Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R29: Geophysical Fluid Dynamics: Sediment Transport |
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Chair: Nicholas Ouellette, Stanford University Room: 310 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R29.00001: Onset and cessation of grain motion in fluid-sheared beds Abe Clark, Julia Salevan, Mark Shattuck, Nick Ouellette, Corey O'Hern We performed molecular dynamics simulations of granular beds driven by a model hydrodynamic shear flow to elucidate general grain-scale mechanisms that determine the onset and cessation of sediment transport. By varying the Shields number (the nondimensional shear stress at the top of the bed) and particle Reynolds number (the ratio of particle inertia to viscous damping), we explore how variations of the fluid flow rate, particle inertia, and fluid viscosity affect the onset and cessation of bed motion. For low to moderate particle Reynolds numbers, a critical boundary separates mobile and static states. Transition times between these states diverge as this boundary is approached both from above and below. At high particle Reynolds number, inertial effects become dominant, and particle motion can be sustained well below flow rates at which mobilization of a static bed occurs. We also find that the onset of bed motion (for both low and high particle Reynolds numbers) is described by Weibullian weakest-link statistics, and thus is crucially dependent on the packing structure of the granular bed, even deep beneath the surface. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R29.00002: Wave-Induced Pressure Under an Internal Solitary Wave and Its Impact at the Bed Gustavo Rivera, Peter Diamesis, James Jenkins, Diego Berzi 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. Particular emphasis is placed on the diffusion into the bed of the pressure field driven by the wake and any near-bed instabilities produced under specific conditions. To this end, a spectral nodal Galerkin approach is used for solving the diffusion equation for the wave-induced pressure. 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 examined. The potential for bed failure upon the passage of the ISW trough and implications for resuspension of bottom particulate matter are both discussed in the context of specific sediment transport models. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R29.00003: Highly-resolved numerical simulations of bed-load transport in a turbulent open-channel flow Bernhard Vowinckel, Tobias Kempe, Vladimir Nikora, Ramandeep Jain, Jochen Fr{\"o}hlich The study presents the analysis of phase-resolving Direct Numerical Simulations of a horizontal turbulent open-channel flow laden with a large number of spherical particles. These particles have a mobility close to their threshold of incipient motion andare transported in bed-load mode. The coupling of the fluid phase with the particlesis realized by an Immersed Boundary Method. The Double-Averaging Methodology is applied for the first time convolutingthe data into a handy set of quantities averaged in time and space to describe the most prominent flow features.In addition, a systematic study elucidatesthe impact of mobility and sediment supply on the pattern formation of particle clusters ina very large computational domain. A detailed description of fluid quantities links the developed particle patterns to the enhancement of turbulence and to a modified hydraulic resistance. Conditional averaging isapplied toerosion events providingthe processes involved inincipient particle motion. Furthermore, the detection of moving particle clusters as well as their surrounding flow field is addressedby a a moving frameanalysis. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R29.00004: Two-dimensional PIV measurements for studying the effect of bed permeability on incipient motion of synthetic sediment particles Heng Wu, Carlo C. Zuniga Zamalloa, Jorge E. San Juan Blanco, Blake J. Landry, Marcelo H. Garcia The experimental study of incipient motion, the regime where particles resting on a granular bed enter a process of sediment transport, can be approached using a single-particle pivoting model. Such pivoting model states that the fundamental mechanism of the incipient motion depends, among other factors, on the local fluid flow, bed-flow interface topology, the geometry, and specific density of the sediment particle; yet it does not specify the effect of the bed permeability. In this work the effect that bed permeability has on the incipient motion of a sediment particle is explored by conducting systematic Particle Image Velocimetry measurements of the flow around cylindrical and spherical particles at incipient motion conditions in a water flume. The permeable bed condition for the flume is achieved by placing a synthetic bed at the bottom which consists of cubically packed, uniformly sized spheres. The impermeable condition is obtained by placing at the bottom of the flume a sheet with rows of hemispheres glued to it, the hemispheres being of the same diameter as the ones in the permeable case. The mean velocity profiles are reported to illustrate the influence of the permeable or impermeable beds. The measured velocity data is also compared with the current pivoting model. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R29.00005: Local and non-local effects of spanwise finite perturbations in erodible river bathymetries Mirko Musa, Craig Hill, Michele Guala Laboratory experiments were performed to study the effect of axial-flow hydrokinetic turbine models on an erodible river bed under live-bed conditions. Results indicate that the presence of an operating turbine rotor creates a blockage in the mean flow which produces a remarkable geomorphic signature in the migrating bedforms. These impacts affect a local area downstream of the turbines when placed symmetrically with respect to the cross section of the channel. On the other hand, more interesting results are observed with an asymmetric installation of the turbines. This configuration demonstrates a stronger effect on the mean flow, resulting in a larger plan-wise distortion of the mean topography and differential migration patterns of bedforms. Different turbine installation arrangements and hub heights above the mean bed were investigated, focusing mainly on the perturbation of sediment transport characteristics influenced by the turbine wake. Additional results with spanwise modulated submerged walls explore the possibility to control river topography harvesting this type of geomorphic destabilization. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R29.00006: Improved resuspension flux model based on local shear stress and particle Reynolds number Mrugesh Shringarpure, S. Balachandar The excess density imposed by suspended sediment drives turbidity currents. The dynamics of the flow is controlled by the rate at which these sediment particles are deposited and/or entrained from the bed. Resuspension flux or entrainment of sediment from the bed is an important aspect of all the mathematical models employed to study and simulate turbidity currents. However, a detailed mathematical description of all the physical process that lead to resuspension is not possible due its inherent complexity. In last several decades, through extensive research and detailed experiments and simulations, critical conditions for sediments to be mobilized have been reported. The abstraction of mobilized sediment to equivalent resuspension flux that is robust and truly based on local flow and bed conditions has not been formulated. Such a resuspension flux will be an improvement over the current models that are based on a correlation between average shear stress and the increase in the sediment load as a turbidity current propagates over a certain span of the bed. In this study we will present an improved model for resuspension flux which is a function of local bed shear stress and particle Reynolds number. This function incorporates the local kinematics of a particle lying on the bed along with turbulence characteristics of the flow such that a spatial average of shear stress and resuspension flux reconciles with the existing models proposed in the literature. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R29.00007: On the intermittency of sediment transport in conditions near the threshold of motion Christian Gonzalez, Cristian Escauriaza, David Richter, Diogo Bolster, Joseph Calantoni The dynamics of sediment particles in a flat bed channel is mainly controlled by the coherent structures of the turbulent boundary layer, as intense velocity fluctuations increase the instantaneous bed shear stress and initiate bedload transport. At low shear stress conditions near the threshold of motion, the bedload transport flux becomes intermittent due to the complex particle motion in close contact with the bed. To understand the physical mechanisms that produce the intermittency, in this investigation we develop a Lagrangian sediment transport model to simulate bedload transport in a flat bed channel. We couple direct numerical simulations (DNS) and the discrete element method (DEM) to solve the particle dynamics using a two-way coupling approach. Numerical results shed light on the nature of the intermittency in the transport flux. We conclude that near the threshold of motion, the cumulative sediment transport is described by a fractal behavior, whose characteristics change as the relative shear stress increases. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R29.00008: Morphodynamics of a granular bed in a water-filled cylinder subjected to perturbed oscillations Matias Duran-Matute, Thijs van Gorp, GertJan van Heijst We study experimentally the morphodynamics of a granular bed at the bottom of an oscillating water-filled cylinder. The granules are translucent PMMA particles with a typical size of 2mm. The bed thickness is measured in real time using a light attenuation technique. As shown already by previous work, the bed remains flat close to the center of the cylinder, and radial ripples form at outer radii. The size of the inner flat region and the number or ripples depend on the frequency and amplitude of the cylinder's oscillation. In the present work, we are interested in the dynamics and control of the bed forms when the primary sinusoidal signal of the oscillation is perturbed by adding a second sinusoidal signal with a relatively small amplitude, a different frequency, and a phase lag. Varying the parameters of the secondary signal results in a signal that can be asymmetric or modulated, for example. These properties translate into the bed producing simple behavior like the propagation of the ripples at a constant speed or more complex behavior like the time dependent coarsening and thinning of the ripples. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R29.00009: Two modes for dune orientation Sylvain Courrech du Pont, Clément Narteau, Xin Gao Earth sand seas experience winds that blow with different strengths and from different directions in line with the seasons. In response, dune fields show a rich variety of shapes from small crescentic barchans to big star and linear dunes. Linear dunes often exhibit complex and compound patterns with different length scales and orientations, which seem difficult to relate to a single wind cycle. We present results of underwater experiments and numerical simulations where a single wind regime can lead to two different dunes orientation depending on sediment availability. Sediment availability selects the overriding mechanism for the formation of dunes: increasing in height from the destabilization of a sand bed or elongating in a finger on a non-erodible ground from a localized sand source. These mechanisms drive the dunes orientation. Therefore, dunes alignment maximizes dunes orthogonality to sand fluxes in the bed instability mode, while dunes are aligned with the sand transport direction in the fingering mode. Then, we derive a model for dunes orientation, which explains the coexistence of bedforms with different alignments and quantitatively predicts the orientation of dunes in Earth deserts. Finally, we explore the phase diagram and the stability of the fingering mode. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R29.00010: Self-similar evolution of 2D aquatic dunes over an erodible bed Delphine Doppler, Pierre Yves Lagr\'ee, Philippe Gondret, Marc Rabaud Scale invariance of shape is a common feature of erosion patterns, such as barchan dunes, sand ripples under shoaling waves or scour holes. Due to their universal and fascinating crescentic shape, barchans dunes have received much attention and scaling laws have been deduced from field observations, satellite images and laboratory experiments. On the other hand, the dynamical long term evolution of ripples and dunes formed over an erodible bed has been far less studied while the temporal behavior of erosion patterns contains substantial information on the physical processes involved. Here, we present experimental results obtained in a linear, quasi-2D closed water channel. When a granular bed is submitted to a uniform shear flow, periodic sand ripples appear all along the channel. We found that the first ripple near the channel inlet exhibit unreported long-term scale-invariant growth. The self-similar dune shape and power-law growth exponent are extracted by image processing for several flow velocity. A simple linear model is built using mass conservation and a granular flux law, so that the bed form is described by a self-similar order 2 linear system. Experimental data fit nicely with the model results. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R29.00011: Large-eddy simulation of sand dune morphodynamics Ali Khosronejad, Fotis Sotiropoulos Sand dunes are natural features that form under complex interaction between turbulent flow and bed morphodynamics. We employ a fully-coupled 3D numerical model (Khosronejad and Sotiropoulos, 2014, Journal of Fluid Mechanics, 753:150-216) to perform high-resolution large-eddy simulations of turbulence and bed morphodynamics in a laboratory scale mobile-bed channel to investigate initiation, evolution and quasi-equilibrium of sand dunes (Venditti and Church, 2005, J. Geophysical Research, 110:F01009). We employ a curvilinear immersed boundary method along with convection-diffusion and bed-morphodynamics modules to simulate the suspended sediment and the bed-load transports respectively. The coupled simulation were carried out on a grid with more than 100 million grid nodes and simulated about 3 hours of physical time of dune evolution. The simulations provide the first complete description of sand dune formation and long-term evolution. The geometric characteristics of the simulated dunes are shown to be in excellent agreement with observed data obtained across a broad range of scales. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R29.00012: Scour of Sand-Gravel Beaches in Front of Seawalls Regis Xharde, Jannette Frandsen, Olivier Gauvin-Tremblay Large-scale physical experiments were conducted in the 5m-wide, 5m-deep and 120m-long wave flume at the Quebec Coastal Laboratory of the national scientific research institute (INRS) to evaluate wave-induced scour depth (d$_{\mathrm{s}})$ at vertical seawalls and on natural beaches. In the initial part of the study, the equilibrium beach profile of a mixed sand-gravel beach with a mean grain size diameter of 12 mm was studied for various beach slopes using regular and irregular waves with intermediate water depths (h$_{\mathrm{0}}\in $ [2.3; 3.8] m) and different wave heights. In the second part of the study, a vertical seawall fronted by a 1:10 sloping mixed sand-gravel beach was tested for more than 50 wave trains using regular and irregular waves with various water depths at the seawall (h$_{\mathrm{w}})$, wave heights and wave periods. The scour depth at the toe of the seawall is highly dependent on the form of wave breaking onto the structure. Sea states where plunging breakers occur directly onto the wall generate jets of water that may penetrate to the seabed and cause a local scour hole immediately adjacent to the seawall. Scour depth is maximum when H$_{\mathrm{b}}$/h$_{\mathrm{w}}$ \textgreater 1 and X$_{\mathrm{b}}$/H$_{\mathrm{b}}$ \textless 1, where H$_{\mathrm{b}}$ is the breaker height and X$_{\mathrm{b}}$ the distance from the seawall of the breaking wave. Comparison with existing semi-empirically derived scour prediction equations was performed. [Preview Abstract] |
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