85 research outputs found
Settling of cohesive sediment: particle-resolved simulations
We develop a physical and computational model for performing fully coupled,
particle-resolved Direct Numerical Simulations of cohesive sediment, based on
the Immersed Boundary Method. The model distributes the cohesive forces over a
thin shell surrounding each particle, thereby allowing for the spatial and
temporal resolution of the cohesive forces during particle-particle
interactions. The influence of the cohesive forces is captured by a single
dimensionless parameter in the form of a cohesion number, which represents the
ratio of cohesive and gravitational forces acting on a particle. We test and
validate the cohesive force model for binary particle interactions in the
Drafting-Kissing-Tumbling (DKT) configuration. The DKT simulations demonstrate
that cohesive particle pairs settle in a preferred orientation, with particles
of very different sizes preferentially aligning themselves in the vertical
direction, so that the smaller particle is drafted in the wake of the larger
one. To test this mechanism in a system of higher complexity, we perform large
simulations of 1,261 polydisperse settling particles starting from rest. These
simulations reproduce several earlier experimental observations by other
authors, such as the accelerated settling of sand and silt particles due to
particle bonding. The simulations demonstrate that cohesive forces accelerate
the overall settling process primarily because smaller grains attach to larger
ones and settle in their wakes. For the present cohesion number values, we
observe that settling can be accelerated by up to 29%. We propose physically
based parametrization of classical hindered settling functions proposed by
earlier authors, in order to account for cohesive forces. An investigation of
the energy budget shows that the work of the collision forces can substantially
modify the relevant energy conversion processes.Comment: 39 page
Consolidation of freshly deposited cohesive and non-cohesive sediment: particle-resolved simulations
We analyze the consolidation of freshly deposited cohesive and non-cohesive
sediment by means of particle-resolved direct Navier-Stokes simulations based
on the Immersed Boundary Method. The computational model is parameterized by
material properties and does not involve any arbitrary calibrations. We obtain
the stress balance of the fluid-particle mixture from first principles and link
it to the classical effective stress concept. The detailed datasets obtained
from our simulations allow us to evaluate all terms of the derived stress
balance. We compare the settling of cohesive sediment to its non-cohesive
counterpart, which corresponds to the settling of the individual primary
particles. The simulation results yield a complete parameterization of the
Gibson equation, which has been the method of choice to analyze self-weight
consolidation.Comment: 16 pages, 9 figures, accepted for Physical Review Fluid
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Eroding Uncertainty: Towards Understanding Flows Interacting with Mobile Sediment Beds Using Grain-Resolving Simulations
Dense particle-laden flows play an important role in many environmental processes, including the shaping of rivers and the formation of landslides. Despite decades of study, researchers have not been able to accurately predict the onset of erosion and the amount of sediment transported by flows, due in part to the difficulty in measuring dense particle-laden flows. Highly-resolved numerical simulations, on the other hand, allow us to study the physics of particle-fluid and particle-particle interactions in much more detail.We develop a code to accurately simulate dense, polydisperse, particle-laden flows as well as methods by which to analyze them. The code solves the Navier-Stokes equations for the fluid phase and resolves the flow around each individual particle using an immersed boundary method. We also develop a collision model to accurately resolve particle-particle interactions within the fluid. We then perform simulations of a pressure-driven flow over a bed of spherical particles that agree with experimental results for particle velocities and flow rates. Using a control volume momentum balance, we analyze fluid and particle stresses within the simulations, which reveal the mechanisms by which the particle bed expands and contracts during changes in flow rates. These same stresses also allow us to measure the rheology of the particle-laden flows, where we find some agreement with existing constitutive models but also reveal the need to develop these models further
Rheology of mobile sediment beds sheared by viscous, pressure-driven flows
We present a detailed comparison of the rheological behaviour of sheared
sediment beds in a pressure-driven, straight channel configuration based on
data that was generated by means of fully coupled, grain-resolved direct
numerical simulations and experimental measurements reviously published by
Aussillous {\it et al.} (J. Fluid Mech., vol. 736, 2013, pp. 594-615). The
highly-resolved simulation data allows to compute the stress balance of the
suspension in the streamwise and vertical directions and the stress exchange
between the fluid and particle phase, which is information needed to infer the
rheology, but has so far been unreachable in experiments. Applying this
knowledge to the experimental and numerical data, we obtain the
statistically-stationary, depth-resolved profiles of the relevant rheological
quantities. The scaling behavior of rheological quantities such as the shear
and normal viscosities and the effective friction coefficient are examined and
compared to data coming from rheometry experiments and from widely-used
rheological correlations. We show that rheological properties that have
previously been inferred for annular Couette-type shear flows with neutrally
buoyant particles still hold for our setup of sediment transport in a
Poiseuille flow and in the dense regime we found good agreement with empirical
relationships derived therefrom. Subdividing the total stress into parts from
particle contact and hydrodynamics suggests a critical particle volume fraction
of 0.3 to separate the dense from the dilute regime. In the dilute regime,
i.e., the sediment transport layer, long-range hydrodynamic interactions are
screened by the porous media and the effective viscosity obeys the Einstein
relation
Confronting Grand Challenges in Environmental Fluid Dynamics
Environmental fluid dynamics underlies a wealth of natural, industrial and, by extension, societal challenges. In the coming decades, as we strive towards a more sustainable planet, there are a wide range of grand challenge problems that need to be tackled, ranging from fundamental advances in understanding and modeling of stratified turbulence and consequent mixing, to applied studies of pollution transport in the ocean, atmosphere and urban environments. A workshop was organized in the Les Houches School of Physics in France in January 2019 with the objective of gathering leading figures in the field to produce a road map for the scientific community. Five subject areas were addressed: multiphase flow, stratified flow, ocean transport, atmospheric and urban transport, and weather and climate prediction. This article summarizes the discussions and outcomes of the meeting, with the intent of providing a resource for the community going forward
Numerical simulation of turbulent sediment transport, from bed load to saltation
Sediment transport is studied as a function of the grain to fluid density
ratio using two phase numerical sim- ulations based on a discrete element
method (DEM) for particles coupled to a continuum Reynolds averaged description
of hydrodynamics. At a density ratio close to unity (typically under water),
vertical velocities are so small that sediment transport occurs in a thin layer
at the surface of the static bed, and is called bed load. Steady, or
'saturated' transport is reached when the fluid borne shear stress at the
interface between the mobile grains and the static grains is reduced to its
threshold value. The number of grains transported per unit surface is therefore
limited by the flux of horizontal momentum towards the surface. However, the
fluid velocity in the transport layer remains almost undisturbed so that the
mean grain velocity scales with the shear velocity u\ast. At large density
ratio (typically in air), the vertical velocities are large enough to make the
transport layer wide and dilute. Sediment transport is then called saltation.
In this case, particles are able to eject others when they collide with the
granular bed, a process called splash. The number of grains transported per
unit surface is selected by the balance between erosion and deposition and
saturation is reached when one grain is statistically replaced by exactly one
grain after a collision, which has the consequence that the mean grain velocity
remains independent of u\ast. The influence of the density ratio is
systematically studied to reveal the transition between these two transport
regimes. Based on the mechanisms identified in the steady case, we discuss the
transient of saturation of sediment transport and in particular the saturation
time and length. Finally, we investigate the exchange of particles between the
mobile and static phases and we determine the exchange time of particles.Comment: 17 pages, 14 figures, submitted to Physics of Fluid
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