Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R17: Geophysical Fluid Dynamics: Sediment Transport |
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Chair: Yi-Ju Chou, National Taiwan University Room: 2002 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R17.00001: Experimental and numerical study of turbulent flow associated with interacting barchan dunes Gianluca Blois, William Anderson, Zhanqi Tang, Julio Barros, James Best, Kenneth Christensen Barchan dunes are naturally occurring three-dimensional topographic features that have been observed on the surface of several planets. They occur both in aeolian and in subaqueous environments. Barchans typically form in fields having a broad distribution in dune size and migration rates. This results in variable bedform spacing and eventually dynamic bedform$-$bedform interactions that involve morphodynamic processes (e.g. collision, merging, splitting). These processes are controlled by complex feedback mechanisms mutually linking three key elements: fluid flow, sediment transport and bed morphology. The aim of this work is to contribute to the understanding of the fluid-flow mechanisms responsible for the formation, migration and interaction of these dunes. To this end, we study the three-dimensional flow generated by the interactions between fixed barchan-dune models arranged in tandem in collision and ejection scenarios via experiments in an optically-accessible flow environment using planar particle-image velocimetry (PIV) measurements of the flow field. These measurements are complemented by targeted large-eddy simulations (LES) meant to provide a three-dimensional view of the flow processes for these fixed dune arrangements. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R17.00002: Erosion of granular bed by a normal jet Sarah Badr, Georges Gauthier, Philippe Gondret We study the local erosion of a horizontal dry granular bed induced by an impinging vertical jet in both laminar and turbulent regimes. The critical velocity of erosion depends on the nature of the bed and on its distance to the jet nozzle. We show that the erosion threshold is governed by a Shields number of inertial origin based on a local velocity according to self-similar jet models. Above threshold, the crater shape is investigated with a laser profilometer for increasing Shields number. Three different crater morphologies are reported and we focus on the processes that made the crater switching from one to another morphology. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R17.00003: Particle Settling in Low Energy Turbulence Rachel Allen, Lissa MacVean, Ian Tse, Laura Mazzaro, Mark Stacey, Evan Variano Particle settling velocities can be altered by turbulence. In turbulence, dense particles may get trapped in convergent flow regions, and falling particles may be swept towards the downward side of turbulent eddies, resulting in enhanced settling velocities. The degree of velocity enhancement may depend on the Stokes number, the Rouse number, and the turbulent Reynolds number. In a homogeneous, isotropic turbulence tank, we tested the effects of particle size and type, suspended sediment concentration, and level of turbulence on the settling velocities of particles typically found in muddy estuaries. Two Acoustic Doppler Velocimeters (ADVs), separated vertically, measured turbulent velocities and suspended sediment concentrations, which yield condition dependent settling velocities, via $\frac{\partial \left\langle C \right\rangle }{\partial t}=-\frac{\partial }{\partial z}\left( {w_{s} \left\langle C \right\rangle +\left\langle {w'C'} \right\rangle } \right)$. These results are pertinent to fine sediment transport in estuaries, where high concentrations of suspended material are transported and impacted by low energy turbulence. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R17.00004: Sedimentation dynamics of disks in a linearly stratified fluid Matthieu Mercier, Justin Pemeja, Patricia Ern The settling dynamics of small objects in a stratified fluid is important to understand the fate of the biomass in lakes or oceanic environments, for industrial applications such as waste-water disposal for instance. Recent numerical and theoretical studies dedicated to freely falling (or rising) bodies in stratified environments have shown some important differences compared to the same problem in a homogeneous fluid. Experimental results are still needed for validation, especially at low and moderate values of the Reynolds number, $Re=Ud/\nu \leq 100$, with $U$ the instantaneous vertical velocity of the object, $d$ its characteristic length, and $\nu$ the kinematic velocity of the fluid. We present original experimental results of freely falling disks of finite thickness in a linearly stratified fluid. Three-dimensional trajectories and the wake of the object are obtained using a pair of cameras visualizing two perpendicular planes, revealing a strong influence of the stratification on the dynamics of the object. In particular, the stratification enhances the steady drag experienced by the disks when falling broadside; and generates a change of stability for the disk orientation (from horizontal to vertical) when the Re number decreases below a threshold value. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R17.00005: Feature tracking for measurement of translational and angular displacements of solid objects in fluid flows with application to saltation Carlo Zuniga Zamalloa, A.M. Hamed, Leonardo P. Chamorro Particle Tracking Velocimetry (PTV) is a well-known technique that is used to obtain translational (Lagrangian) paths of particles within a fluid media. In a number of phenomena, the dynamics of free moving particles is of major significance. In addition to purely translational motions, 3-axis rotation can provide major insights into the dynamics of the particle. This feature can be achieved by tracking optical texture added to the moving particles. In the present work we show how this addition of optical texture to a coarse particle moving in a flow yields accurate angular displacements of the particle by means of feature tracking. The three dimensional rotation is obtained from a rotational matrix that is calculated from correspondence of features between image pairs that are consecutive in time. This technique and its resulting rotational information give rich insights into problems such as saltation of coarse particles. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R17.00006: Large Eddy Simulation of suspended sediment transport in wall bounded turbulence Vincenzo Armenio, Marjan Dallali Large eddy simulation is used to investigate suspended sediment transport and its effect on the dynamic of the turbulent boundary layer. We use an Euler-Euler methodology based on single-phase approach. Sediment-induced buoyancy on momentum is considered through a buoyancy term in the Boussinesq form of the \textit{3D} Navier-Stokes equations. We consider four sediment sizes and the simulations are performed for both one-way and two-way coupling approach. The level of stratification for each particle size is qualified by the bulk Richardson number which increases by decreasing the grain size. We calculated first and second order statistics and compared our results with available literature experimental studies. The analysis reveals that the reduction of sediment size produces a larger resuspension and sediment concentration in the flow field, due to the concurrence of increased available concentration at the wall and reduced deposition velocity. Our study also shows that the one-way coupling approach is valid for relatively large sediments, that on the other hand, are more likely transported according to the bed-load mode. For smaller particles, transported according to the suspension-load mode, the two-way coupling approach should be preferred. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R17.00007: Numerical simulation of sediment processes - An approach for a suspension-bed-load model Markus Burkow Current driven sediment transport causes the evolution of bedforms like dunes, ripples or scour marks. In this study we use a numerical simulation of the three dimensional fluid flow and the simultaneous transport to reproduce these sediment processes. To solve the instatioary incompressible Navier-Stokes equations we use NaSt3D as fluid solver for incompressible two-phase flow problems in three dimensions. Fifth order WENO schemes are applied for spatial discretization. For temporal discretization we apply Runge-Kutta schemes up to third order. The free surface between both fluid phases is tracked with a level set technique. The main parts in sediment transport are bed load and suspension load. A common way to model the change of the sediment surface by the rearrangement of the sediment is the Exner equation. Bed load is calculated from the fluid velocities. Additionally an advection-diffusion equation is used to compute the transport of the suspension load in the fluid. Using the conservation of mass the interchange of sediment mass between bed load and suspension load is assured. Single phase examples like dunes and ripples as well as two-phase phenomena like scouring at an obstacle illustrate the large variety of sediment processes, which can be reproduced by this model. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R17.00008: Sediment transport modeling using highly resolved Euler-Lagrange LES Sunil Arolla, Olivier Desjardins We use an explicitly volume-filtered Euler-Lagrange large eddy simulation methodology to investigate the detailed dynamics of turbulent liquid-solid slurry flows through a horizontal pipe. A series of simulations have been performed by varying the superficial liquid velocity to be consistent with the available experimental data by Danielson (2007). From our numerical simulations, the critical deposition velocity below which a static sand bed starts forming is predicted and compared with the experiments. We discuss the dynamics of liquid-solid slurry flow in connection with the Shields diagram. Depending on the Shields number, patterns develop at the surface of the particle bed, in close analogy with patterns discussed in sediment transport research. We also present statistics extracted to evaluate and improve recently proposed RANS based closure modeling ideas in the context of Euler-Euler formalism. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R17.00009: Physical Modeling of the Cross-Shore Sediment Transport on a Sand-Gravel Beach Regis Xharde, Corinne Brunelle, Jannette Frandsen The aim of the study is to investigate the cross-shore evolution of a nourished beach profile under storm wave conditions with specific emphasis on sediment transport within the breaking zone. To investigate the underlying mechanisms of the coastal transport processes, a physical model of the beach was built at scale 1:3 in the new Quebec Coastal Physics Laboratory (QCPL), Canada. The modeled beach is 4.2 m high, 5 m wide and 40 m long with a mean slope of 1:10. The beach is formed of a mixture of sediment with grain sizes ranging from 0.65 mm up to 20 mm. The stability of the beach is tested for operational and storm waves. We report on run-up and run-down processes via wave gages, video records of waves and ultrasonic water level measurements. Sediment transport processes within the surf zone and on the beach face are monitored using acoustic Doppler profilers and optical backscattering sensors. The beach profile is surveyed prior and after each test series using a topographic laser scanner. Initial results show that sand is transported off-shore to a breaker bar while cobbles are pushed on the upper beach by run-up. Details of the underlying mechanism of different breaker types and impact on sediment transport will be presented. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R17.00010: Incipient Motion of Surf Zone Sediments Donya Frank, Diane Foster, In Mei Sou, Joseph Calantoni A combined incipient motion formulation was evaluated to determine the role of the fluid shear stresses and pressure gradients at the onset of sediment motion in oscillatory flows. Small-scale incipient motion experiments were conducted in an oscillating flow tunnel with natural gravel, acetate beads and coarse gravel-sized electronic sediment grains. The effects of sediment characteristics on incipient motion were also investigated. Results suggest that the onset of sediment motion was dominated by the pressure gradients for flows with small orbital excursion amplitudes; by the shear stresses for flows with large orbital excursion amplitudes and by the combined effects for intermediate flows. The critical threshold of the combined parameter was dependent on the static coefficient of friction and the packing concentration of the mobile bed layer. The denser, more angular gravel required greater forcing to trigger sediment motion than the spherical and less dense acetate beads and electronic sediments. The combined incipient motion parameter may be more applicable in the nearshore environment. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R17.00011: Bottom shear stress and pressure perturbations under an internal solitary wave Gustavo Rivera, Peter Diamessis The bottom boundary layer (BBL) under a mode-1 internal solitary wave (ISW) of depression propagating against an oncoming model barotropic current is examined using 2-D direct numerical simulation based on a spectral multidomain penalty method model. Use of a postprocessing projection onto a modified set of divergence-free basis functions enables investigation of wave-based Reynolds numbers within the range $[10^5,10^6]$. At sufficiently high ISW amplitude, the BBL undergoes a global instability which produces intermittent vortex shedding from within the separation bubble in the lee of the wave. The interplay between the bottom shear stress field and pressure perturbations during vortex ejection events and the subsequent evolution of the vortices is the focus of this presentation. Implications for resuspension of bottom particulate matter are discussed in the context of specific sediment transport models. [Preview Abstract] |
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