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Wenxin Huai - One of the best experts on this subject based on the ideXlab platform.
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analytical solutions of velocity profile in flow through Submerged Vegetation with variable frontal width
Journal of Hydrology, 2019Co-Authors: Weijie Wang, Wenxin Huai, Yufei Wang, Ping Wang, Jiao ZhangAbstract:Abstract Flow within Vegetation is one of the main driving forces for material exchange and energy transfer in wetland systems. Impacted by Vegetation, the flow velocity profile illustrates distortions to the classic logarithmic velocity profile and has attracted much attention among researchers. Different from analytical models of velocity distribution in literature, which is mainly suitable for Vegetation with uniform frontal width, this paper establishes new analytical solutions of the velocity profile for Vegetation such as shrub and sedge that have a variable frontal width in the vertical direction. A new shape function is proposed under these conditions in which the frontal width exhibits a gradual increase in the vertical direction from bottom up in the Vegetation. Along with different closure models for eddy viscosity in the Vegetation layer and surface layer, analytical solutions of the velocity profile are derived from the momentum equations. Good agreement between calculated and measured data shows our analytical model is effective in predicting velocity profiles.
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predicting the bulk average velocity of open channel flow with Submerged rigid Vegetation
Journal of Hydrology, 2019Co-Authors: Haoran Shi, Wenxin Huai, Xuerong Liang, Yufei WangAbstract:Abstract Predicting the bulk cross-sectional average flow velocity in open channels with Submerged Vegetation is an important topic in river engineering. Researchers have proposed numerous theoretical and empirical formulae, but the accuracy and physical basis of their solutions still need improvement. This study separates the flow into Vegetation layer and surface layer, following conventional two-layer approach, and estimates the average velocities in these two layers separately. In the Vegetation layer, force balance equation provides the basement of the estimation. And in the surface layer, we use genetic programming (GP), a data-driven method. A Darcy–Weisbach-coefficient-like parameter is proposed for the surface layer, which is related to other parameters through the GP algorithm. The maximum dissimilarity algorithm (a data-clustering algorithm) is used to separate the existing data sets in the training, validation, and testing groups to feed GP algorithm. Finally, by weighted combination, a new velocity formula with high accuracy and physical basis is proposed for Submerged vegetated flow.
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roughness height of Submerged Vegetation in flow based on spatial structure
Journal of Hydrodynamics, 2018Co-Authors: Weijie Wang, Wenqi Peng, Wenxin Huai, Fei Dong, Jian FengAbstract:The classic hydraulic resistance formulas, such as those in the Darcy-Weisbach methods, perform well in the hydraulic design with the characteristic roughness height ks smaller than the flow depth, which can be linked with the momentum roughness height based on the turbulent boundary-layer theory with the log-law formulation. However, when the roughness scale is of the same order as the flow depth, the traditional log-law formulation cannot provide satisfactory results because the flow structure is complicated and the vortices in different layers are dominated by various principles, such as the Karman streets near the channel bed, the mixing layer near the Vegetation top, and a canonical turbulent boundary layer above the Vegetation layer. Thus, the distribution of the streamwise velocity in the vegetated flow is a combination of the velocity profile linked with the dominant vortex and shows significant differences as compared with the traditional log-law distribution. This paper proposes a new characteristic roughness height of Vegetation kv by linking Vegetation attributes, especially the characteristics of the cross section in the flow within the Vegetation. The power law resistance formula is derived based on a large amount of experimental data. Results show that the new formula is applicable to shallow flows with Vegetation.
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hydrodynamics of discontinuous rigid Submerged Vegetation patches in open channel flow
Journal of Hydro-environment Research, 2016Co-Authors: Fang Zhao, Wenxin HuaiAbstract:Abstract We investigate the effects of discontinuous rigid Submerged Vegetation patches on flow turbulence. Two laboratory flume experiments are performed to validate the large eddy simulation (LES) model. The obtained LES data are in good agreement with the experimental data. They are also highly accurate in capturing the secondary peaks of the mean velocity near the channel bed. The coherent vortices, which are generated by the shear between the slower canopy flow and the faster overlying flow, are associated with the velocity inflection and maximum Reynolds stress around the interface. The mean velocity in the gap regions is evidently slower than that in the canopy regions. A high canopy density and Reynolds number are more conducive for the fully developed flow state of discontinuous Vegetation patches. The velocity distinctly increases within the first two patches with a high canopy density. The velocity profile in the large gaps is more stable than that in the small gaps below the Vegetation height, whereas the effect of patch distribution is not evident in the overlying flow layer. A spectral analysis shows that two vortex scales, namely, stem-scale and shear-scale vortices, influence the turbulence of flow through discontinuous Vegetation patches. The power spectral densities are consistent with Kolmogorov theory for a −5/3 slope when the dominant eddy frequency exceeds 0.04 Hz.
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analytical solution for vertical profile of streamwise velocity in open channel flow with Submerged Vegetation
Environmental Fluid Mechanics, 2013Co-Authors: Wenxin Huai, Jie HanAbstract:A three-zone model was constructed and applied to study vertical profiles of streamwise velocity in steady uniform, open-channel flows with Submerged Vegetation. Three zones are examined—lower Vegetation, upper Vegetation and non-vegetated. Dominant forces acting on the water body were mainly gravity, Vegetation drag and Reynolds stress. The latter was estimated by mixing length theory. A power series method was used to solve the governing differential equation of the upper Vegetation zone. Other governing equations for the remaining two zones were directly solved analytically, deriving formulas for calculating the streamwise velocities. Values calculated with the formulas agreed well with measured experimental data, which demonstrates the practical applicability of the model.
Heidi Nepf - One of the best experts on this subject based on the ideXlab platform.
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wake structure and sediment deposition behind models of Submerged Vegetation with and without flexible leaves
Advances in Water Resources, 2018Co-Authors: Jiarui Lei, Chao Liu, Heidi NepfAbstract:Abstract This laboratory study explored the flow structure and pattern of deposition directly downstream of Submerged patches of Vegetation, focusing on whether, or not, the presence of flexible trailing leaves enhanced deposition. Both leaves of different length and patches of different geometry (circular and channel-spanning) were considered. The study defined the length of wake within which the velocity was diminished and net deposition was enhanced. The model sediment represented an organic or mineral solid smaller than fine sand. For a channel-spanning, Submerged patch, recirculation or turbulent diffusion in the x-z (streamwise-vertical) plane set the wake length within which velocity was diminished and deposition was enhanced. This length was greater for patches with lower stem densities, because greater flow through the patch displaced the recirculating eddy farther downstream. In addition, for a channel-spanning patch, the presence of flexible trailing leaves extended the wake length, which in turn increased the length of the deposition region. In contrast, for a circular patch, the wake contained oscillations in the x-y (streamwise – lateral) plane. The onset of the wake unsteadiness set the length of the deposition region. Because the presence of flexible trailing leaves on a circular patch did not affect the formation distance for the wake oscillation, the length of the deposition region was unchanged with the addition of the flexible trailing leaves. For both circular and channel-spanning patches, a longer deposition region was associated with a larger deposition mass.
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vortex structure and sediment deposition in the wake behind a finite patch of model Submerged Vegetation
Journal of Hydraulic Engineering, 2018Co-Authors: Chao Liu, Jiarui Lei, Heidi NepfAbstract:AbstractA Submerged patch of finite-width Vegetation may produce vortex structures in its wake in both the vertical and horizontal planes. This experimental study used velocity and deposition measu...
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mean and turbulent velocity fields near rigid and flexible plants and the implications for deposition
Journal of Geophysical Research, 2013Co-Authors: Andrew D Ashton, Alejandra C Ortiz, Heidi NepfAbstract:[1] The transport of fine sediment and organic matter plays an important role in the nutrient dynamics of shallow aquatic systems, and the fate of these particles is closely linked to Vegetation. We describe the mean and turbulent flow near circular patches of synthetic Vegetation and examine how the spatial distribution of flow is connected to the spatial distribution of suspended sediment deposition. Patches of rigid, emergent, and flexible, Submerged Vegetation were considered, with two different stem densities. For the rigid emergent Vegetation, flow adjustment was primarily two-dimensional, with flow deflected in the horizontal plane. Horizontal shear layers produced a von Karman vortex street. Flow through the patch shifted the vortex street downstream, resulting in a region directly downstream of the patch in which both the mean and turbulent velocities were diminished. Net deposition was enhanced within this region. In contrast, for the flexible, Submerged Vegetation, flow adjustment was three-dimensional, with shear layers formed in the vertical and horizontal planes. Because of strong vertical circulation, turbulent kinetic energy was elevated directly downstream of the patch. Consistent with this, deposition was not enhanced at any point in the wake. This comparison suggests that morphological feedbacks differ between Submerged and emergent Vegetation. Further, enhanced deposition occurred only in regions where both turbulent and mean velocities were reduced, relative to the open channel. Reduced deposition (indicating enhanced resuspension) occurred in regions of high turbulence kinetic energy, regardless of local mean velocity. These observations highlight the importance of turbulence in controlling deposition.
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Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies
Transport in Porous Media, 2009Co-Authors: Marco Ghisalberti, Heidi NepfAbstract:Aquatic flow over a Submerged Vegetation canopy is a ubiquitous example of flow adjacent to a permeable medium. Aquatic canopy flows, however, have two important distinguishing features. Firstly, Submerged Vegetation typically grows in shallow regions. Consequently, the roughness sublayer, the region where the drag length scale of the canopy is dynamically important, can often encompass the entire flow depth. In such shallow flows, vortices generated by the inflectional velocity profile are the dominant mixing mechanism. Vertical transport across the canopy–water interface occurs over a narrow frequency range centered around f _ v (the frequency of vortex passage), with the vortices responsible for more than three-quarters of the interfacial flux. Secondly, Submerged canopies are typically flexible, coupling the motion of the fluid and canopy. Importantly, flexible canopies can exhibit a coherent waving (the monami ) in response to vortex passage. This waving reduces canopy drag, allowing greater in-canopy velocities and turbulent stresses. As a result, the waving of an experimental canopy reduces the canopy residence time by a factor of four. Finally, the length required for the set-up and full development of mixing-layer-type canopy flow is investigated. This distance, which scales upon the drag length scale, can be of the same order as the length of the canopy. In several flows adjacent to permeable media (such as urban canopies and reef systems), patchiness of the medium is common such that the fully developed condition may not be representative of the flow as a whole.
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flow and transport in channels with Submerged Vegetation
Acta Geophysica, 2008Co-Authors: Heidi Nepf, Marco GhisalbertiAbstract:This paper reviews recent work on flow and transport in channels with Submerged Vegetation, including discussions of turbulence structure, mean velocity profiles, and dispersion. For Submerged canopies of sufficient density, the dominant characteristic of the flow is the generation of a shear-layer at the top of the canopy. The shear-layer generates coherent vortices by Kelvin-Helmholtz (KH) instability. These vortices control the vertical exchange of mass and momentum, influencing both the mean velocity profile, as well as the turbulent diffusivity. For flexible canopies, the passage of the KH vortices generates a progressive wave along the canopy interface, termed monami. The KH vortices formed at the top of the canopy penetrate a distance δe into the canopy. This penetration scale segregates the canopy into an upper layer of rapid transport and a lower layer of slow transport. Flushing of the upper canopy is enhanced by the energetic shear-scale vortices. In the lower layer turbulence is limited to length-scales set by the stem geometry, and the resulting transport is significantly slower than that of the upper layer.
Stefan E. B. Weisner - One of the best experts on this subject based on the ideXlab platform.
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dynamics of Submerged macrophyte populations in response to biomanipulation
Freshwater Biology, 2001Co-Authors: John A. Strand, Stefan E. B. WeisnerAbstract:1. A 6-year study (1992-97) of changes in Submerged Vegetation after biomanipulation was carried out in the eutrophicated Lake Finjasjon, Southern Sweden. Ten sites around the lake were revisited each year. At each site five samples of above-ground biomass were taken at 10 cm water depth intervals. An investigation of the seed bank at the 10 sites, and a grazing experiment where birds and large fish were excluded was also conducted. 2. Between 1992 and 1996, in shallow areas (water depth 95% of the increase in biomass and plant cover. The following year (1997), however, cover and above-ground biomass decreased, mainly attributable to the total disappearance of E. canadensis. Secchi depth increased after biomanipulation until 1996, but decreased again in 1997. 3. Total and mean number of Submerged species increased after biomanipulation, probably as a result of the improved light climate. However, after the initial increase in species number there was a decrease during the following years, possibly attributed to competition from the rapidly expanding E. canadensis and M. spicatum. The lack of increase in species number after the disappearance of E. canadensis in 1997 implies that other factors also affected species richness. 4. A viable seed bank was not necessary for a rapid recolonization of Submerged macrophytes, nor did grazing by waterfowl or fish delay the re-colonization of Submerged macrophytes. 5. Submerged macrophytes are capable of rapid recolonization if conditions improve, even in large lakes such as Finjasjon (11 km(2)). Species that spread by fragments will increase rapidly and probably outcompete other species. 6. The results indicate that after the initial Secchi depth increase, probably caused by high zooplankton densities, Submerged Vegetation further improved the light climate. The decrease in macrophyte biomass in 1997 may have caused the observed increase in phosphorus and chlorophyll a, and the decrease in Secchi depth. We suggest that nutrient competition from periphyton, attached to the macrophytes, may be an important factor in limiting phytoplankton production, although other factors (e.g. zooplankton grazing) are also of importance, especially as triggers for the shift to a clear-water state.
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mechanisms regulating abundance of Submerged Vegetation in shallow eutrophic lakes
Oecologia, 1997Co-Authors: Stefan E. B. Weisner, John A. Strand, Hakan SandstenAbstract:Shallow eutrophic lakes tend to be either in a turbid state dominated by phytoplankton or in a clear-water state dominated by Submerged macroVegetation. Recent studies suggest that the low water turbidity in the clear-water state is maintained through direct and indirect effects of the Submerged Vegetation. This study examined what mechanisms may cause a recession of the Submerged Vegetation in the clear-water state, and thereby a switch to the turbid state. The spatial distribution of Submerged Vegetation biomass was investigated in two shallow eutrophic lakes in the clear-water state in southern Sweden. Biomass of Submerged Vegetation was positively correlated with water depth and wave exposure, which also were mutually correlated, suggesting that mechanisms hampering Submerged Vegetation were strongest at shallow and/or sheltered locations. The growth of Myriophyllum spicatum, planted in the same substrate and at the same water depth, was compared between sheltered and wave exposed sites in two lakes. After 6 weeks the plants were significantly smaller at the sheltered sites, where periphyton production was about 5 times higher than at the exposed sites. Exclosure experiments were conducted to evaluate the effects of waterfowl grazing on macrophyte biomass. Potamogeton pectinatus growth was decreased by grazing, whereas M. spicatum was not affected. The effects were greater at a sheltered than at a wave-exposed site, and also negatively related to distance from the reed belt. These results suggest that competition from epiphytes and waterfowl grazing hamper the development of Submerged Vegetation at sheltered and/or shallow locations. An increased strength of these mechanisms may cause a recession of Submerged Vegetation in shallow eutrophic lakes in the clear-water state and thereby a switch to the turbid state.
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Indirect effects of fish community structure on Submerged Vegetation in shallow, eutrophic lakes: an alternative mechanism
Hydrobiologia, 1992Co-Authors: Christer Brönmark, Stefan E. B. WeisnerAbstract:The loss of Submerged macrophytes during eutrophication of shallow lakes is a commonly observed phenomenon. The proximate reason for this decline is a reduction of available light due to increasing phytoplankton and/or epiphyton biomass. Here we argue that the ultimate cause for the transition from a macrophyte-dominated state to a phytoplankton-dominated state is a change in fish community structure. A catastrophic disturbance event (e.g. winterkill) acting selectively on piscivores, cascades down food chains, eventually reducing macrophyte growth through shading by epiphyton, an effect that is reinforced by increasing phytoplankton biomass. The transition back from the phytoplankton to the macrophyte state depends on an increase in piscivore standing stock and a reduction of planktivores. A conceptual model of these mechanisms is presented and supported by literature data and preliminary observations from a field experiment.
Yufei Wang - One of the best experts on this subject based on the ideXlab platform.
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analytical solutions of velocity profile in flow through Submerged Vegetation with variable frontal width
Journal of Hydrology, 2019Co-Authors: Weijie Wang, Wenxin Huai, Yufei Wang, Ping Wang, Jiao ZhangAbstract:Abstract Flow within Vegetation is one of the main driving forces for material exchange and energy transfer in wetland systems. Impacted by Vegetation, the flow velocity profile illustrates distortions to the classic logarithmic velocity profile and has attracted much attention among researchers. Different from analytical models of velocity distribution in literature, which is mainly suitable for Vegetation with uniform frontal width, this paper establishes new analytical solutions of the velocity profile for Vegetation such as shrub and sedge that have a variable frontal width in the vertical direction. A new shape function is proposed under these conditions in which the frontal width exhibits a gradual increase in the vertical direction from bottom up in the Vegetation. Along with different closure models for eddy viscosity in the Vegetation layer and surface layer, analytical solutions of the velocity profile are derived from the momentum equations. Good agreement between calculated and measured data shows our analytical model is effective in predicting velocity profiles.
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predicting the bulk average velocity of open channel flow with Submerged rigid Vegetation
Journal of Hydrology, 2019Co-Authors: Haoran Shi, Wenxin Huai, Xuerong Liang, Yufei WangAbstract:Abstract Predicting the bulk cross-sectional average flow velocity in open channels with Submerged Vegetation is an important topic in river engineering. Researchers have proposed numerous theoretical and empirical formulae, but the accuracy and physical basis of their solutions still need improvement. This study separates the flow into Vegetation layer and surface layer, following conventional two-layer approach, and estimates the average velocities in these two layers separately. In the Vegetation layer, force balance equation provides the basement of the estimation. And in the surface layer, we use genetic programming (GP), a data-driven method. A Darcy–Weisbach-coefficient-like parameter is proposed for the surface layer, which is related to other parameters through the GP algorithm. The maximum dissimilarity algorithm (a data-clustering algorithm) is used to separate the existing data sets in the training, validation, and testing groups to feed GP algorithm. Finally, by weighted combination, a new velocity formula with high accuracy and physical basis is proposed for Submerged vegetated flow.
Hanqing Zhao - One of the best experts on this subject based on the ideXlab platform.
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spectral shortcut in turbulence energy transfer in open channel flow over Submerged Vegetation
Journal of Hydro-environment Research, 2020Co-Authors: Hanqing Zhao, Hongwu Tang, Jing Yan, Dongfang Liang, Jinyu ZhengAbstract:Abstract This study explores the characteristics of the spectral shortcut in the turbulence kinetic energy transfer in experimental open channel flows with the presence of Submerged Vegetation flow. The Vegetation layer was simulated by arrays of rigid vertical cylinders, distributed uniformly in the channel bed. Results indicate that there are dual inertial subranges (ISRs) in the spectral distribution of turbulence energy in the penetration layer, where the Kelvin-Helmholtz (KH) and wake vortices coexist. The lower-frequency and higher-frequency ISRs reflect the energy cascading of the KH and wake-scale vortices, respectively. Spectral shortcut narrows the ISR for the KH vortex and contributes to the ISR for the wake-scale vortices, because such an action transfers a significant amount of turbulent energy directly from the large-scale eddies to the wake-scale vortices. We study the influence of spectral shortcut on energy transfer according to the turbulence kinetic energy budget equation for shear turbulence. The transferred energy is found to account for 58–71% of the shear turbulent energy and contributes considerably to the wake-scale turbulence. The strength of the energy transfer increases with the increase in the Vegetation density and the mean bulk velocity and the decrease in the relative submergence.
