1,721,094 research outputs found
Hydrodynamics of vegetated channels
Full text linkThis paper highlights some recent trends in vegetation hydrodynamics, focusing on conditions within channels and spanning spatial scales from individual blades, to canopies or vegetation patches, to the channel reach. At the blade scale, the boundary layer formed on the plant surface plays a role in controlling nutrient uptake. Flow resistance and light availability are also influenced by the reconfiguration of flexible blades. At the canopy scale, there are two flow regimes. For sparse canopies, the flow resembles a rough boundary layer. For dense canopies, the flow resembles a mixing layer. At the reach scale, flow resistance is more closely connected to the patch-scale vegetation distribution, described by the blockage factor, than to the geometry of individual plants. The impact of vegetation distribution on sediment movement is discussed, with attention being paid to methods for estimating bed stress within regions of vegetation. The key research challenges of the hydrodynamics of vegetated channels are highlighted.National Science Foundation (U.S.) (Grant No. EAR0309188)National Science Foundation (U.S.) (EAR0125056)National Science Foundation (U.S.) (EAR0738352)National Science Foundation (U.S.) (OCE0751358
Drag, turbulence, and diffusion in flow through emergent vegetation
No full textAquatic 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.National Science Foundation (U.S.) (CAREER Award)National Science Foundation (U.S.) (grant EAR 962925
Thermally driven exchange flow between open water and an aquatic canopy
No full textDifferential solar heating can result from shading by rooted emergent aquatic plants, producing a temperature difference between vegetated and unvegetated regions of a surface water body. This temperature difference will promote an exchange flow between the vegetation and open water. Drag associated with the submerged portion of the plants modifies this exchange, specifically, changing the dominant velocity scale. Scaling analysis predicts several distinct flow regimes, including inertia-dominated, drag-dominated and energy-limiting regimes. After a constant heat source is initiated, the flow is initially inertial, but quickly transitions to the drag-dominated regime. The energy-limiting regime is not likely to occur in the presence of rooted vegetation. Laboratory experiments describe the exchange flow and confirm the scaling analysis. Particle Imaging Velocimetry (PIV) was used to quantify the velocity field. Once the exchange flow enters the drag-dominated regime, the intrusion velocity uV is steady. The intrusion velocity decreases with increasing density of vegetation. The thickness of the intruding layer is set by the length scale of light penetration.National Science Foundation (U.S.) (Grant EAR0509658
Prediction of near-field shear dispersion in an emergent canopy with heterogeneous morphology
No full textThe evaluation of longitudinal dispersion in aquatic canopies is necessary
to predict the behavior of dissolved species and suspended particles in marsh and wetland
systems. Here we consider the influence of canopy morphology on longitudinal
dispersion, focusing on transport before constituents have mixed over depth. Velocity
and longitudinal dispersion were measured in a model canopy with vertically varying
canopy density. The vertical variation in canopy morphology generates vertical variation
in the mean velocity profile, which in turn creates mean-shear dispersion. We
develop and verify a model that predicts the mean-shear dispersion in the near field
from morphological characteristics of the canopy, such as stem diameter and frontal
area. Close to the source, longitudinal dispersion is dominated by velocity heterogeneity
at the scale of individual stems. However, within a distance of approximately
1m, the shear dispersion associated with velocity heterogeneity over depth increases
and eclipses this smaller-scale process.National Defense Science and Engineering Graduate FellowshipNational Science Foundation (U.S.). Graduate Research Fellowship ProgramNational Science Foundation (U.S.). (Grant EAR 0309188
Thermal Mediation by Littoral Wetlands and Impact on Lake Intrusion Depth
No full textLake inflow dynamics can be affected by the thermal mediation provided by shallow littoral regions such as wetlands. In this study, wetland thermal mediation is evaluated using a linearized dead-zone model. Its impact on lake inflow dynamics is then assessed by applying the model sequentially to the river reach, wetland, and lake. Our results suggest that littoral wetlands can dramatically alter the inflow dynamics of reservoirs located in small or forested watersheds, for example, by raising the temperature of the inflow during the summer and creating surface intrusions when a plunging inflow would otherwise exist. Consequently, river-borne nutrients, contaminants, and pathogens enter directly into the epilimnion, where they enhance eutrophication and the risk of human exposure. The addition of a littoral wetland has less significant effects in larger watersheds, where the water has already equilibrated with the atmosphere upon reaching the wetland and sun shading is less prominent.National Science Foundation (U.S.) (Superfund Basic Research Program, grant P42-ES04675
Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies
No full textAquatic 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
Wave-induced dynamics of flexible blades
No full textIn this paper, we present an experimental and numerical study that describes the motion of flexible blades, scaled to be dynamically similar to natural aquatic vegetation, forced by wave-induced oscillatory flows. For the conditions tested, blade motion is governed primarily by two dimensionless variables: (i) the Cauchy number, Ca, which represents the ratio of the hydrodynamic forcing to the restoring force due to blade stiffness, and (ii) the ratio of the blade length to the wave orbital excursion, L. For flexible blades with Ca≫. 1, the relationship between drag and velocity can be described by two different scaling laws at the large- and small-excursion limits. For large excursions (L≪. 1), the flow resembles a unidirectional current and the scaling laws developed for steady-flow reconfiguration studies hold. For small excursions (L≫. 1), the beam equations may be linearized and a different scaling law for drag applies. The experimental force measurements suggest that the small-excursion scaling applies even for intermediate cases with L~. O(1). The numerical model employs the well-known Morison force formulation, and adequately reproduces the observed blade dynamics and measured hydrodynamic forces without the use of any fitted parameters. For Ca≫. 1, the movement of the flexible blades reduces the measured and modeled hydrodynamic drag relative to a rigid blade of the same morphology. However, in some cases with Ca~. O(1), the measured hydrodynamic forces generated by the flexible blades exceed those generated by rigid blades, but this is not reproduced in the model. Observations of blade motion suggest that this unusual behavior is related to an unsteady vortex shedding event, which the simple numerical model cannot reproduce. Finally, we also discuss implications for the modeling of wave energy dissipation over canopies of natural aquatic vegetation. Keywords: flexible vegetation; reconfiguration; large amplitude deformation; oscillatory flows; wave energy dissipationNational Science Foundation (U.S.) (Grant OCE0751358
Density-driven exchange flow between open water and an aquatic canopy
No full textDifferences in water density can drive an exchange flow between the vegetated and open regions of surface water systems. A laboratory experiment has been conducted to investigate this exchange flow, using a random array of rigid, emergent cylinders to represent the canopy region. The flow pattern was captured using a CCD camera. The velocity of the current entering the canopy and the volume discharge both decrease with increasing vegetative drag and also decrease gradually over time. Theoretical predictions for velocity and discharge rate are developed and verified with experimental observations. Extensions to field conditions are also discussed.National Science Foundation (U.S.) (grant EAR0509658
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