to CSL momentum transfer is commonly neglected in model calculations and in nearly all field measurement interpreta- tions. Recent wind-tunnel studies suggest that these fluxes may be important in the lower layers of the canopy; however, no one study considered their importance across all regions of the canopy and for a wide range of canopy roughness densities. Using detailed laser Doppler anemometry measurements conducted in a model canopy composed of cylinders within a large flume, we demonstrate that the dispersive fluxes are only significant (i.e., > 10%) for sparse canopies. These fluxes are in the same direction as the turbulent flux in the lower layers of the canopy but in the opposite direction near the canopy top. For dense canopies, we show that the dispersive fluxes are < 5% at all heights. These results appear to be insensitive to the Reynolds number (at high Reynolds numbers).
130 citations