Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C38: Geophysical Fluid Dynamics Sediment Transport |
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Chair: Bruce Sutherland, University of Alberta Room: 620 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C38.00001: Dispersion and transport of tracer particles at an upper ocean front Vicky Verma, Sutanu Sarkar Turbulence resolving simulations of upper-ocean fronts show the formation of thin filament structures during the evolution of baroclinic instability. The filaments develop large vertical vorticity and vertical velocity; furthermore, long filaments roll up to form coherent submesoscale eddies. The role of coherent structures, i.e. the vortex filaments and the eddies, in vertical and lateral transport and dispersion of particles has been studied for a submesoscale mixed layer front using tracer particles which advect with the local fluid velocity. The time evolution of tracer particles reveals stirring across the front, as well as vertical transport directed by the coherent structures. There are vortex filaments on both the heavier and lighter sides of the front that provide direct pathways for vertical transport. The filament on the heavier side downwells the local (heavier) fluid particles to the bottom, spiraling around the eddy, while the filament at the lighter side upwells the local (lighter) fluid particles to the surface. Subsequently, the downwelled and upwelled particles slowly adjust to a smooth stable profile. The effect of coherent structures on the dispersion of particles is studied using single-particle, two-particle and multi-particle dispersion statistics. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C38.00002: Sediment transport and morphodynamics induced by a translating vortex Matias Duran-Matute, Samuel Gonzalez-Vera, GertJan Van Heijst We present experimental results on the effect of a translating monopolar vortex on a sediment bed. Experiments took place inside a water-filled square tank with a flat layer of small spherical particles at the bottom and placed on top of a rotating table. A vortex is generated on the tip of a plate perpendicular to one of the sidewalls by drastically changing the speed of the rotating table from a state of solid body rotation. This change was designed such that a residual current remained, advecting the vortex away from the plate. Strong-enough vortices bring sediment into suspension and transport it along their path. As the vortices weaken, the sediment settles back into the bed. This mechanism produces changes in the bed. Measurements of the flow were obtained through particle image velocimetry (PIV). The region of suspended sediment was reconstructed using images of the particles illuminated by a vertically oscillating horizontal laser sheet, and the changes in the bed thickness were measured with a photogrammetric technique. The strength of a vortex is the main parameter governing the capture and suspension of particles with similar settling velocity. A power law was found between the vortex strength and the net displaced particle volume. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C38.00003: Two-way coupled direct numerical simulations of sinuous-crested bedforms. Nadim Zgheib, S Balachandar We present results from direct numerical simulations on the transition from straight-crested to sinuous-crested bedforms. The numerical setup is representative of turbulent open channel flow over an erodible sediment bed at a shear Reynolds number of \textit{Re}$_{\tau }=_{\mathrm{\thinspace }}$180 \begin{figure}[htbp] \centerline{\includegraphics[width=0.72in,height=0.19in]{090720191.eps}} \label{fig1} \end{figure} . The immersed boundary method accounts for the presence of the sediment bed. The simulations are two-way coupled in the sense that the turbulent flow can erode and modify the sediment bed, and in turn, the sediment bed modifies the overlying flow. The coupling from the flow to the sediment bed occurs through the Exner equation, while the back coupling from the bed to the flow is achieved by imposing the no-slip and no-penetration condition at the immersed boundary. The simulation setup is similar to that by Zgheib et al. (\underline {https://doi.org/10.1002/2017JF004398}) except for the presence of sidewalls to better mimic laboratory flume conditions. Sidewalls are observed to significantly increase bedform sinuosity. We also investigate the effect of domain size and a zero sediment flux boundary condition on crestline sinuosity. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C38.00004: Dune-dune repulsion Karol Bacik, Sean Lovett, Colm-cille Caulfield, Nathalie Vriend Dunes are coherent sedimentary structures which arise spontaneously due to the dynamical interplay between granular matter and the flow of the overlaying fluid. Natural dunes rarely occur in isolation. Aeolian dunes form vast dune fields and subaqueous bedforms occur in groups. As of now, the mechanisms which regulate the large scale organisation of dune fields are poorly understood. In particular it is unclear if the dune configurations we observe are stable or transient. Here we investigate the dynamics of a quasi-2D dune corridor using a subaqueous experiment in an annular geometry. We show that the corridor structure appears to be robust and that stabilisation is achieved by long range dune-dune interactions. Our experiments reveal that by altering the flow, dunes strongly affect the shape and the migration rate of their downstream neighbours which leads to an effective dune-dune repulsion. Here, we discuss the physical origin of the repulsion mechanism and explore its consequences for the system-level dynamics of the dune corridor. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C38.00005: Aggregate particle settling from a laboratory river plume David Deepwell, Brianna Mueller, Bruce Sutherland Motivated by understanding how particles in sediment bearing river plumes ultimately settle, we present laboratory experiments of a constant-flux fresh water surface gravity current containing a low ($<1$\%) concentration of dense particles which initially overrides a saline ambient. Two classes of experiments were performed: 1) with a uniform density saline ambient fluid and 2) a two-layer ambient fluid. In both classes of experiments, collective settling combined with ambient recirculating flows result in particles settling out predominantly as a large single vertical plume. As the plume falls, the centre of the plume travels faster than the fluid around it causing an entrainment of neighbouring particles leading to a narrowing of the plume with depth. In a two-layer stratification with micrometer-sized particles, the settling is delayed at the interface between the upper and lower layers as a result of lighter fluid being dragged down with each particle in its surrounding viscous boundary layer thereby decreasing its effective density. The width and location of the sedimentation plume as well as the enhanced settling velocity of particles in the plume is characterized with respect to particle density and concentration. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C38.00006: Strain-hardening evolution on sediment transport under cyclic laminar shear Fernando Cúñez, Erick M. Franklin, Morgane Houssais, Paulo E. Arratia, Douglas J. Jerolmack Fluid-driven granular flows are common in nature and industry, yet difficult to understand. We present experiments using an annular flume that mimics an infinitely-long river, filled with refractive-index matched, sedimenting particles sheared by a laminar flow; this allows visualization of particle transport throughout the bed. We introduce stepped cycles of fluid shear and examine the downward propagation of an ``unjamming'' front, which marks the boundary between a granular fluid and a creeping solid. Repeated shear cycles give rise to strain hardening, as the fluidized layer thins over progressive cycles to an eventual steady state. Strain hardening results from two effects: isotropic compaction, and formation of anisotropic grain fabric structures aligned with the applied shear. When shear direction is reversed, the bed recovers some (but not all) of its initial mobility. Strain hardening, associated compaction and grain-fabric formation, occur in the observed creep regime. Beyond a critical shear rate, grains are fluidized resulting in dilation and destruction of granular fabrics. Results explain puzzling observations of bed-load hysteresis in natural rivers and delineate the transport regimes under which memory is created and destroyed in sedimentary beds. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C38.00007: LES of a large-scale river with arbitrarily complex bathymetry and wall-mounted structures Kevin Flora, Ali Khosronejad High-fidelity numerical simulation of flow in natural riverine environments is essential for understanding the potential impacts of flood events on infrastructure and the potential morphological changes to the channel. However, due to the bathymetric complexity and unique shapes of man-made structures located in the flow, it is very challenging to create high-fidelity models which accurately depict the dominant three-dimensional flow structures. Such flow dynamics often lead to the erosion of the banks, scour of infrastructure foundations and overall transport of sediment in the flow. As a result, simplified models are often used in engineering practice, but these models fail to accurately simulate important features of flow. We employ a large eddy simulation (LES) code to model flow around four bridges in the American River in California to study the qualitative and quantitative differences in the resulting data. Specifically, this study compares the results of the LES code with more simplified hydrodynamic models to assess the difference in the estimated bed shear stress distribution and the overall impacts of the bridge structures on the turbulence. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C38.00008: Fluid-driven transport of spherical sediment particles: from discrete simulations to continuum modeling Qiong Zhang, Ken Kamrin, Eric Deal, Taylor Perron, Jeremy Venditti, Santiago Benavides, Matthew Rushlow Empirical bedload transport expressions commonly over- or underpredict sediment flux by more than a factor of two, even under controlled laboratory conditions. In this work, the Discrete Element Method and Lattice Boltzmann Method are coupled together to simulate 3D fluid-driven transport problems, in which the spherical sediment particles are fully resolved. After comparisons with flume experiments are made to test the numerical simulations, the grain-scale physics is studied, such as the flow field around individual particles and higher order descriptions of the granular motion. A more robust continuum model, unifying empirical models under various conditions and in different regimes, is further proposed based on the new grain-scale understanding of the mechanisms. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C38.00009: Three-phase flow LES of flash floods in a real-life desert stream Ali Khosronejad, Kevin Flora We present a fully coupled three-phase flow model of air-water-sediment to simulate numerically the propagation of flash floods in field-scale dry-bed desert streams. The turbulent flow and free surface of the flash flood are computed using large-eddy simulation (LES) level-set method, respectively. The evolution of stream morphology, due to the propagating flood on the mobile bed, is calculated using an Eulerian morphodynamics model based on the curvilinear immersed boundary method. We demonestarte the capabilities of our numerical framework by applying it to simulate a flash flood event in a 0.65 km-long reach of a desert stream in California. The simulated region of the stream includes a number of bridge foundations. Simulation results of the model for the flash flood event revealed the formation of highly complex flow field and scour patterns within the stream. Moreover, our simulation results show that most of the scour processes takes place during the steady phase of the flash flood. The transient phase of the flash flood is rather short and contributes to a very limited amount of erosion within the desert stream. \textbf{Acknowledgment:} support for this work is provided by NSF award EAR-1823121. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C38.00010: Euler-Lagrange Simulations of Bedload Transport and Bedform Generation Liheng Guan, Jorge Sebastian Salinas, S. Balachandar Bedload transport is a subset of sediment transport in which the sediment particles rolls, slides and saltates under the influence of the overlying flow field. The particles in the bedload occasionally gets lifted and gets suspended at high enough flow rate. In this work, the bedload tranport problem is investigated by performing numerical simulations with implementation of Euler-Lagrange point-particle approach. The fluid field is solved on the Eulerian reference frame while the motion of sediment particles are tracked on the Lagrangian reference frame. Moreover, the collisions between particles are handled using a soft-sphere collision model which includes both a normal collision force and a tangential collision force. Additionally, the rotational motion of particles is also considered in this work. In the bedload tranport simulation, a fully developed turbulent channel flow is imposed over an initially flat bed. Various bedform structures, from longitudinal streaks to ripples, arise as the sediment bed evolves. The results are compared with those using Exner's equation. [Preview Abstract] |
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