arXiv subjects

Eckart Meiburg

Publications and source records attributed to Eckart Meiburg.

12 recordsLinked to original sources

arXiv2026

Aggregation, breakup, and size-dependent transport in a turbulent channel flow with cohesive particles

Due to attractive inter-particle forces, cohesive particles suspended in turbulence undergo a complex process of aggregation, breakup, and restructuring. Despite a growing body of knowledge on the ``flocculation'' of cohesive granular materials suspended in homogeneous isotropic turbulence, little focus has so far been placed on wall-bounded flows where turbulence and shear are inhomogeneous. This study presents a first investigation of a fully developed wall-bounded flow of resolved cohesive particles. Five direct numerical simulations of turbulent channel flows laden with finite-sized particles at successively increasing cohesive strength are performed. A population balance equation (PBE) framework is used to analyze aggregate dynamics. When integrated over the full domain, the PBE is closed by aggregation and breakup alone. However, this balance is found to not hold locally in the wall-normal direction, where regions of net aggregate production and depletion are identified. This imbalance is shown to be compensated by the size-dependent wall-normal transport of aggregates, revealing a mean circulation: larger aggregates are preferentially produced in the channel center and migrate toward the wall where they break, while smaller aggregates are transported away from the wall, grow, and reenter the cycle.

physics.flu-dyn↗

arXiv2026

On the biogenic hydrodynamic transport of upward and downward cruising copepods

Mesozooplankton aggregations undergoing vertical migrations in the upper ocean have been hypothesized to have an important role in the redistribution of carbon, nutrients, and oxygen via biogenic hydrodynamic transport (BHT). While laboratory studies have demonstrated how swarm-induced hydrodynamic instabilities can drive large-scale transport in strongly stratified environments, measurements are usually performed with model organisms that differ in morphology and swimming mode from ecologically relevant marine species. To bridge this gap, we conducted experiments with copepods and analyzed upward and downward trajectories to identify differences in flow fields, force distribution, and BHT for these two scenarios. Using two-dimensional bright-field Particle Image Velocimetry (PIV), we quantified the near-body velocity field and found that the average downward swimming speed significantly exceeds the average upward swimming speed, with the flow fields exhibiting direction-dependent characteristics. We incorporated these findings into a continuum squirmer model to estimate the swimmer-induced drift volume and mixing efficiency, focusing on the effects of the reduced gravity of the swimmers and the density stratification of the surrounding fluid. Our simulations reveal that both the excess weight of the organisms and the fluid stratification strongly constrain the net BHT. This study provides a critical step toward integrating lab-based models of marine mesozooplankton with remote sensing data to incorporate vertical migrations into global ocean models with realistic biogeochemistry and assess their ecological significance in actively sustaining local ecosystems.

physics.flu-dyn↗

arXiv2025

Gravity-driven flux of particles through apertures

The gravity-driven discharge of granular material through an aperture is a fundamental problem in granular physics and is classically described by empirical laws with different fitting parameters. In this Letter, we disentangle the mass flux into distinct velocity and packing contributions by combining three-dimensional experiments and simulations. We define a dimensionless flux ratio that captures confinement-driven deviations from a free-fall limit, which is recovered when the aperture is large compared to the grain size. For spherical cohesionless grains, the deviations from the free-fall limit are captured by a single exponential correction factor over a characteristic length scale of $\sim$ 10-15 grain diameters. This is shown to be the scale over which the packing structure is modified due to the boundary. Building on the $\sqrt{gD}$ exit-velocity scaling, we propose a kinematic framework that explains the universality of granular discharge beyond empirical descriptions.

cond-mat.soft↗

arXiv2025

Gravity current propagating against constant and pulsating counter flows

This paper describes the evolution of two-dimensional (2D) gravity currents that flow against a horizontally uniform laminar pulsating flow. We study the effect of opposing mean flow amplitude and the oscillatory velocity amplitude on the evolution of the gravity current, the emergence of instabilities due to shear at the interface of heavy and light fluid and unstable density stratification near the bottom wall, and the associated density redistributions. The velocity amplitudes and the oscillation frequency are reminiscent of tidal estuarine flows. This study revealed two key processes affecting the horizontal density transport of the heavy fluid, in addition to the buoyancy-driven propagation of the gravity current. The first process concerns the presence of shear-driven Kelvin-Helmholtz (KH) billows, depending on the strength of the opposing mean flow and the thickness of the gravity current. These KH billows are generated in the inertial phase of gravity current propagation and are responsible for coherent advective transport of heavy-fluid patches away from the gravity current head. The second process is related to the lifting of the gravity current head due to differential advection near the bottom wall when the propagation direction of the gravity current and the oscillating externally imposed flow are in the same direction. It generates a layer of light fluid below the heavy fluid of the gravity current head and becomes unstable when the ambient flow opposes the gravity current propagation, generating Rayleigh-Taylor-like (RT-like) instabilities. This results in a strong vertical redistribution of light and heavy fluid. Non-hydrostatic effects, such as the presence of KH billows and RT-like instabilities, with associated vertical density transport, have significant implications for large-scale horizontal density transport and modeling of salt intrusions in rivers and estuaries.

physics.flu-dyn↗

arXiv2025

Long-term microgravity experiments reveal a new mechanism for particle aggregation in suspension

Microgravity experiments on board the International Space Station, combined with particle-resolved direct numerical simulations, were conducted to investigate the long-term flocculation behavior of clay suspensions in saline water in the absence of gravity. After an initial homogenization of the suspensions, different clay compositions were continuously monitored for 99 days, allowing a detailed analysis of aggregate growth through image processing. The results indicate that the onboard oscillations (g-jitter) may have accelerated the aggregation process. Aggregate growth driven by these oscillations is found to occur at a faster rate than aggregation caused by Brownian motion. This effect is further confirmed by numerical simulations, which also demonstrated that parameters such as the oscillation amplitude and the solid volume fraction influence growth acceleration. These findings highlight that oscillations may act as a previously unrecognized mechanism that contributes to particle aggregation in fluids.

cond-mat.soft↗

arXiv2024

Settling of two rigidly connected spheres

Laboratory experiments and particle-resolved simulations are employed to investigate the settling dynamics of a pair of rigidly connected spherical particles of unequal size. They yield a detailed picture of the transient evolution and the terminal values of the aggregate's orientation angle and its settling and drift velocities as functions of the aspect ratio and the Galileo number $Ga$, which denotes the ratio of buoyancy and viscous forces acting on the aggregate. At low to moderate values of $Ga$, the aggregate's orientation and velocity converge to their terminal values monotonically, whereas for higher $Ga$-values the aggregate tends to undergo a more complex motion. If the aggregate assumes an asymmetric terminal orientation, it displays a nonzero terminal drift velocity. For diameter ratios much larger than one and small $Ga$, the terminal orientation of the aggregate becomes approximately vertical, whereas when $Ga$ is sufficiently large for flow separation to occur, the aggregate orients itself such that the smaller sphere is located at the separation line. Empirical scaling laws are obtained for the terminal settling velocity and orientation angle as functions of the aspect ratio and $Ga$ for diameter ratios from 1 to 4 and particle-to-fluid density ratios from 1.3 to 5. An analysis of the accompanying flow field shows the formation of vortical structures exhibiting complex topologies in the aggregate's wake, and it indicates the formation of a horizontal pressure gradient across the larger sphere, which represents the main reason for the emergence of the drift velocity.

physics.flu-dyn↗

arXiv2024

Pairwise interaction of spherical particles aligned in oscillatory flow

We present a systematic simulation campaign to investigate the pairwise interaction of two mobile, monodisperse particles submerged in a viscous fluid and subjected to monochromatic oscillating flows. To this end, we employ the immersed boundary method to geometrically resolve the flow around the two particles in a non-inertial reference frame. We neglect gravity to focus on fluid-particle interactions associated with particle inertia and consider particles of three different density ratios aligned along the axis of oscillation. We systematically vary the initial particle distance and the frequency based on which the particles show either attractive or repulsive behavior by approaching or moving away from each other, respectively. This behavior is consistently confirmed for the three density ratios investigated, although particle inertia dictates the overall magnitude of the particle dynamics. Based on this, threshold conditions for the transition from attraction to repulsion are introduced that obey the same power law for all density ratios investigated. We furthermore analyze the flow patterns by suitable averaging and decomposition of the flow fields and find competing effects of the vorticity induced by the fluid-particle interactions. Based on these flow patterns, we derive a circulation-based criterion that provides a quantitative measure to categorize the different cases. It is shown that such a criterion provides a consistent measure to distinguish the attractive and repulsive arrangements.

physics.flu-dyn↗

arXiv2022

Particle-resolved simulations of four-way coupled, polydispersed, particle-laden flows

We present a collocated-grid framework for Direct Numerical Simulations of polydisperse particles submerged in a viscous fluid. The fluid-particle forces are coupled with the Immersed Boundary Method (IBM) while the particle-particle forces are modeled with a combination of contact and lubrication models, adapted for collocated grids. Our method is modified from the staggered-grid IBM of previous authors to a collocated-grid IBM by adapting the fluid and particle solvers. The method scales well on high-performance parallel computing platforms. It has been validated against various cases and is able to reproduce experimental results. Tuning parameters have been thoroughly calibrated to ensure the accuracy of the method. Finally, we demonstrate the capability of the method to simulate both monodispersed and bidispersed fluidized beds and reproduce the power law relationship between the inflow velocity and the porosity.

physics.flu-dyn↗

arXiv2022

A simple criterion and experiments for onset of flocculation in kaolin clay suspensions

Cohesive effects between fine-grained sediment particles greatly influence their effective settling rate and erodibility. Many studies have observed a qualitative difference in settling dynamics between clays in freshwater, where particles remain dispersed, and in saltwater, where aggregates form and settle rapidly. The critical coagulation concentration (CCC) of salt that separates the two regimes however remains under-investigated, even though knowledge of the CCC is crucial to understanding aggregation in settings such as estuaries, where large salt concentration gradients occur. Furthermore, no simple criterion exists to predict the CCC for clay suspensions. In this study, systematic experiments are performed to determine the CCC, by measuring transmitted light intensity through clay suspensions. To investigate the effect of ion valence, sodium chloride (NaCl) and calcium chloride (CaCl$_2$) are used. For kaolin clay, the results show a CCC of 0.6mM NaCl ($\approx 0.04$ppt NaCl $=$ 0.04 PSU), and of 0.04mM CaCl$_2$ ($\approx 0.004$ppt CaCl$_2$). Because these salinities are lower than those commonly observed in nature, these findings indicate that kaolin clay should flocculate in nearly all natural aquatic environments. Furthermore, due to the fact that tap water often has salinities higher than this threshold, these results imply that great care is needed in experiments, especially in large facilities where using distilled water is not feasible. In addition, a simple criterion to estimate the CCC for a kaolin clay suspension is derived. This criterion predicts that flocculation occurs at extremely low salt concentrations and is approximately independent of clay concentration, in agreement with the experimental observations and consistent with experimental evidence from the literature.

physics.flu-dyn↗

arXiv2019

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.

cond-mat.soft↗

arXiv2017

A collision model for grain-resolving simulations of flows over dense, mobile, polydisperse granular sediment beds

We present a collision model for phase-resolved Direct Numerical Simulations of sediment transport that couple the fluid and particles by the Immersed Boundary Method. Typically, a contact model for these types of simulations comprises a lubrication force for particles in close proximity to another solid object, a normal contact force to prevent particles from overlapping, and a tangential contact force to account for friction. Our model extends the work of previous authors to improve upon the time integration scheme to obtain consistent results for particle-wall collisions. Furthermore, we account for polydisperse spherical particles and introduce new criteria to account for enduring contact, which occurs in many sediment transport situations. This is done without using arbitrary values for physically-defined parameters and by maintaining the full momentum balance of a particle in enduring contact. We validate our model against several test cases for binary particle-wall collisions as well as the collective motion of a sediment bed sheared by a viscous flow, yielding satisfactory agreement with experimental data by various authors.

physics.flu-dyn↗

arXiv2012

Particle-laden currents interacting with complex bottom topography: a numerical investigation

We conduct depth-resolved three-dimensional Direct Numerical Simulations (DNS) of bi-disperse turbidity currents interacting with complex bottom topography in the form of a Gaussian bump. Several flow characteristics such as suspended particle mass, instantaneous wall shear stress, transient deposit height are shown via videos. Furthermore, we investigate the influence of the obstacle on the vortical structure and sedimentation of particles by comparing the results against the same setup and but with a flat bottom surface. We observe that the obstacle influences the deposition of coarse particles mainly in the vicinity of the obstacle due to lateral deflection, whereas for the sedimentation of fine particles the effects of topographical features are felt further downstream. The results shown in this fluid dynamics video help us develop a fundamental understanding of the dynamics of turbidity currents interacting with complex seafloor topography.

physics.flu-dyn↗