Three Dimensional Non Hydrostatic Modeling Of Free Surface Turbulent Flows And Transport Of Cohesive Sediment

The flow after the rupture of a dam on an inclined plane of arbitrary slope and the induced transport of non-cohesive sediment are analysed using the shallow-water approximation. We observe the development of free-surface instabilities in the numerical results, hereafter called roll waves. Subsequently, the present monograph presents a novel Continuum Mechanics model which allows us to study the transport of sediment both in laminar and turbulent, non-hydrostatic free-surface flow, avoiding the intrinsic limitations of flow depth averaged models. Finally, this model is applied to solve the dam-break problem against an isolated obstacle and to predict the transport of sediment after the rupture of a horizontal dam. It is demonstrated that models based on depth-averaged variables (e.g. generalisations of the one-dimensional Saint-Venant equations to predict morphological changes) are superseded by more sophisticated and accurate procedures valid for non-hydrostatic shallow water flows over bed of arbitrary bottom slopes (e.g. the model described herein).

Comprehensive text on the fundamentals of modeling flow and sediment transport in rivers treating both physical principles and numerical methods for various degrees of complexity. Includes 1-D, 2-D (both depth- and width-averaged) and 3-D models, as well as the integration and coupling of these models. Contains a broad selection

To better understand the turbulent coherent structures in the coastal environments, we report 3D Large-eddy simulation (LES) study of wave breaking processes in the surf zone, and shear instabilities in an idealized buoyant plume at high Reynolds number. For the simulation of breaking waves, the numerical model is implemented using the open-source CFD toolbox, OpenFOAM®, in which the incompressible three-dimensional filtered Navier-Stokes equations for the water and air phases are solved with a finite volume scheme. A Volume of Fluid (VOF) method is used to capture the evolution of the water-air interface. The numerical model is validated with measured free surface elevation, turbulence averaged flow velocity, turbulent intensity for solitary wave and periodic wave conditions in a laboratory wave flume and near-prototype scale large wave flume. For the first time, simulated intermittency of breaking wave turbulence over barred beach are compared with observation. Simulation results show that during the initial overturning of the breaking wave, 2-D horizontal rollers are generated, and can further evolve into a couple of 3-D hairpin vortices. Some of these vortices are sufficiently intense to impinge onto the bed. These hairpin vortices possess counter-rotating and downburst features, which are key characteristics of obliquely descending eddies (ODEs) observed by earlier laboratory studies using Particle Image Velocimetry. Simulation results also confirm that as the ODEs approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated for a periodic wave train. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results further suggest that about 60~70% of coherent bottom stress events are associated with surface-generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave-breaking turbulence approaching the bed. Shear instabilities are responsible for major turbulent mixing in estuaries. However, most estuarine models adopt hydrostatic pressure assumption and the grid resolution used are not sufficient to resolve shear instabilities. In this study, the numerical investigation focused on resolving shear instabilities and their surface signature in an idealized buoyant plume at high Reynolds number using the non-hydrostatic surface and terrain-following coastal circulation model NHWAVE. The Reynolds number of the resolved shear instabilities in the simulation exceed 1.2× 106, which is similar to that observed in the Connecticut River plume. Using 80 million grid points with grid size approaching the observed Ozmidov length scale, simulation with standard Smagorinsky closure can reproduce the observed shear instabilities with a similar length scale and turbulent dissipation rate. By examining the resolved energy spectrum, about one order of magnitude of energy cascade ( -5/3 slope) is resolved. Moreover, model results show high turbulence in braids instead of cores of shear instabilities, which is similar to the field observations. By computing the horizontal surface divergence to represent surface signatures, model results clearly indicate that shear instabilities can leave unique surface signatures which may be detected by remote sensing imagery. An additional simulation with k-? closure captures bulk features, but the shear instability and surface signature are lacking. Simulations with lower resolution suggest that the resolved energy spectra start to deviate from the expected -5/3 slope when the grid size is significantly greater than the Ozmidov length scale.