Stephane Vincent

Numerical modelling of finite-size particle collisions in a viscous fluid

A general model is presented for short-range hydrodynamic interactions and head-on particle-particle/wall collisions. The model has been embedded in two distinct numerical methods for fully resolved simulation of finite-size particles in a viscous fluid. It accounts for the material properties of the particles and lubrication effects prior to collision that cannot be fully resolved on a fixed grid. We demonstrate that the model is able to reproduce experimental data for the coefficient of restitution of particle-wall collisions over a wide range of Stokes number based on the particle impact velocity. The set of model parameters we selected and more generally the modelling approach we propose can be efficiently used for fully resolved simulations of moderately dense solid-liquid suspensions.

LES Modeling with a Multifield Approach

In nuclear power plants, flow studies with a Computational Fluid Dynamics (CFD) approach require the ability of dealing with inclusions of different sizes and shapes and turbulence effects. For this purpose, multifield methods have been developed to simulate separately the small spherical bubbles and the large deformable ones. In this article, we consider an approach in which the first range of bubbles are followed by an Eulerian dispersed method and the second structures are tracked by interface tracking methods within a two-fluid model. To deal with these large inclusions, we present and validate, in this paper, a model, called the Large Bubble Model, introduced for the simulation of large deformable interfaces between two continuous fields. The Large Bubble Model includes a surface tension model, a new drag force expression to couple the velocity of the two fields at the interface and the resolution of an interface sharpening equation to limit the numerical smearing induced by the two-fluid model. To take into account the turbulence effects, an a priori two-phase LES filtering is proposed with the two-fluid equations and the interfacial forces of the Large Bubble Model. This filtering highlights new subgrid terms compared to previous works done on the single-fluid model. Finally, DNS simulations are performed with a phase inversion test case to evaluate the order of magnitude of these terms and to compare five different turbulence models.

A Priori Study for the Modeling of LES Subgrid Scale Terms in Resolved Scale Multiphase Flows

Modeling accurately the energy transfer across the interface in multiphase flows is difficult. To deal with this phenomenon, the derivation of the governing equations for two-phase flows have been formulated. A priori tests are used in order to evaluate the relative magnitude of unclosed LES specific terms to multiphase flows. There consist in the explicit filtering of 3D Direct Numerical Simulation in order to find LES models appropriated to the different subgrid contributions. In our study, explicit volume filtering and phase weighted filtering have been used in a case of phase separation flow in a cubic closed box between water, the heavier fluid and oil, the lighter fluid, in order to understand the effect of the filtering process on the subgrid contributions.

Analysis of Unsteady Lagrangian and Eulerian Characteristics of a Liquid Fluidized Bed by Direct Numerical Simulation

The characterization of fluidized beds is still a challenging task for macroscopic modeling issues and industrial applications. The macroscopic models require to be fed with parameters or laws that are not well understood or even impossible to estimate as soon as the solid fraction is larger than 0.1. The aim of the present work is to investigate Direct Numerical Simulation [1] of unsteady particle flows in order to solve all the time and space scales of the flow and the particles and to allow for the estimate of unknown macroscopic or stochastic characteristics of the flow. In the DNS, the particles are fully resolved, i.e. the particle diameter is larger than the grid size and to the smallest hydrodynamic scale. A benchmark experimental fluidized bed [2] is simulated and analyzed in terms of macroscopic and Lagrangian characteristics. Comparisons of numerical solutions to measurements are achieved with success.