Microscopic origins of shear stress in dense fluid-grain mixtures

A numerical model is used to simulate rheometer experiments at constant normal stress on dense suspensions of spheres. The complete model includes sphere-sphere contacts using a soft contact approach, short range hydrodynamic interactions defined by frame-invariant expressions of forces and torques in the lubrication approximation, and drag forces resulting from the poromechanical coupling computed with the DEM-PFV technique. Series of simulations in which some of the coupling terms are neglected highlight the role of the poromechanical coupling in the transient regimes. They also reveal that the shear component of the lubrication forces, though frequently neglected in the literature, has a dominant effect in the volume changes. On the other hand, the effects of lubrication torques are much less significant. The bulk shear stress is decomposed into contact stress and hydrodynamic stress terms whose dependency on a dimensionless shear rate - the so called viscous number $I_v$ - are examined. Both contributions are increasing functions of $I_v$, contacts contribution dominates at low viscous number ($I_v$ 0.15, consistently with a phenomenological law infered by other authors. Statistics of microstructural variables highlight a complex interplay between solid contacts and hydrodynamic interactions. In contrast with a popular idea, the results suggest that lubrication may not necessarily reduce the contribution of contact forces to the bulk shear stress. The proposed model is general and applies directly to sheared immersed granular media in which pore pressure feedback plays a key role (triggering of avalanches, liquefaction).Comment: to appear in Granular Matte

A three-dimensional numerical model for dense granular flows based on the μ (I) rheology

International audienceThis paper presents a three-dimensional implementation of the so-called μ(I) rheology to accurately and efficiently compute steady-state dense granular flows. The tricky pressure dependent visco-plastic behaviour within an incompressible flow solver has been overcome using a regularisation technique along with a complete derivation of the incremental formulation associated with the Newton-Raphson algorithm. The computational accuracy and efficiency of the proposed numerical model have been assessed on two representative problems that have an analytical solution. Then, two application examples dealing with actual lab experiments have also been considered: the first one concerns a granular flow on a heap and the second one deals with the granular flow around a cylinder. In both configurations the obtained computational results are in good agreement with available experimental data

A two-phase model for sheet flow regime based on dense granular flow rheology

International audienceA two-phase model having a μ(I) rheology for the intergranular stresses and a mixing length approach for the turbulent stresses is proposed to describe the sheet flow regime of sediment transport. In the model two layers are considered, a dilute suspension layer and a dense sediment bed layer. The concentration profile is obtained from the dilatancy law Φ(I) in the sediment bed layer and from a Rouse profile in the suspension layer. The comparison of velocity profile, concentration profile and macroscopic parameters (sediment flux, thickness and roughness) with experimental data shows a good agreement. These comparisons demonstrate that the dense granular rheology is relevant to describe intense bed-load transport in turbulent regime (sheet flow). The transition from the dense static bed to the dilute suspension is well described by the present model. Also, the different regimes of the dense granular rheology seems to be able to capture the transition between collision dominant and turbulent fluctuations dominant sheet flows, depending on the particles characteristic

From elasto-plasticity to visco-elasto-plasticity for saturated granular materials

A recent extension of the discrete element method is reported for the simulation of dense mixtures of non-colloidal particles and viscous fluids in the non-inertial regime. The numerical model includes sphere-sphere contacts using a soft contact ap- proach [2], short range hydrodynamic interactions defined by frame-invariant expressions of forces and torques in the lubrication approximation, and drag forces resulting from the poromechanical coupling computed with the DEM-PFV technique [3]. The proposed model is general and applies directly to sheared satured granular media in which pore pressure feedback plays a key role. A partitioned solver makes the algorithm trivially parallel, which enables the coupled problems to be solved with nearly the same wall-clock time as uncoupled dry materials simulations. The shear stress in a dense suspension is analyzed, and decomposed into contact stress and hydrodynamic stress. Both contributions are shown to be increasing functions of a dimensionless shear rate Iv, in agreement with experimental results [4]. In contrast with a popular idea, the results suggest that lubrication may not necessarily reduce the contribution of contact forces to the bulk shear stress

INVESTIGATION OF THE MULTI-SCALE INTERACTIONS BETWEEN AN OFFSHORE WIND TURBINE WAKE AND THE OCEAN-SEDIMENT DYNAMICS IN A INDEALIZED FRAMEWORK

International audienceA coupled two dimensional idealized numerical model of the ocean and sediment layers, forced by an offshore wind turbine wake is used to investigate the complex interactions between the wake, the ocean and the sediment layers, together with the retro-action on the wind energy. Results show that the turbine wake has an impact on both, the ocean and the sediment layers. The turbine wake impacts the ocean surface and generates instabilities or vortex streets for some parameter values. Shallow ocean layers (typically below 15m) are laminar. When water depth is higher, large scale instabilities are generated, leading to a turbulent dynamic in the ocean layer. The size of the generated vortices in the ocean increases with water depth and decreases with the quadratic-law bottom friction coefficient. Considering the morphodynamics three cases are observed, depending on whether the ocean dynamics is laminar (i), has a localized (ii) or domain wide (iii) turbulent behavior. In the first case, changes in seabed elevation are around a few millimeters per month. Results are similar for the localized turbulence case with small spatial variations. For the domain wide turbulence case (iii), instantaneous seabed changes are of the order of a few millimeters per month, whereas the transport averaged over several days decreases to a few tenths of millimeter per month. This behavior is easily explained by the oscillating local velocity which transports sediments back and forth. The above emphasizes that the water depth is a key parameter for the coupled atmosphere-ocean-sediment system around wind turbines. Furthermore, considering the ocean velocity in the atmospheric forcing at the ocean surface leads to a decrease of 4 % of the power lost by friction at the atmosphere-ocean interface. Ocean dynamics could thus have a non-negligible feedback on the wind power available for the turbines and its variability

An Eulerian two-phase model for steady sheet flow using large-eddy simulation methodology

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Advances in Water Resources 111 (2018): 205-223, doi:10.1016/j.advwatres.2017.11.016.A three-dimensional Eulerian two-phase flow model for sediment transport in sheet flow conditions is presented. To resolve turbulence and turbulence-sediment interactions, the large-eddy simulation approach is adopted. Specifically, a dynamic Smagorinsky closure is used for the subgrid fluid and sediment stresses, while the subgrid contribution to the drag force is included using a drift velocity model with a similar dynamic procedure. The contribution of sediment stresses due to intergranular interactions is modeled by the kinetic theory of granular flow at low to intermediate sediment concentration, while at high sediment concentration of enduring contact, a phenomenological closure for particle pressure and frictional viscosity is used. The model is validated with a comprehensive high-resolution dataset of unidirectional steady sheet flow (Revil-Baudard et al., 2015, Journal of Fluid Mechanics, 767, 1–30). At a particle Stokes number of about 10, simulation results indicate a reduced von Kármán coefficient of κ ≈ 0.215 obtained from the fluid velocity profile. A fluid turbulence kinetic energy budget analysis further indicates that the drag-induced turbulence dissipation rate is significant in the sheet flow layer, while in the dilute transport layer, the pressure work plays a similar role as the buoyancy dissipation, which is typically used in the single-phase stratified flow formulation. The present model also reproduces the sheet layer thickness and mobile bed roughness similar to measured data. However, the resulting mobile bed roughness is more than two times larger than that predicted by the empirical formulae. Further analysis suggests that through intermittent turbulent motions near the bed, the resolved sediment Reynolds stress plays a major role in the enhancement of mobile bed roughness. Our analysis on near-bed intermittency also suggests that the turbulent ejection motions are highly correlated with the upward sediment suspension flux, while the turbulent sweep events are mostly associated with the downward sediment deposition flux.This study was supported by National Science Foundation (OCE-1635151; OCE- 958 1537231) and Office of Naval Research (N00014-16-1-2853). J. Chauchat was supported by the Region Rhones-Alpes (COOPERA project and Explora Pro grant) and the French national programme EC2CO-LEFE MODSED. The authors would also like to acknowledge the support from the program on "Fluid-Mediated Particle Transport in Geophysical Flows" at the Kavli Institute for Theoretical Physics, Santa Barbara, USA

Discrete element modelling of bedload transport

International audienceWe present a model for the description of bed load transport at the particle scale. The granular phase was modelled using discrete element method while the fluid phase was characterized by a fluid profile taken from the experiment. The coupling between the two phases was done considering only the effect of the fluid on the particle, through the drag force. The results of the model were compared to particular experimental results. A good agreement was obtained on the particle velocity and solid volume fraction in function of the depth considering the simplicity of the coupling

Charriage de particules dans un écoulement cisaillé

Colloque avec actes et comité de lecture. Internationale.National audienceUn lit de particules soumis à un écoulement de fluide, par exemple le lit d'une rivière, se met en mouvement quand les forces hydrodynamiques deviennent supérieures à une fraction du poids apparent des particules. Nous étudions expérimentalement le transport de particules dans un tube à section rectangulaire. Nous comparons les résultats aux prédictions d'un modèle continu à deux phases, dans lequel nous utilisons une rhéologie granulaire pour la contrainte solide

Eddy interaction model for turbulent suspension in Reynolds-averaged Euler-Lagrange simulations of steady sheet flow

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Advances in Water Resources 111 (2018): 435-451, doi:10.1016/j.advwatres.2017.11.019.A Reynolds-averaged Euler–Lagrange sediment transport model (CFDEM-EIM) was developed for steady sheet flow, where the inter-granular interactions were resolved and the flow turbulence was modeled with a low Reynolds number corrected turbulence closure modified for two-phase flows. To model the effect of turbulence on the sediment suspension, the interaction between the turbulent eddies and particles was simulated with an eddy interaction model (EIM). The EIM was first calibrated with measurements from dilute suspension experiments. We demonstrated that the eddy-interaction model was able to reproduce the well-known Rouse profile for suspended sediment concentration. The model results were found to be sensitive to the choice of the coefficient, C0, associated with the turbulence-sediment interaction time. A value was suggested to match the measured concentration in the dilute suspension. The calibrated CFDEM-EIM was used to model a steady sheet flow experiment of lightweight coarse particles and yielded reasonable agreements with measured velocity, concentration and turbulence kinetic energy profiles. Further numerical experiments for sheet flow suggested that when C0 was decreased to C0  1.0). Additional simulations for a range of Shields parameters between 0.3 and 1.2 confirmed that CFDEM-EIM was capable of predicting sediment transport rates similar to empirical formulations. Based on the analysis of sediment transport rate and transport layer thickness, the EIM and the resulting suspended load were shown to be important when the fall parameter is less than 1.25.Z. Cheng and T.-J. Hsu were supported by the U.S. Office of Naval Research (N00014- 16-1-2853) and National Science Foundation (OCE- 1537231). J. Chauchat was supported by the Région Rhones-Alpes (COOPERA project and Explora Pro grant) and the French national programme EC2CO-LEFE MODSED. J. Calantoni was supported under base funding to the U.S. Naval Research Laboratory from the U.S. Office of Naval Research. The authors would also like to acknowledge the support from the program on "Fluid- Mediated Particle Transport in Geophysical Flows" at the Kavli Institute for Theoretical Physics, Santa Barbara, USA

Investigation of the mobile granular layer in bedload transport by laminar shearing flows

International audienceThe mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid and for a wide range of fluid flow rates above incipient motion. A full three-dimensional investigation of the flow field inside the mobile layer is also provided. These experimental observations are compared to the predictions of a two-phase continuum model having a frictional rheology to describe particle-particle interactions. Different rheological constitutive laws having increasing degrees of sophistication are tested and discussed