1,624 research outputs found
A turbulence-resolving numerical investigation of wave-supported gravity flows
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 125(2), (2020): e2019JC015220, doi:10.1029/2019JC015220.Waveâsupported gravity flows (WSGFs) have been identified as a key process driving the offshore delivery of fine sediment across continental shelves. However, our understanding on the various factors controlling the maximum sediment load and the resulting gravity current speed remains incomplete. We adopt a new turbulenceâresolving numerical model for fine sediment transport to investigate the formation, evolution, and termination of WSGFs. We consider the simplest scenario in which fine sediments are supported by the waveâinduced fluid turbulence at a low critical shear stress of erosion over a flat sloping bed. Under the energetic wave condition reported on the Northern California Coast with a shelf slope of 0.005, simulation results show that WSGFs are transitionally turbulent and that the sediment concentration cannot exceed 30âkg/m urn:x-wiley:jgrc:media:jgrc23843:jgrc23843-math-0001 (g/L) due to the attenuation of turbulence by the sedimentâinduced stable density stratification. Wave direction is found to be important in the resulting gravity current intensity. When waves are in crossâshelf direction, the downslope current has a maximum velocity of 1.2 cm/s, which increases to 2.1 cm/s when waves propagate in the alongâshelf direction. Further analysis on the waveâaveraged momentum balance confirms that when waves are parallel to the slope (crossâshelf) direction, the more intense waveâcurrent interaction results in larger waveâaveraged Reynolds shear stress and thus in a smaller current speed. Findings from this study suggest that the more intense crossâshelf gravity current observed in the field may be caused by additional processes, which may enhance the sedimentâcarrying capacity of flow, such as the ambient current or bedforms.This study is supported by NSF (OCEâ1537231 and OCEâ1924532) and Office of Naval Research (N00014â17â1â2796). Numerical simulations presented in this study were carried out using the Mills and Canviness clusters at University of Delaware, and the SuperMIC cluster at Louisiana State University via XSEDE (TGâOCE100015). Z. Cheng would like to express thanks for the support of a postdoctoral scholarship from Woods Hole Oceanographic Institution. The source code and the case setup to reproduce the same results are publicly available via the repository maintained by GitHub: https://github.com/yueliangyi/TURBID (source code) and https://github.com/yueliangyi/TURBID/tree/master/spike/wave_supported_gravity_flow (case setup), respectively.2020-08-0
Wave modelling - the state of the art
This paper is the product of the wave modelling community and it tries to make a picture of the present situation in this branch of science, exploring the previous and the most recent results and looking ahead towards the solution of the problems we presently face. Both theory and applications are considered.
The many faces of the subject imply separate discussions. This is reflected into the single sections, seven of them, each dealing with a specific topic, the whole providing a broad and solid overview of the present state of the art. After an introduction framing the problem and the approach we followed, we deal in sequence with the following subjects: (Section) 2, generation by wind; 3, nonlinear interactions in deep water; 4, white-capping dissipation; 5, nonlinear interactions in shallow water; 6, dissipation at the sea bottom; 7, wave propagation; 8, numerics. The two final sections, 9 and 10, summarize the present situation from a general point of view and try to look at the future developments
Numerical Modeling of Wave- and Current-supported Turbidity Currents over Erodible Bed
The physical processes that route sediments from nearshore to the continental margin provide vital information to the global assessment of the geochemically important matter and the life in the ocean. Therefore, understanding these processes at the fundamental level will help develop accurate models that can be integrated into operational ocean models. Wave- and current-supported turbidity currents (WCSTCs) are one of the mechanisms that deliver sediments to the continental margin. WCSTCs are slow-moving turbidity currents where near-bed turbulence driven by strong surface waves and/or currents, tide- and/or wind-driven, maintain the turbidity current in motion. This study investigates the along-shelf current-supported turbidity currents (ACSTCs) over an erodible bed, where only the along-shelf current drives the flow, and sediment suspension is sourced from the ephemeral fine sediment deposits. To mimic ACSTCs, direct numerical simulations of a flow in a steady, turbulent, sediment-laden channel with a mild spanwise slope were conducted over an erodible bed. The primary focus of this study is to determine the effect of various sediment settling velocity, erosion parameters, and associated sediment-induced density stratification on total suspended sediment concentration, velocity structure, and turbulent characteristics of the ACSTCs. Specifically, this study aims to analytically and numerically investigates the transition of alongshore current-supported turbidity currents to self-sustaining turbidity currents over erodible seabed composed of fine sediment. Thus, a simplified depth-integrated dynamic equation is developed for suspended sediment concentration. The stability of the developed equation is analyzed both in itself and through temporal linear stability analysis. The analyses find two criteria for the inception of the aforementioned transition. Both criteria indicate that transition is found to reflect the competition between erosion flux, enhanced by the cross-shelf motion of alongshore current-supported turbidity currents, and the deposition flux. In addition, drag coefficient associated with cross-shelf motion of ACSTCs is formulated as a function of the Reynolds number, sediment concentration, sediment settling velocity, and the bed slope
Threeâdimensional turbulenceâresolving simulations of the plunge phenomenon in a tilted channel
Hyperpycnal flows are produced when the density of a fluid flowing in a relatively quiescent basin is greater than the density of the fluid in the basin. The density differences can be due to the difference in temperatures, salinity, turbidity, concentration, or a combination of them. When the inflow momentum diminishes, the inflowing fluid eventually plunges under the basin fluid and flows along the bottom floor as an underflow density current. In the present work, 3âD turbulenceâresolving simulations are performed for an hyperpycnal flow evolving at the bottom floor of a tilted channel. Using advanced numerical techniques designed for supercomputers, the incompressible NavierâStokes and transport equations are solved to reproduce numerically the experiments of Lamb et al. (2010, https://doi.org/10.1130/B30125.1) obtained inside a flume with a long tilted ramp. This study focuses on presenting and validating a new numerical framework for the correct reproduction and analysis of the plunge phenomenon and its associated flow features. A very good agreement is found between the experimental data of Lamb et al. (2010), the analytical models of Parker and Toniolo (2007, https://doi.org/10.1061/(ASCE)0733-9429(2007)133:6(690)), and the present turbulenceâresolving simulations. The mixing process between the ambient fluid and the underflow density current is also analyzed thanks to visualizations of vortical structures at the interface
Recent advances in nearshore wave, circulation, and sediment transport modeling
Significant advances in the modeling of nearshore processes have occurred over recent decades as a result of developments in both computational approaches and theoretical understanding. This review examines the present state of progress primarily from a hydrodynamics standpoint, followed by a brief discussion of applications to sediment transport and morphological evolution. Wave-averaged formulations of the wave-current interaction problem and resulting models for wave-induced currents are reviewed in order to compare and contrast radiation stress and vortex force approaches. Waveresolving approaches are then discussed, with an emphasis on the recent rapid development of 3D nonhydrostatic models and their application to a wide range of physical problems. The recently developed understanding of the importance of vorticity generation mechanisms at wave-resolved scales, and their contribution to transport and mixing processes, are discussed
An Eulerian two-phase model for steady sheet flow using large-eddy simulation methodology
Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Advances in Water Resources 111 (2018): 205-223, doi:10.1016/j.advwatres.2017.11.016.A three-dimensional Eulerian two-phase flow model for sediment transport in sheet flow conditions is presented. To resolve turbulence and turbulence-sediment interactions, the large-eddy simulation approach is adopted. Specifically, a dynamic Smagorinsky closure is used for the subgrid fluid and sediment stresses, while the subgrid contribution to the drag force is included using a drift velocity model with a similar dynamic procedure. The contribution of sediment stresses due to intergranular interactions is modeled by the kinetic theory of granular flow at low to intermediate sediment concentration, while at high sediment concentration of enduring contact, a phenomenological closure for particle pressure and frictional viscosity is used. The model is validated with a comprehensive high-resolution dataset of unidirectional steady sheet flow (Revil-Baudard et al., 2015, Journal of Fluid Mechanics, 767, 1â30). At a particle Stokes number of about 10, simulation results indicate a reduced von KĂĄrmĂĄn coefficient of ÎșâŻââŻ0.215 obtained from the fluid velocity profile. A fluid turbulence kinetic energy budget analysis further indicates that the drag-induced turbulence dissipation rate is significant in the sheet flow layer, while in the dilute transport layer, the pressure work plays a similar role as the buoyancy dissipation, which is typically used in the single-phase stratified flow formulation. The present model also reproduces the sheet layer thickness and mobile bed roughness similar to measured data. However, the resulting mobile bed roughness is more than two times larger than that predicted by the empirical formulae. Further analysis suggests that through intermittent turbulent motions near the bed, the resolved sediment Reynolds stress plays a major role in the enhancement of mobile bed roughness. Our analysis on near-bed intermittency also suggests that the turbulent ejection motions are highly correlated with the upward sediment suspension flux, while the turbulent sweep events are mostly associated with the downward sediment deposition flux.This study was supported by National Science Foundation (OCE-1635151; OCE-
958 1537231) and Office of Naval Research (N00014-16-1-2853). J. Chauchat was supported by the Region Rhones-Alpes (COOPERA project and Explora Pro grant) and
the French national programme EC2CO-LEFE MODSED. The authors would also like to acknowledge the support from the program on "Fluid-Mediated Particle Transport in
Geophysical Flows" at the Kavli Institute for Theoretical Physics, Santa Barbara, USA
The future of coastal and estuarine modeling: Findings from a workshop
This paper summarizes the findings of a workshop convened in the United States in 2018 to discuss methods in coastal and estuarine modeling and to propose key areas of research and development needed to improve their accuracy and reliability. The focus of this paper is on physical processes, and we provide an overview of the current state-of-the-art based on presentations and discussions at the meeting, which revolved around the four primary themes of parameterizations, numerical methods, in-situ and remote-sensing measurements,and high-performance computing. A primary outcome of the workshop was agreement on the need to reduce subjectivity and improve reproducibility in modeling of physical processes in the coastal ocean. Reduction of subjectivity can be accomplished through development of standards for benchmarks, grid generation, and validation, and reproducibility can be improved through development of standards for input/output, coupling and model nesting, and reporting. Subjectivity can also be reduced through more engagement with the applied mathematics and computer science communities to develop methods for robust parameter estimation anduncertainty quantification. Such engagement could be encouraged through more collaboration between thef orward and inverse modeling communities and integration of more applied math and computer science into oceanography curricula. Another outcome of the workshop was agreement on the need to develop high-resolution models that scale on advanced HPC systems to resolve, rather than parameterize, processes with horizontal scales that range between the depth and the internal Rossby deformation scale. Unsurprisingly,more research is needed on parameterizations of processes at scales smaller than the depth, includingparameterizations for drag (including bottom roughness, bedforms, vegetation and corals), wave breaking, and airâsea interactions under strong wind conditions. Other topics that require significantly more work to better parameterize include nearshore wave modeling, sediment transport modeling, and morphodynamics. Finally, it was agreed that coastal models should be considered as key infrastructure needed to support research, just like laboratory facilities, field instrumentation, and research vessels. This will require a shift in the way proposals related to coastal ocean modeling are reviewed and funded
Impact assessment for the improved four boundary conditions (at bed, free-surface, land-boundary and offshore-boundary) on coastal hydrodynamics and particulate transport
The FIELD_AC project aims at providing an improved operational service for coastal areas and at generating added value for shelf and regional scale predictions. Coastal-zone oceanographic predictions seldom appraise the land discharge as a boundary condition. River fluxes are sometimes considered, but neglecting their 3D character, while the "distributed" continental run-off is not taken into consideration. Moreover, many coastal scale processes, particularly those relevant in geographically restricted domains (coast with harbors or river mouth areas), are not well parametrized in present simulations.Work package 3 dedicated to Boundary Fluxes aims to establish and use the best possible boundary conditions for coastal water quality modelling. On this scale, all boundaries become important. For the land boundary side the needed products are distributed and point wise run-off both quantitatively and qualitatively. For the offshore boundary condition, 3D current, water quality field, and wave spectra will be used. For the atmospheric boundary, products from local scale meteorological models (wind, atmospheric pressure and rainfall) are needed. For the seabed, boundary information on sediment composition, bedforms and bathymetry and bio-geo-chemical parameters is essential.This report addresses the impact assessment for improvements in the four boundary conditions (boundary fluxes from land, free-surface boundary condition, seabed boundary condition and open boundary fluxes) on coastal hydrodynamics and particulate transport. The description of the improved four boundary conditions is followed by examples of concrete impact assessment of the theory into the Catalan coast, Liverpool Bay, German Bight and Gulf of Venice
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments
with Dynamics (ROCKE-3D) is a 3-Dimensional General Circulation Model (GCM)
developed at the NASA Goddard Institute for Space Studies for the modeling of
atmospheres of Solar System and exoplanetary terrestrial planets. Its parent
model, known as ModelE2 (Schmidt et al. 2014), is used to simulate modern and
21st Century Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing
effort to expand the capabilities of ModelE2 to handle a broader range of
atmospheric conditions including higher and lower atmospheric pressures, more
diverse chemistries and compositions, larger and smaller planet radii and
gravity, different rotation rates (slowly rotating to more rapidly rotating
than modern Earth, including synchronous rotation), diverse ocean and land
distributions and topographies, and potential basic biosphere functions. The
first aim of ROCKE-3D is to model planetary atmospheres on terrestrial worlds
within the Solar System such as paleo-Earth, modern and paleo-Mars,
paleo-Venus, and Saturn's moon Titan. By validating the model for a broad range
of temperatures, pressures, and atmospheric constituents we can then expand its
capabilities further to those exoplanetary rocky worlds that have been
discovered in the past and those to be discovered in the future. We discuss the
current and near-future capabilities of ROCKE-3D as a community model for
studying planetary and exoplanetary atmospheres.Comment: Revisions since previous draft. Now submitted to Astrophysical
Journal Supplement Serie
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