10,862 research outputs found
Fully coupled simulations of non-colloidal monodisperse sheared suspensions
In this work we investigate numerically the dynamics of sheared suspensions in the limit of vanishingly small fluid and particle inertia. The numerical model we used is able to handle the multi-body hydrodynamic interactions between thousands of particles embedded in a linear shear flow. The presence of the particles is modeled by momentum source terms spread out on a spherical envelop forcing the Stokes equations of the creeping flow. Therefore all the velocity perturbations induced by the moving particles are simultaneously accounted for.
The statistical properties of the sheared suspensions are related to the velocity fluctuation of the particles. We formed averages for the resulting velocity fluctuation and rotation rate tensors. We found that the latter are highly anisotropic and that all the velocity fluctuation terms grow linearly with particle volume fraction. Only one off-diagonal term is found to be non zero (clearly related to trajectory symmetry breaking induced by the non-hydrodynamic repulsion force). We also found a strong correlation of positive/negative velocities in the shear plane, on a time scale controlled by the shear rate (direct interaction of two particles). The time scale required to restore uncorrelated velocity fluctuations decreases continuously as the concentration increases. We calculated the shear induced self-diffusion coefficients using two different methods and the resulting diffusion tensor appears to be anisotropic too.
The microstructure of the suspension is found to be drastically modified by particle interactions. First the probability density function of velocity fluctuations showed a transition from exponential to Gaussian behavior as particle concentration varies. Second the probability of finding close pairs while the particles move under shear flow is strongly enhanced by hydrodynamic interactions when the concentration increases
Large-scale simulation of steady and time-dependent active suspensions with the force-coupling method
We present a new development of the force-coupling method (FCM) to address
the accurate simulation of a large number of interacting micro-swimmers. Our
approach is based on the squirmer model, which we adapt to the FCM framework,
resulting in a method that is suitable for simulating semi-dilute squirmer
suspensions. Other effects, such as steric interactions, are considered with
our model. We test our method by comparing the velocity field around a single
squirmer and the pairwise interactions between two squirmers with exact
solutions to the Stokes equations and results given by other numerical methods.
We also illustrate our method's ability to describe spheroidal swimmer shapes
and biologically-relevant time-dependent swimming gaits. We detail the
numerical algorithm used to compute the hydrodynamic coupling between a large
collection () of micro-swimmers. Using this methodology, we
investigate the emergence of polar order in a suspension of squirmers and show
that for large domains, both the steady-state polar order parameter and the
growth rate of instability are independent of system size. These results
demonstrate the effectiveness of our approach to achieve near continuum-level
results, allowing for better comparison with experimental measurements while
complementing and informing continuum models.Comment: 37 pages, 21 figure
Dynamics of bidisperse suspensions under stokes flows: linear shear flow and eedimentation
Sedimenting and sheared bidisperse homogeneous suspensions of non-Brownian particles are investigated by numerical simulations in the limit of vanishing small Reynolds number and negligible inertia of the particles. The numerical approach is based on the solution of the three-dimensional Stokes equations forced by the presence of the dispersed phase. Multi-body hydrodynamic interactions are achieved by a low order multipole expansion of the velocity perturbation. The accuracy of the model is validated on analytic solutions of generic flow configurations involving a pair of particles.
The first part of the paper aims at investigating the dynamics of monodisperse and bidisperse suspensions embedded in a linear shear flow. The macroscopic transport properties due to hydrodynamic and non hydrodynamic interactions (short range repulsion force) show good agreement with previous theoretical and experimental works on homogeneous monodisperse particles. Increasing the volumetric concentration of the suspension leads to an enhancement of particle fluctuations and self-diffusion. The velocity fluctuation tensor scales linearly up to 15% concentration. Multi-body interactions weaken the correlation of velocity fluctuations and lead to a diffusion like motion of the particles. Probability density functions show a clear transition from Gaussian to exponential tails while the concentration decreases. The behavior of bidisperse suspensions is more complicated, since the respective amount of small and large particles modifies the overall response of the flow. Our simulations show that, for a given concentration of both species, when the size ratio varies from 1 to 2.5, the fluctuation level of the small particles is strongly enhanced. A similar trend is observed on the evolution of the shear induced self-diffusion coefficient. Thus for a fixed and total concentration, increasing the respective volume fraction of large particles can double the velocity fluctuation of small particles.
In the second part of the paper, the sedimentation of a single test particle embedded in a suspension of monodisperse particles allows the determination of basic hydrodynamic interactions involved in a bidisperse suspension. Good agreement is achieved when comparing the mean settling velocity and fluctuations levels of the test sphere with experiments. Two distinct behaviors are observed depending on the physical properties of the particle. The Lagrangian velocity autocorrelation function has a negative region when the test particle has a settling velocity twice as large as the reference velocity of the surrounding suspension. The test particle settles with a zig-zag vertical trajectory while a strong reduction of horizontal dispersion occurs. Then, several configurations of bidisperse settling suspensions are investigated. Mean velocity depends on concentration of both species, density ratio and size ratio. Results are compared with theoretical predictions at low concentration and empirical correlations when the assumption of a dilute regime is no longer valid. For particular configurations, a segregation instability sets in. Columnar patterns tend to collect particles of the same species and eventually a complete separation of the suspension is observed. The instability threshold is compared with experiments in the case of suspensions of buoyant and heavy spheres. The basic features are well reproduced by the simulation model
Dynamic simulation of hydrodynamically interacting suspensions
A general method for computing the hydrodynamic interactions among an infinite suspension of particles, under the condition of vanishingly small particle Reynolds number, is presented. The method follows the procedure developed by O'Brien (1979) for constructing absolutely convergent expressions for particle interactions. For use in dynamic simulation, the convergence of these expressions is accelerated by application of the Ewald summation technique. The resulting hydrodynamic mobility and/or resistance matrices correctly include all far-field non-convergent interactions. Near-field lubrication interactions are incorporated into the resistance matrix using the technique developed by Durlofsky, Brady & Bossis (1987). The method is rigorous, accurate and computationally efficient, and forms the basis of the Stokesian-dynamics simulation method. The method is completely general and allows such diverse suspension problems as self-diffusion, sedimentation, rheology and flow in porous media to be treated within the same formulation for any microstructural arrangement of particles. The accuracy of the Stokesian-dynamics method is illustrated by comparing with the known exact results for spatially periodic suspensions
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 - are examined. Both contributions are increasing
functions of , contacts contribution dominates at low viscous number
( 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
Spectral Ewald Acceleration of Stokesian Dynamics for polydisperse suspensions
In this work we develop the Spectral Ewald Accelerated Stokesian Dynamics
(SEASD), a novel computational method for dynamic simulations of polydisperse
colloidal suspensions with full hydrodynamic interactions. SEASD is based on
the framework of Stokesian Dynamics (SD) with extension to compressible
solvents, and uses the Spectral Ewald (SE) method [Lindbo & Tornberg, J.
Comput. Phys. 229 (2010) 8994] for the wave-space mobility computation. To meet
the performance requirement of dynamic simulations, we use Graphic Processing
Units (GPU) to evaluate the suspension mobility, and achieve an order of
magnitude speedup compared to a CPU implementation. For further speedup, we
develop a novel far-field block-diagonal preconditioner to reduce the far-field
evaluations in the iterative solver, and SEASD-nf, a polydisperse extension of
the mean-field Brownian approximation of Banchio & Brady [J. Chem. Phys. 118
(2003) 10323]. We extensively discuss implementation and parameter selection
strategies in SEASD, and demonstrate the spectral accuracy in the mobility
evaluation and the overall computation scaling. We
present three computational examples to further validate SEASD and SEASD-nf in
monodisperse and bidisperse suspensions: the short-time transport properties,
the equilibrium osmotic pressure and viscoelastic moduli, and the steady shear
Brownian rheology. Our validation results show that the agreement between SEASD
and SEASD-nf is satisfactory over a wide range of parameters, and also provide
significant insight into the dynamics of polydisperse colloidal suspensions.Comment: 39 pages, 21 figure
Turbulent channel flow of dense suspensions of neutrally-buoyant spheres
Dense particle suspensions are widely encountered in many applications and in
environmental flows. While many previous studies investigate their rheological
properties in laminar flows, little is known on the behaviour of these
suspensions in the turbulent/inertial regime. The present study aims to fill
this gap by investigating the turbulent flow of a Newtonian fluid laden with
solid neutrally-buoyant spheres at relatively high volume fractions in a plane
channel. Direct Numerical Simulation are performed in the range of volume
fractions Phi=0-0.2 with an Immersed Boundary Method used to account for the
dispersed phase. The results show that the mean velocity profiles are
significantly altered by the presence of a solid phase with a decrease of the
von Karman constant in the log-law. The overall drag is found to increase with
the volume fraction, more than one would expect just considering the increase
of the system viscosity due to the presence of the particles. At the highest
volume fraction here investigated, Phi=0.2, the velocity fluctuation
intensities and the Reynolds shear stress are found to decrease. The analysis
of the mean momentum balance shows that the particle-induced stresses govern
the dynamics at high Phi and are the main responsible of the overall drag
increase. In the dense limit, we therefore find a decrease of the turbulence
activity and a growth of the particle induced stress, where the latter
dominates for the Reynolds numbers considered here.Comment: Journal of Fluid Mechanics, 201
- …