1,473 research outputs found
The effect of particle anisotropy on the modulation of turbulent flows
We investigate the modulation of turbulence caused by the presence of
finite-size dispersed particles. Bluff (isotropic) spheres vs slender
(anisotropic) fibers are considered to understand the influence of the object
shape on altering the carrier flow. While at a fixed mass fraction - but
different Stokes number - both objects provide a similar bulk effect
characterized by a large-scale energy depletion, a scale-by-scale analysis of
the energy transfer reveals that the alteration of the whole spectrum is
intrinsically different. For bluff objects, the classical energy cascade is
shrinked in its extension but unaltered in the energy content and its typical
features, while for slender ones we find an alternative energy flux which is
essentially mediated by the fluid-solid coupling.Comment: 11 pages, 6 figure
Multi-Particle Collision Dynamics -- a Particle-Based Mesoscale Simulation Approach to the Hydrodynamics of Complex Fluids
In this review, we describe and analyze a mesoscale simulation method for
fluid flow, which was introduced by Malevanets and Kapral in 1999, and is now
called multi-particle collision dynamics (MPC) or stochastic rotation dynamics
(SRD). The method consists of alternating streaming and collision steps in an
ensemble of point particles. The multi-particle collisions are performed by
grouping particles in collision cells, and mass, momentum, and energy are
locally conserved. This simulation technique captures both full hydrodynamic
interactions and thermal fluctuations. The first part of the review begins with
a description of several widely used MPC algorithms and then discusses
important features of the original SRD algorithm and frequently used
variations. Two complementary approaches for deriving the hydrodynamic
equations and evaluating the transport coefficients are reviewed. It is then
shown how MPC algorithms can be generalized to model non-ideal fluids, and
binary mixtures with a consolute point. The importance of angular-momentum
conservation for systems like phase-separated liquids with different
viscosities is discussed. The second part of the review describes a number of
recent applications of MPC algorithms to study colloid and polymer dynamics,
the behavior of vesicles and cells in hydrodynamic flows, and the dynamics of
viscoelastic fluids
A monolithic fluid-structure interaction formulation for solid and liquid membranes including free-surface contact
A unified fluid-structure interaction (FSI) formulation is presented for
solid, liquid and mixed membranes. Nonlinear finite elements (FE) and the
generalized-alpha scheme are used for the spatial and temporal discretization.
The membrane discretization is based on curvilinear surface elements that can
describe large deformations and rotations, and also provide a straightforward
description for contact. The fluid is described by the incompressible
Navier-Stokes equations, and its discretization is based on stabilized
Petrov-Galerkin FE. The coupling between fluid and structure uses a conforming
sharp interface discretization, and the resulting non-linear FE equations are
solved monolithically within the Newton-Raphson scheme. An arbitrary
Lagrangian-Eulerian formulation is used for the fluid in order to account for
the mesh motion around the structure. The formulation is very general and
admits diverse applications that include contact at free surfaces. This is
demonstrated by two analytical and three numerical examples exhibiting strong
coupling between fluid and structure. The examples include balloon inflation,
droplet rolling and flapping flags. They span a Reynolds-number range from
0.001 to 2000. One of the examples considers the extension to rotation-free
shells using isogeometric FE.Comment: 38 pages, 17 figure
Simulating structured fluids with tensorial viscoelasticity
We consider an immersed elastic body that is actively driven through a
structured fluid by a motor or an external force. The behavior of such a system
generally cannot be solved analytically, necessitating the use of numerical
methods. However, current numerical methods omit important details of the
microscopic structure and dynamics of the fluid, which can modulate the
magnitudes and directions of viscoelastic restoring forces. To address this
issue, we develop a simulation platform for modeling viscoelastic media with
tensorial elasticity. We build on the lattice Boltzmann algorithm and
incorporate viscoelastic forces, elastic immersed objects, a microscopic
orientation field, and coupling between viscoelasticity and the orientation
field. We demonstrate our method by characterizing how the viscoelastic
restoring force on a driven immersed object depends on various key parameters
as well as the tensorial character of the elastic response. We find that the
restoring force depends non-monotonically on the rate of diffusion of the
stress and the size of the object. We further show how the restoring force
depends on the relative orientation of the microscopic structure and the
pulling direction. These results imply that accounting for previously neglected
physical features, such as stress diffusion and the microscopic orientation
field, can improve the realism of viscoelastic simulations. We discuss possible
applications and extensions to the method.Comment: 17 pages, 11 figure
Modelling the dynamics of a sphere approaching and bouncing on a wall in a viscous fluid
The canonical configuration of a solid particle bouncing on a wall in a viscous fluid is considered here, focusing on rough particles as encountered in most of the laboratory experiments or applications. In that case, the particle deformation is not expected to be significant prior to solid contact. An immersed boundary method (IBM) allowing the fluid flow around the solid particle to be numerically described is combined with a discrete element method (DEM) in order to numerically investigate the dynamics of the system. Particular attention is paid to modelling the lubrication force added in the discrete element method, which is not captured by the fluid solver at very small scale. Specifically, the proposed numerical model accounts for the surface roughness of real particles through an effective roughness length in the contact model, and considers that the time scale of the contact is small compared to that of the fluid. The present coupled method is shown to quantitatively reproduce available experimental data and in particular is in very good agreement with recent measurement of the dynamics of a particle approaching very close to a wall in the viscous regime St <O(10), where St is the Stokes number which represents the balance between particle inertia and viscous dissipation. Finally, based on the reliability of the numerical results, two predictive models are proposed, namely for the dynamics of the particle close to the wall and the effective coefficient of restitution. Both models use the effective roughness height and assume the particle remains rigid prior to solid contact. They are shown to be pertinent to describe experimental and numerical data for the whole range of investigated parameters
On the damped oscillations of an elastic quasi-circular membrane in a two-dimensional incompressible fluid
We propose a procedure - partly analytical and partly numerical - to find the
frequency and the damping rate of the small-amplitude oscillations of a
massless elastic capsule immersed in a two-dimensional viscous incompressible
fluid. The unsteady Stokes equations for the stream function are decomposed
onto normal modes for the angular and temporal variables, leading to a
fourth-order linear ordinary differential equation in the radial variable. The
forcing terms are dictated by the properties of the membrane, and result into
jump conditions at the interface between the internal and external media. The
equation can be solved numerically, and an excellent agreement is found with a
fully-computational approach we developed in parallel. Comparisons are also
shown with the results available in the scientific literature for drops, and a
model based on the concept of embarked fluid is presented, which allows for a
good representation of the results and a consistent interpretation of the
underlying physics.Comment: in press on JF
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