Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R28: Geophysical Fluid Dynamics: Sediment Transport |
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Chair: Mohammad Farazmand, North Carolina State University Room: 251 F |
Monday, November 25, 2024 1:50PM - 2:03PM |
R28.00001: Bedload sediment transport in the turbulent flow over a rough bed with an array of boulders Maria Magdalena Barros, Ivana Vinkovic, Cristian Escauriaza Bedload transport is a complex phenomenon significantly influenced by near-wall turbulence, where coherent structures and velocity fluctuations can cause the onset of motion and mobilize sediments downstream. Namely, particles are subject to the competition of instantaneous local stresses with resistive forces and particle collisions. Transport is also affected when large immobile boulders, induce flow separation and large-scale unsteady vortices that can either hide or expose smaller mobile grains to the local flow. With the objective of improving our understanding of the effects that an array of boulders placed on a rough bed produces on bedload transport, we perform Large Eddy Simulations (LES) coupled with an immersed boundary method for the boulders and the rough bed. Based on the double-averaged (DA) methodology we explain the effects caused by the turbulence generated at the scale of the roughness elements through the new terms on the DA momentum and energy budgets. We also evaluate the effective shear stress to estimate bedload fluxes, considering form-induced stresses and excluding drag. The turbulent flow is then coupled to the Lagrangian tracking of sediment particles based on the discrete element method (DEM) to evaluate how spatial flow variations influence transport. Throughout this work we characterize the spatial variability of bedload fluxes and other sediment quantities such as particle velocity and activity in regard to local flow properties for two different mobility conditions. |
Monday, November 25, 2024 2:03PM - 2:16PM |
R28.00002: The interplay between turbulence structures and forces on bed spheres in open-channel flow through boulder arrays at various Froude numbers Yan Liu, Zhengdao Tang The Froude number (Fr) is a key parameter affecting turbulence structures, bedload transport, and bedforms in mountainous rivers. In this paper, the interplay between turbulence structures and forces on bed spheres in open-channel flow through boulder arrays at a Froude number range from 0.15 to 0.89 are reported. At low and intermediate Fr, the boulder top is above the water surface and time-averaged streamwise flow velocity, Reynolds shear stresses, and the turbulent kinetic energy (TKE) are relatively low in the wake of boulders. Conversely, at high Fr values, the boulders are submerged, hence the flow separates at the boulder crest, creates vertical recirculation and reattaches on the bed downstream, resulting in an area of elevated Reynolds shear stresses and TKE downstream of the boulders. Two dominant turbulence structures are observed: (i) flapping of boulder wakes with a characteristic length of 2.1 times the boulder diameter (D) at low and intermediate Fr and (ii) an upstream oriented hairpin vortex with a length scale of 1.0D at high Fr. These turbulence structures influence hyporheic exchange downstream of boulders within a limited region of x/D < 2.0. In other locations, hyporheic flow is driven by downwelling flow immediately upstream of boulders with a wavelength larger than 2.9D. Finally, the normalised time-averaged hyporheic flux increases with increasing Fr, but decreases at higher Fr values once overtopping flow disrupts the formation of the boulder wake. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R28.00003: Particle-resolved simulation of sediment transport over vibrating bed Yuanqing Liu, Chloe Seibert, Chris Paola, Cecilia McHugh, Leonardo Seeber, Lian Shen To understand surface-particle remobilization during seismic events, we performed particle-resolved direct numerical simulations (PR-DNS) on sediment transport over a vibrating bed. We considered varying oscillation conditions in horizontal and/or vertical directions and conducted both time-averaged and phase-averaged analyses on different cases. Time-averaged analyses revealed smaller mean velocities and reduced Reynolds normal stresses in all cases involving oscillations. Phase-averaged analyses suggested that fluid velocity and Reynolds normal stresses are significantly influenced by the oscillation phase. Regarding sediment transport rate, time-averaged analyses showed that mean sediment transport rates increased in all cases involving oscillations, while phase-averaged analyses highlighted different sediment transport patterns across varying oscillation conditions. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R28.00004: Taylor dispersion of particles under surface waves Andrew Russell, Michelle Heather DiBenedetto The transport of buoyant particles in the ocean, such as microplastics, oil droplets, and bubbles, is largely influenced by the free surface boundary layer, including surface waves and turbulence. These particles accumulate at the free surface, but turbulence can mix them downward, distributing them throughout the vertically sheared flow. This causes particles to move at different relative speeds, enhancing their streamwise spreading, commonly known as Taylor dispersion. The rate and magnitude of this dispersion is primarily controlled by the wave parameters, particle rise velocity, and turbulent diffusivity profile. In this talk, we investigate the Taylor dispersion of particles suspended under waves, separating the effects of the unsteady, fast orbital motion from the slow Stokes drift. We derive an analytical model to estimate the influence of wave steepness, relative wave depth, and Peclet number characterized by the rise velocity and wave amplitude. Lastly, we contrast our results with a numerical simulation, which shows similar trends to the analytical model. |
Monday, November 25, 2024 2:42PM - 2:55PM |
R28.00005: Diffusion-limited settling of highly porous particles in density-stratified fluids Robert Hunt, Roberto Camassa, Richard M McLaughlin, Daniel M Harris The vertical transport of solid material in a stratified medium is fundamental to a number of environmental applications, with implications for the carbon cycle and nutrient transport in marine ecosystems. In this work, we study the diffusion-limited settling of highly porous particles in a density-stratified fluid through a combination of experiment, analysis, and numerical simulation. By delineating and appealing to the diffusion-limited regime wherein buoyancy effects due to mass adaptation dominate hydrodynamic drag, we derive a simple expression for the steady settling velocity of a sphere as a function of the density, size, and diffusivity of the solid, as well as the density gradient of the background fluid. In this regime, smaller particles settle faster, in contrast with most conventional hydrodynamic drag mechanisms. Furthermore, we outline a general mathematical framework for computing the steady settling speed of a body of arbitrary shape in this regime and compute exact results for the case of general ellipsoids. Using hydrogels as a highly porous model system, we validate the predictions with laboratory experiments in linear stratification for a wide range of parameters. Lastly, we show how the predictions can be applied to arbitrary slowly varying background density profiles and demonstrate how a measured particle trajectory can be used to reconstruct the background density profile. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R28.00006: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 3:08PM - 3:21PM |
R28.00007: Fate of positively buoyant particles in a scaled swash flow Carlos Abarca, Tong Ling, Michelle Heather DiBenedetto Microplastics are ubiquitous throughout the world's oceans including their coastlines. These coastlines act as both a source and a sink for ocean microplastics, a necessary boundary condition for modelling their transport. On beaches, the swash zone controls the transport of microplastics across the coastal boundary. And yet, there is still no definitive model to predict the fate of microplastics once within the swash zone. To address this problem we performed a dimensional analysis to identify the key non-dimensional parameters. Next, we designed laboratory experiments to explore this parameter space, where we tracked particles that were initially at rest on a beach within the swash zone. We observe that the smallest particles that start lowest on the beach are more likely to end up higher on the beach after a solitary swash event, whereas larger particles that start higher on the beach are more likely to beach lower or wash off entirely. To characterize these results, we calculate the Stokes number at the point of initial wave impact. We find that this Stokes number is well correlated with particle final position in our data, likely because the particle's ultimate position in the wave and its subsequent transport by the wave is set by how it is initially picked up. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R28.00008: Extreme firebrand transport by atmospheric waves in wildfires Mohammad M Farazmand, Louisa Ebby In wildfires, burning pieces of wood (i.e. firebrand) are transported downstream by wind. Once they land, these firebrands can start a secondary fire far away from the main fire. This process is referred to as spotting and the secondary fires are called spot fires. In this talk, we first show numerically that atmospheric waves can lead to extremely large spotting distances, sometimes reaching several kilometers. Then we present a dynamical systems theory which explains and quantifies these large spotting distances. In essence, our theory identifies time-dependent trapping regions with relatively high lift which delay the landing of some firebrands. |
Monday, November 25, 2024 3:34PM - 3:47PM |
R28.00009: Deposition in Biologically Cohesive Sediment within Emergent Canopies HYOUNGCHUL PARK, Heidi Nepf Extracelluar Polymetric Substances (EPS) produced by microorganisms are ubiquitous in aquatic vegetated channels, playing a crucial role in geophysical processes. Biofilms formed by EPS bind sediments together, enhancing the stability of the channel bed and reducing erosion. The change in sediment porosity and cohesiveness due to biofilm can also affect deposition processes within the vegetated canopies, but previous studies on vegetated conditions have focused on abiotic (non-cohesive) sediment. Therefore, this study investigated the role of biofilm in deposition process within a vegetated channel through laboratory experiments. Mangrove pneumatophores were modelled using cylinder dowels, and a biologically cohesive sediment bed was created by mixing non-cohesive sand with commercial EPS (Xanthan gum), ranging from 0 to 0.5 % of dry sand weight. A fixed amount of fluorescent particles were released in suspension, and the deposition rate was measured by counting the number of particles deposited to the bed over a fixed time. The deposition rate was inversely proportional to the velocity. At the same velocity, the deposition rate decreased with increasing cohesion (increasing EPS). This reduction in deposition rate was attributed to the decrease in porosity caused by increased EPS, which weakened the hiding effect and enhanced exposure effect. The critical Shields parameter for the deposited particles decreased with increasing EPS, which was used to develop a new deposition probability model. |
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