An official website of the United States government
Abstract Laboratory experiments explored the impact of vegetation patchiness on channel‐averaged turbulence and sediment transport. Stems were clustered into 16 randomly distributed circular patches of decreasing diameter. For the same channel velocity, the sediment transport increased with total stem number but decreased as stems were clustered into smaller patch diameters, occupying a smaller fraction of the bed area. The channel‐averaged turbulence, which also declined with increased clustering, was shown to be a good predictor for sediment transport at the channel scale. Previous models for uniform vegetation were adapted to predict both the channel‐averaged turbulence and sediment transport as a function of the total number of stems and degree of clustering, represented by the fraction of bed covered by patches. This provides a way for numerical modelers to represent the impact of subgrid‐scale vegetation patchiness on sediment transport.
more »
« less
A volume penalization immersed boundary method for flow interactions with aquatic vegetation
Abstract: A volume-penalization immersed boundary (VPIB) method was developed to study flow interactions with aquatic vegetation. The model has been validated with data from laboratory experiments and previous high-fidelity models with satisfactory results. Sensitivity analyzes on both penalty parameter and thickness parameter were conducted, and optimal values for these parameters are recommended. The validated model has been applied to study the effects of swaying motion of vegetation stems on the flow dynamics at both vegetate-stem scale and patch scale. The swaying motion of the vegetation stem is prescribed following a cubic law that peaks at the top and decreases to zero at the bottom. At stem-scale, the hydrodynamics depend on the Keulegan Carpenter number (KC), which is defined as the maximum excursion of the vegetation stem to the diameter of the stem. Simulations with three KC values were carried out. For KC≥1, the flow turbulence is significantly enhanced by the swaying motion of the stem, and turbulence becomes more isotropic in the wake. The swaying motion of vegetation stems caused a 5% increase of the bottom shear stress at the shoulders of the stem, and the effect is negligible in the wake. At patch-scale, the hydrodynamics depend on the effective Keulegan Carpenter number based on the patch size of the vegetation patch, and the solid volume fraction for dense vegetation canopy. Solid volume fraction was varied while maintaining the same effective Keulegan Carpenter in the simulations. When the effective Keulgen Carpenter number is small (KC<1), effects of the swaying motion of vegetation stems on the large patch-scale dynamics are not significant, including both the turbulence statistics and the bottom stress.
more »
« less
- Award ID(s):
- 1945685
- PAR ID:
- 10314488
- Date Published:
- Journal Name:
- Advances in water resources
- Volume:
- 161
- ISSN:
- 0309-1708
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
A three-dimensional Eulerian two-phase flow solver, SedFoam, has been developed for various sediment transport applications. The solver has demonstrated success in modeling sheet flow and bedforms driven by oscillatory flows using a Reynolds-averaged Navier–Stokes (RANS) formulation. However, the accuracy of the RANS formulation for more complex flows, such as scour around structures, requires further evaluation. SedFoam has recently been enhanced to incorporate two-phase large-eddy simulation (LES) capability. In this study, RANS and LES approaches are tested via a three-dimensional case of wave-induced local scour around a single vertical circular pile. Two laboratory experiments, one with an erodible bed and the other with a rigid bed, were chosen for simulation, with both experiments having a Keulegan-Carpenter (KC) number of 10. The k-ω turbulence closure was selected for the RANS simulation, and the dynamic Lagrangian subgrid closure was chosen for the LES simulation. Numerical results reveal that both RANS and LES simulations can resolve lee-wake vortices, although the vortices are significantly weaker in the RANS simulation. In comparison with the LES results, the RANS approach fails to predict horseshoe vortex with sufficient intensity, leading to an underestimation of scour hole depth development. Although the scour depths develop at a very similar rate in the early stage, the scour depth predicted by the RANS simulation quickly reaches equilibrium, while the LES simulation follows the measured trend. These findings indicate that a turbulence-resolving methodology, i.e. LES, is necessary for accurate scour simulations.more » « less
-
Abstract An immersed boundary‐finite element with soft‐body dynamics has been implemented to study steady flow over a finite patch of submerged flexible aquatic vegetation. The flow structure interaction model can resolve the flow interactions with flexible vegetation, and hence the reconfiguration of vegetation blades to ambient flow. Flow dynamics strongly depend on two dimensionless parameters, namely vegetation density and Cauchy number (defined as the ratio of the fluid drag force to the elastic force). Five different flow patterns have been identified based on vegetation density and Cauchy number, including the limited reach, swaying, “monami” A, “monami” B with slow moving interfacial wave, and prone. The “monami” B pattern occurred at high vegetation density and is different from “monami” A, in which the passage of Kelvin‐Helmholtz billows strongly affects the vegetation interface. With soft‐body dynamics, blade‐to‐blade interactions can also be resolved. At high vegetation density, the hydrodynamic interactions play an important role in blade‐to‐blade interactions, where adjacent vegetation blades interact via the interstitial fluid pressure. At low vegetation density, direct contacts among vegetation blades play important roles in preventing unphysical penetration of vegetation blades.more » « less
-
In the coastal ocean, interactions of waves and currents with large roughness elements, similar in size to wave orbital excursions, generate drag and dissipate energy. These boundary layer dynamics differ significantly from well-studied small-scale roughness. To address this problem, we derived spatially and phase-averaged momentum equations for combined wave–current flows over rough bottoms, including the canopy layer containing obstacles. These equations were decomposed into steady and oscillatory parts to investigate the effects of waves on currents, and currents on waves. We applied this framework to analyse large-eddy simulations of combined oscillatory and steady flows over hemisphere arrays (diameter $$D$$ ), in which current ( $$U_c$$ ), wave velocity ( $$U_w$$ ) and period ( $$T$$ ) were varied. In the steady momentum budget, waves increase drag on the current, and this is balanced by the total stress at the canopy top. Dispersive stresses from oscillatory flow around obstacles are increasingly important as $$U_w/U_c$$ increases. In the oscillatory momentum budget, acceleration in the canopy is balanced by pressure gradient, added-mass and form drag forces; stress gradients are small compared to other terms. Form drag is increasingly important as the Keulegan–Carpenter number $$KC=U_wT/D$$ and $$U_c/U_w$$ increase. Decomposing the drag term illustrates that a quadratic relationship predicts the observed dependences of steady and oscillatory drag on $$U_c/U_w$$ and $KC$ . For large roughness elements, bottom friction is well represented by a friction factor ( $$f_w$$ ) defined using combined wave and current velocities in the canopy layer, which is proportional to drag coefficient and frontal area per unit plan area, and increases with $KC$ and $$U_c/U_w$$ .more » « less
-
This work investigates the steady-state nonlinear dynamics of a large-deformation flexible beam model under oscillatory flow. A flexible beam dynamics model combined with hydrodynamic loading is employed using large deformation beam theory. The equations of motion discretised using the high-order finite element method (FEM) are solved in the time domain using the efficient Galerkin averaging-incremental harmonic balance (EGA-IHB) method. The arc-length continuation method and Hsu’s method trace stable and unstable solutions. The numerical results are in accordance with the physical experimental results and reveal multiple resonance phenomena. Low-order resonances exhibit hardening due to geometric nonlinearity, while higher-order resonances transition from softening to hardening influenced by inertia and geometric nonlinearity. A strong coupling between tensile and bending deformation is observed. The axial deformation is dominated by inertia, while bending resonance is influenced by an interplay between inertia, structure stiffness, and fluid drag. Finally, the effects of two dimensionless parameters, Keulegan and Carpenter number (KC) and Cauchy number (Ca), on the response of the flexible beam are discussed.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.advwatres.2021.104120
