Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A13: Geophysical Fluid Dynamics: Sediment Transport & Gravity Currents |
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Chair: Justin Finn, University of Liverpool Room: C124 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A13.00001: Coupled large-eddy simulation and morphodynamics of a large-scale river under extreme flood conditions Ali Khosronejad, Fotis Sotiropoulos We present a coupled flow and morphodynamic simulations of extreme flooding in 3 km long and 300 m wide reach of the Mississippi River in Minnesota, which includes three islands and hydraulic structures. We employ the large-eddy simulation (LES) and bed-morphodynamic modules of the VFS-Geophysics model to investigate the flow and bed evolution of the river during a 500 year flood. The coupling of the two modules is carried out via a fluid-structure interaction approach using a nested domain approach to enhance the resolution of bridge scour predictions. The geometrical data of the river, islands and structures are obtained from LiDAR, sub-aqueous sonar and in-situ surveying to construct a digital map of the river bathymetry. Our simulation results for the bed evolution of the river reveal complex sediment dynamics near the hydraulic structures. The numerically captured scour depth near some of the structures reach a maximum of about 10 m. The data-driven simulation strategy we present in this work exemplifies a practical simulation-based-engineering-approach to investigate the resilience of infrastructures to extreme flood events in intricate field-scale riverine systems. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A13.00002: Analysis of Reynolds stress budgets in LES of Langmuir supercells under crosswind currents in a coastal ocean Andres Tejada-Martinez, Jie Zhang Langmuir supercells (LSCs) in coastal oceans consist of parallel counter rotating vortices engulfing the water column in unstratified conditions. These cells have been observed in continental shelf regions 15-30 meters deep during the passage of storms. LSCs are aligned roughly in the wind direction and are generated via interaction of the wind-driven shear current and Stokes drift velocity induced by surface gravity waves. LSCs have been determined to be an important contributor to the suspension of sediments and their overall transport across shelves. It has also been shown that tidal forcing distorts and weakens LSCs, inhibiting their potential for sediment suspension. Large-eddy simulations of LSCs in flows driven by a surface wind stress and a constant crosswind pressure gradient (representative of crosswind tidal forcing) have been performed. Although a crosswind tidal current stronger than the wind-driven current is able to break up the LSCs giving rise to smaller scale, weaker Langmuir cells (LCs), analysis of Reynolds shear stress budgets reveals that non-local transport remains significant relative to flow without LCs. This demonstrates the need for a non-local transport term in Reynolds shear stress and turbulent scalar flux closures for coastal flows with LCs. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A13.00003: Regimes of sediment-turbulence interaction and guidelines for simulating the multiphase bottom boundary layer Justin Finn, Ming Li Characterizing the interaction of mobile sediments with a turbulent boundary layer driven by waves and/or currents represents an important scientific and engineering challenge. To approach this, Balachandar’s scaling relations for particle Reynolds number and Stokes number (IJMF, vol. 35, pg 801–110, 2009) are recast in terms of Shields parameter, $\Theta$, particle Galileo number, $Ga$, and particle-to-fluid density ratio, $s$. This allows the modified Shields diagram to be partitioned into at least five regimes, where distinct primary mechanisms of sediment-turbulence interaction can be identified. Additionally, practical guidelines for selecting an appropriate multiphase modeling strategy for direct and large eddy simulation (DNS/LES) of the bottom boundary layer are proposed based on the results. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A13.00004: Partial-depth lock-release and related phenomena: a vorticity-based analysis Mohammad Amin Khodkar, Mohamad Nasr-azadani, Eckart Meiburg We present a vorticity-based model for partial-depth Boussinesq lock exchange flows, without empirical closure assumptions. Upon release of the lock fluid, experiments and DNS simulations show that a gravity current front forms, followed by a rarefaction wave. For sufficiently large ratios of lock height to tank height, a left-propagating internal bore will be generated as well. By employing the conservation of mass in each fluid, along with the overall vorticity, we propose a set of ODEs that predicts the velocity distribution within the rarefaction wave, in addition to the velocity and height of the gravity current front. The model furthermore predicts when a bore is produced, and how it influences the dynamics of the flow. The model predictions are compared to DNS results, as well as to theoretical and experimental results of earlier investigations, and good agreement is observed for all flow properties. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A13.00005: The dynamics of bottom-boundary gravity currents propagating over a submerged array of cylinders Jian Zhou, Tim Williams, Megan Ball, Claudia Cenedese, Subhas Venayagamoorthy, Roger Nokes The structure and propagation of lock-exchange bottom-boundary gravity currents (BBGC) in a rectangular horizontal channel containing a submerged array of cylindrical obstacles are investigated using experiments and large eddy simulations. Excellent agreement on the front velocity between the experimental and numerical results is found. A broad-range three-dimensional parametric study is performed in which the solid volume fraction $\phi$ of the array is varied continuously from 0 (flat-bed case) to 1(solid-slab case), and the submergence ratio is varied from 1 (emergent) to 10 (deeply submerged). Both in-line and staggered cylinder arrangements are considered. The various flow regimes arising from the current-array interaction and their mutual transitions are investigated in detail in terms of front velocity, density, vorticity, turbulent mixing and global energy budget. Our analysis provides a new framework for predicting the front velocity of BBGCs propagating over a submerged array of cylinders under the influence of array inhomogeneity. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A13.00006: Sediment morpho-dynamics induced by a swirl-flow: an experimental study Alfredo Gonzalez-Vera, Matias Duran-Matute, GertJan van Heijst This research focuses on a detailed experimental study of the effect of a swirl-flow over a sediment bed in a cylindrical domain. Experiments were performed in a water-filled cylindrical rotating tank with a bottom layer of translucent polystyrene particles acting as a sediment bed. The experiments started by slowly spinning the tank up until the fluid had reached a solid-body rotation at a selected rotation speed ($\Omega_i$). Once this state was reached, a swirl-flow was generated by spinning-down the system to a lower rotation rate ($\Omega_f$). Under the flow's influence, particles from the bed were displaced, which changed the bed morphology, and under certain conditions, pattern formation was observed. Changes in the bed height distribution were measured by utilizing a Light Attenuation Technique (LAT). For this purpose, the particle layer was illuminated from below. Images of the transmitted light distribution provided quantitative information about the local thickness of the sediment bed. The experiments revealed a few characteristic regimes corresponding to sediment displacement, pattern formation and the occurrence of particle pick-up. Such regimes depend on both the Reynolds ($Re$) and Rossby ($Ro$) numbers. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A13.00007: A study of saltation process using hydrogel particles. Stella Wang, Yuchen Zhao, Robert Behringer Saltation is a natural process for sediments transported by flow, and occurs in situations such as wind-driven sand dunes in the dessert, and rivers or streams where fluid motion drives gravel. The onset of grain motions is set by the strength of the shear, and grains exhibit rolling, successive jumping where they are lifted by the turbulence. It is an open issue as to how the grain size affects saltation transport, particle velocities and mass fluxes, etc[1]., and also how the inelastic collision between grains affects saltation. Here, we describe a new saltation experiment using hydrogel particles immersed in uniform flow of water. Because the refraction indexes of particles and the fluid are nearly matched, the hydrogel particles can be imaged by a parallel light source, resulting in overlapping dark rings that not only reflect lateral positions, but also depths in one 2D image at one time. Mono-disperse particles are used and their size is adjusted by changing salt concentrations in the fluid. Preliminary results show that the softness of hydrogel particles leads to relatively large collisional losses. This property allows us to explore the phase diagram of saltation transport in the in-elastic collision regime. [Preview Abstract] |
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