Session T33: Geophysical Fluid Dynamics: Sediment Transport

Linking turbulent eddies to suspended sediments in open channel flows

Modeling suspended sediment concentration in the turbulent open channel flow continues to draw significant research attention given its relevance to multidisciplinary problems across geophysics and hydrology. In terms of contemporary and emerging challenges, suspended sediments have been shown to interact with nano/microplastics via aggregation and settling processes in rivers and other water systems. Early studies linking turbulent diffusivity to suspended sediments were conducted based on an effective mixing length theory developed for turbulence. However, the energetics of eddies, a significant characteristic of turbulence, were not explicitly represented in these sediment models. To bridge this gap, a new perspective will be offered for representing suspended sediment concentration in such applications by providing a formulation that accommodates all scales of turbulent eddies. The approach is to implement a co-spectral budget (CSB) model that can relate the turbulent sediment flux to the local shear stress and the sediment properties. The CSB considers the pressure-redistribution term as a revised linear Rotta scheme in spectral form, resolving the isotropozation of mechanical production and particle-turbulence interaction with a scale-dependent de-correlation relaxation time. Provided a commonly accepted vertical velocity spectrum and standard constants from turbulence models, the CSB result can resolve all the effects from turbulent eddies of multiple length scales, Reynolds number, and Schmidt number.

    

Reconstruction of aeolian saltation enabled by measurements of particle accelerations

Aeolian saltation critically affects geo-morphological processes and Earth's climate, but our understanding of it is incomplete. The dynamics close to the bed are challenging to characterize experimentally, due to the combination of rapid small-scale processes happening at relatively dense concentrations. The process is also difficult to model numerically, due to the vast range of scales at play in the two-way coupling between the turbulent boundary layer and the microscopic particles. In this study, imaging experiments on size-selected glass microspheres are conducted in a large atmospheric wind tunnel, obtaining extensive data sets at high spatio-temporal resolution. Particle tracking velocimetry is applied to measure the saltating particle kinematics, including accelerations, in the millimetres above the bed. Assuming established drag formulations, the data is used to estimate the mean fluid velocity, recovering a logarithmic profile. Also, assuming a quasi-ballistic motion in the vertical direction, constraints on the vertical energy distribution are derived and the trajectory shape for a given ejection energy is reconstructed. This allows evaluating the statistical distribution of the saltation length and height, which are crucial for the modelling of the process.   

Sediment Erosion And Deposition In The Wake Flow Of Flexible Vegetation

Sediment erosion and deposition downstream of thin rectangular flexible plates mimicking aquatic vegetations with aspect ratios ranging from 5 to 15, and width-based Reynolds numbers ranging from 2500 to 3900 were experimentally investigated. The sediment bedform and topography were characterized using a high-resolution surface scanner whose data was later utilized to calculate net erosion volume. The impacts of wake flow dynamics on sediment erosion patterns were further illustrated using time-resolved Three-dimensional Particle Image Velocimetry (3D-PIV). The results indicate that enhanced local turbulent intensities due to the presence of flexible plates caused sediment erosion right downstream of the obstacles. Sediment erosion rate was highly influenced by both aspect ratio and Reynolds numbers. Specifically, the growth of Reynolds number resulted in increased erosion rates. However, the trend was opposite with the growth of plate aspect ratio. Further investigation of three-dimensional flow statistics highlighted that higher aspect ratio led to larger Cauchy number and inclination angles at the base of flexible plates, which was accompanied with lower turbulent kinetic energy levels and therefore mitigated local sediment erosion.

    

Direct Numerical Simulations of Oscillatory Boundary Layers Over Moving Spherical Particles

     Oscillating turbulent flows contribute to the coastal erosion that affects natural and human-made structures located in the coastal regions of the ocean. To study this phenomenon, Direct Numerical Simulations of the oscillating turbulent boundary layer were performed. As an initial benchmark, a first set of simulations were completed where the oscillatory flow was studied over a bed composed of fixed spherical particles. With increasing Reynolds number, it was observed that the turbulent stresses reached their maximum values at different phases of the oscillation. At the same time, at each given Reynolds number, a phase delay was present between the maximum stress and wall oscillation. As a consequence, imbalances in the turbulent kinetic energy budgets were generated during the oscillation. 

     In the second set of simulations, the motion of the particles was computed by solving the rigid body equation using the hydrodynamic forces of the flow. These forces were calculated from the flow velocity and pressure acting around the particles. The effect of particles on the near wall velocity field will be discussed at the conference presentation. 

    

How does polydispersity of grains affect barchan-barchan interactions?

Barchans are crescent-shaped dunes found in several environments such as deserts, bottom of rivers, oil pipelines, and also on the surface of Mars. They belong to dune fields, where interactions between dunes of different sizes and speeds regulate the dynamics of the entire field. Previous studies reported interactions between barchans composed of monodisperse particles, but dunes in nature consist of polydisperse mixtures of grains. In this study, we investigate experimentally the binary interactions of barchans consisting of either (i) bidisperse mixtures or (ii) for each dune, different monodisperse grains. We measured both the dune morphologies and motion of individual grains as two subaqueous barchans interacted in a water channel. We observed that the interaction patterns vary with the grains forming the dunes, with new patterns arising when each barchan consists of monodisperse grains of different size. In addition, we computed the sediment flux over barchans during different interaction patterns. Some of the results shown in this presentation are reported in Assis et al., JGR:Earth Surf., 2022.   

The influence of permeability on the settling behavior of porous flocs

Currently, the Stokes-based relation for a drag coefficient is used in the majority of experimental studies to calculate the settling velocity of flocculated sediments. Despite the fact that there are more sophisticated models to calculate settling velocity, it is usually assumed that the main mechanism affecting the settling velocity is the reduced density of the sediment. In the present work, we, therefore, focus on the influence of permeability of porous sediments. We analyze available models for the settling velocity of sediments and compare them with existing experimental measurements of various sediment types. We show that even though various settling models yield similar results to capture experimental data of single flocs settling in quiescent fluid, the mechanisms of drag reduction are different. To this end, we carry out particle resolved Direct Numerical Simulations that treat porous flocs as permeable objects. Using this approach to simulate more complex scenarios such as settling in a stratified fluid and hindered settling, we illustrate that permeability of flocs is a key parameter that strongly affects the settling behavior and cannot be neglected.   

Drag enhancement by the addition of weak and strong waves to a wave-current boundary layer over bumpy walls

We present a comparison of direct numerical simulation results of two wave-current boundary layers over bumpy walls: one in the current-dominated flow regime (wave-driven to steady current ratio of 0.34) and the other in the wave-dominated flow regime (wave-driven to steady current ratio of 1.25), giving respective wave Reynolds numbers of 351 and 4780. The turbulent, wave-current channel flow has a friction Reynolds number of 350 for both cases. At the lower boundary, a bumpy wall is introduced with a direct forcing immersed boundary method, while the top wall has a free-slip boundary condition. For the current-dominated case, despite the hydraulically smooth nature of the wave-driven flow, the addition of waves to the steady flow enhances the turbulent kinetic energy (TKE) dissipation and results in an increase in the relative roughness of 1.4 times the physical roughness and an increase in the drag coefficient by 10%. For the wave-dominated case, the relative roughness increases by a factor of 8 times the physical roughness and the drag coefficient increases by 72%. These results agree with the theoretical predictions by Grant and Madsen (1979), even in the current-dominated regime which should not apply to the theoretical predictions. By analyzing the TKE and Reynolds Stress budgets, we observe that pressure-strain rate correlations for the bumpy wall cases play a significant role in redistributing TKE from the streamwise directions to the other two coordinate directions. Additionally, we observe enhanced second quadrant events responsible for streak lifting or breakdown events resulting in larger dissipation.   

Non-hydrostatic RANS simulation of nearshore Langmuir circulation and sediment transport

Langmuir turbulence in the coastal ocean is driven by winds and waves and is characterized by Langmuir cells (LCs) that can span the full depth of unstratified water columns in the coastal ocean. LCs consist of wind-aligned parallel counter-rotating vortices, which when spanning the full-depth of the water column can result in significant sediment resuspension and subsequent lateral transport. Non-hydrostatic Reynolds-averaged Navier-Stokes (RANS) simulations of LCs in the surf-shelf transition zone of the coastal ocean will be presented. In this region, the structure of the LCs is dependent on depth-refraction of the gravity surface waves generating the LCs, surface wave breaking, and the alongshore current generated by the wave-breaking. Overall, the present simulations account for effects of nearshore processes on LCs, unlike earlier simulations which have focused on LCs in the open ocean away from coastal boundaries. In addition to LCs, the resolved flow is characterized by a wind-driven, onshore-directed surface shear current and a compesating undertow. Given this neashore circulation, lighter sediments suspended by LCs will be transported onshore and heavier suspened sediments will be transported offshore. This sediment transport behavior will be investigated in terms of various sediment classes characterized by their density and settling velocity.   

Dynamics of an oscillatory boundary layer over a cohesionless bed of particles at increasing Reynolds number in Eulerian-Lagrangian simulations.

We investigate the effects of oscillating pressure gradients over a cohesionless particle bed on the formation of bedforms and grain entrainment using high-fidelity Eulerian-Lagrangian simulations. The bed is created by allowing sand particles of size 550[endif]--> to sediment onto a solid wall, forming a sediment layer of height approximately equal to 25 particle diameters. Four cases are considered where the period of oscillations is maintained constant at 7s and the magnitude of the pressure gradient is varied to yield the Reynolds numbers 100, 200, 400 and 800. To understand the role of the sediment grains in modifying flow features, auxiliary simulations with no particles, i.e., simulations of oscillatory boundary layers on a smooth flat wall, are conducted at the four same Reynolds numbers and compared with the simulations. The first three cases yield an oscillatory boundary layer in the disturbed laminar, whereas the fourth case falls under the intermittently turbulent regime. The sediment transport modes, i.e saltation and suspension are analyzed in all cases, as well as the bedform generated by the oscillatory flow. With the presence of a bed, the flow at Reynolds 100 is seen to largely behave as a laminar oscillatory boundary layer with a shift in the wall location corresponding to the bed height. Whereas the flow remains laminar at Reynolds 200 and 400 without the particle bed, intermittent vortex shedding is observed when the particle bed is included, caused by small bedforms. At Reynolds 800, vortex shedding intensifies and is observed throughout the period of oscillation. Further, the bedform shows dynamic particle dunes, with the top layer of particles saltating across the bed at phases with the highest vorticity, before returning to rest as the intensity of the vortical structures is reduced.

    

Stability of subglacial sediment systems

Glacier surges are quasiperiodic episodes of fast ice flow that occur principally due to increased slip at the ice-bed interface. However, mechanisms that trigger glacier surges are poorly understood. We study deformable subglacial sediment systems by drawing on earthquake literature to formulate a rate-and-state description of subglacial sediment and coupling it to an overlying viscous ice layer. Our framework models saturated sediment and accounts for the evolution of water pressure. The formulation is shown to produce a positive feedback process between the dynamics of the granular sediment and the viscous flow of the glacier, revealing an instability mechanism. We explore the stability of the purely time-dependent system, constructing a full regime diagram showing the conditions for stability. On incorporating vertical dependence into the system, we find that the sediment layer deepens, and sediment flux is enhanced during surge events, corresponding to observations of surging glaciers.   

Quantifying rare events in spotting: How far do wildfires spread?

Spotting refers to the transport of burning pieces of firebrand by wind which, at the time of landing, may ignite new fires beyond the direct ignition zone of the main fire. Spot fires that occur far from the original burn unit are rare but have consequential ramifications since their prediction and control remains challenging. We examine three methods for quantifying the landing distribution of firebrands: crude Monte Carlo simulations, importance sampling, and large deviation theory (LDT). We propose an LDT method that accurately quantifies the low probability events at the tail of the landing distribution. In contrast, Monte Carlo and importance sampling methods are most efficient in quantifying the high probability landing distances near the mode of the distribution, but become computationally intractable for quantifying the tail of the distribution due to the large sample size required. Defining the relative landed mass as the proportion of mass landed at a given distance from the main fire, we derive an explicit formula which allows computing this quantity as a function of the landing distribution at a negligible computational cost. We numerically demonstrate our findings on two prescribed wind fields.   

Interface-resolved simulation of particles in sediment transport

Sediment transport is a common phenomenon happening in nature and it constantly affects the landscape and human lives. The movement and collision of sediment particles modify the flow field around them. Among the flow configurations and particle properties, the shape of particles plays an important role in the fluid-particle interaction process. In this talk, we present numerical simulations of sedimentation transport for spherical particles, oblate particles and their mixture. We use the immersed boundary method to resolve the interactions between the flow and particles. Collisions between particles are modelled by a linear spring-dashpot system. The Adaptive Collision Time Model is employed to calculate the contact force. These numerical tools enable us to study realistic flows where point particle model is not applicable, providing further insights into the sediment transport process.