Eckart Meiburg

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 analyse 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 centre and migrate toward the wall where they break, while smaller aggregates are transported away from the wall, grow and re-enter the cycle.

Direct numerical simulations (DNS) are used to investigate the impact of flow instabilities, nonhydrostatic mixing, and phase-dependent flow behavior on gravity currents propagating against a pulsating flow in a lock-exchange configuration. Two regimes are considered: (i) laminar pulsating flow, with two-dimensional (2D) and three-dimensional (3D) DNS at Re=3000 using eight combinations of Froude numbers Frm and Fro, representing dimensionless velocity amplitudes of the opposing mean flow and the oscillatory component, respectively; and (ii) turbulent pulsating flow, with 3D DNS at Re=1.2×104, Frm=0.46, and Fro=0.5. Across both regimes, the gravity current evolution is strongly phase dependent and shaped by nonhydrostatic processes: Kelvin–Helmholtz instabilities, lobe-cleft structures, lifting of the gravity current, Rayleigh–Taylor-like instabilities, and their impact on 3D mixing. We show that long-term processes and large-scale phenomena can be captured with 2D DNS. Other properties, related to nonhydrostatic processes, such as instabilities and lifting, active on shorter time scales (smaller than the oscillation period) or small-scale flow phenomena, require full 3D DNS. Turbulent ambient flow conditions modulate these short-time and small-scale processes significantly, thus always requiring full 3D DNS (or large-eddy simulation) and having a clear impact over the full oscillation cycle. Together, the presented results clarify which aspects of evolving gravity currents under pulsating flow conditions can be captured with 2D models and identify the 3D mechanisms that must be represented when parameterizing mixing and entrainment in estuarine salt-intrusion applications.

This study employed field observations to investigate the evolution of deep‐sea sand ripples under the influence of internal waves. An observational system was deployed at a depth of 720 m in Dongsha, where cameras captured four distinct types of sand ripples: linear, curved, transitional, and covered ripples. A joint analysis of the sand ripple images and the observed hydrodynamic conditions revealed that internal waves facilitate the transformation between different sand ripples. Additionally, analysis of seabed echo intensity indicated that internal waves can cause the resuspension of seabed sediments, resulting in the emergence of covered sand ripples. Furthermore, under the same bottom flow velocity conditions, different types of sand ripples as bedforms result in varying concentrations of suspended sediments. Specifically, the sediment resuspension caused by transitional sand ripples is greater than that caused by curved sand ripples, which in turn is greater than that caused by linear sand ripples. These direct observations not only confirm the evolution of sand ripples under the influence of internal waves but also provide new insights into the mechanisms of sediment resuspension in the study area.

Understanding viscoplastic fluid flow in discrete fracture networks is essential for natural and industrial processes, yet the hydraulic resistance at fracture intersections remains poorly characterized. This study investigates Herschel–Bulkley fluid flow through two intersecting fractures using two approaches: a full Navier–Stokes model and a reduced Hele–Shaw approximation. The Hele–Shaw model accurately predicts hydraulic resistance for Reynolds numbers up to O(10) and Bingham numbers spanning 10−2 to 1. Resistance is largely insensitive to Bingham number for Bn≲0.1, and pressure distributions are well captured by the approximation. The power-law index significantly influences resistance and pressure, with strong shear-thinning behavior increasing resistance variability by an order of magnitude and inducing channelization in the downstream fracture.

Flocculation of suspended cohesive particles in turbulence plays a crucial role in various natural and industrial processes. In this study, we carry out direct numerical simulations with a cohesive discrete element method to investigate the evolution of cohesive particle flocs in homogeneous isotropic turbulence. We focus on how particle flocs are formed and destructed in turbulence, and the influences of cohesive strength, particle inertia, particle volume fraction and turbulence intensity. The statistics show that the formation and destruction of cohesive particle flocs vary across different flocculation stages and conditions. At the early developing stage, flocs primarily form through coagulation, where individual primary particles collide and adhere together. As the system approaches equilibrium, floc formation may also proceed via separation, where small flocs detach from larger ones under the conditions of strong cohesive strength, low particle inertia or weak turbulent shear. Under these same conditions, floc destruction occurs predominantly through absorption across all stages, in which smaller flocs are incorporated into larger ones. In contrast, under the opposite conditions, floc destruction shifts to disintegration, where a floc breaks apart into primary particles or two-particle flocs. The probability distribution of the floc lifetime follows an exponential decay. Furthermore, flocs exhibit shorter lifetimes under stronger turbulent shear, larger particle inertia or weaker cohesion, as these conditions promote more frequent destruction. A higher particle volume fraction leads to more frequent floc formation and destruction, thus producing shorter lifetimes. Meanwhile, cohesion between particles markedly extends the floc lifetime compared to lifetimes of non-cohesive particle clusters. Moreover, floc compactness is positively correlated with its lifetime. These findings advance our understanding of the flocculation dynamics of cohesive particles in turbulent flow.

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 ∼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.

Resuspension of fine‐grained bottom sediment under wind‐driven currents and waves is a key process in shaping nearshore environments. Commonly, resuspension is quantified for predicting the dispersion of contaminants and nutrients affecting water quality by numerical modeling and field measurements. Although a large body of research deals with this topic, unique field observations from hypersaline environments coupled with conceptual‐quantitative description of the process are lacking. Here, we present high‐resolution direct measurements of winds, waves, currents, and turbidity conducted along the Dead Sea shores and derivations of an integrated 1D‐numerical model based on mass and momentum conservation laws. Comparing the model predictions and the observations determine, for the first time, that depth‐averaged turbulent viscosity during Dead Sea storms is of order of 10 ⁻³ m ² s ⁻¹ . Resuspension of bottom clay to fine sand is governed primarily by waves inducing shear stress three orders of magnitude larger than current‐induced shear stress, a ratio which is rather constant during Dead Sea storms. The observed spatiotemporal turbidity pattern is reproduced and accounts for the effect of grain‐size distributions on the lake floor. Additionally, we highlight the importance of wave‐induced resuspension as an additional source of sediment involved in the formation of thin, muddy layers that are traditionally interpreted as indicators of inflowing sediment plumes. The novelty of the manuscript lies in the combination of rare observations and modeling, which provides comprehensive physics of the studied processes, an approach that can be used in other nearshore environments of lakes or oceans.