4,653 research outputs found
Accurate lubrication corrections for spherical and non-spherical particles in discretized fluid simulations
Discretized fluid solvers coupled to a Newtonian dynamics method are a
popular tool to study suspension flow. As any simulation technique with finite
resolution, the lattice Boltzmann method, when coupled to discrete particles
using the momentum exchange method, resolves the diverging lubrication
interactions between surfaces near contact only insufficiently. For spheres, it
is common practice to account for surface-normal lubrication forces by means of
an explicit correction term. A method that additionally covers all further
singular interactions for spheres is present in the literature as well as a
link-based approach that allows for more general shapes but does not capture
non-normal interactions correctly. In this paper, lattice-independent
lubrication corrections for aspherical particles are outlined, taking into
account all leading divergent interaction terms. An efficient implementation
for arbitrary spheroids is presented and compared to purely normal and
link-based models. Good consistency with Stokesian dynamics simulations of
spheres is found. The non-normal interactions affect the viscosity of
suspensions of spheres at volume fractions \Phi >= 0.3 but already at \Phi >=
0.2 for spheroids. Regarding shear-induced diffusion of spheres, a distinct
effect is found at 0.1 <= \Phi <= 0.5 and even increasing the resolution of the
radius to 8 lattice units is no substitute for an accurate modeling of
non-normal interactions.Comment: 19 pages, 10 figure
Helicity Transfer in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. We show
that a Reynolds-independent helicity flux is present in the model when the
large scale forcing breaks inversion symmetry. The equivalent in Shell Models
of the ``2/15 law'', obtained from helicity conservation in Navier-Stokes eqs.,
is derived and tested. The odd part of helicity flux statistic is found to be
dominated by a few very intense events. In a particular model, we calculate
analytically leading and sub-leading contribution to the scaling of triple
velocity correlation.Comment: 4 pages, LaTex, 2 figure
Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence
We present a numerical study of two-particle dispersion from point-sources in
3D incompressible Homogeneous and Isotropic turbulence, at Reynolds number Re
\simeq 300. Tracer particles are emitted in bunches from localized sources
smaller than the Kolmogorov scale. We report the first quantitative evidence,
supported by an unprecedented statistics, of the deviations of relative
dispersion from Richardson's picture. Deviations are due to extreme events of
pairs separating much faster than average, and of pairs remaining close for
long times. The two classes of events are the fingerprint of complete different
physics, the former being dominated by inertial subrange and large-scale
fluctuations, while the latter by the dissipation subrange. A comparison of
relative separation in surrogate white-in-time velocity field, with correct
viscous-, inertial- and integral-scale properties allows us to assess the
importance of temporal correlations along tracer trajectories.Comment: 5 pages, 6 figure
Intermittency in Turbulence: Multiplicative random process in space and time
We present a simple stochastic algorithm for generating multiplicative
processes with multiscaling both in space and in time. With this algorithm we
are able to reproduce a synthetic signal with the same space and time
correlation as the one coming from shell models for turbulence and the one
coming from a turbulent velocity field in a quasi-Lagrangian reference frame.Comment: 23 pages, 12 figure
Helicity advection in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. In
particular, we study the scaling behavior of helicity transfer in a dynamical
model of turbulence lacking inversion symmetry. We present some
phenomenological and numerical support to the idea that Helicity becomes -at
scale small enough- a passively-advected quantity.Comment: 6 pages, 2 figures, contribution to the proceedings of the
conference: Disorder and Chaos, in honour of Giovanni Paladin, September
22-24, 1997, Rom
Towards a continuum model for particle-induced velocity fluctuations in suspension flow through a stenosed geometry
Non-particulate continuum descriptions allow for computationally efficient
modeling of suspension flows at scales that are inaccessible to more detailed
particulate approaches. It is well known that the presence of particles
influences the effective viscosity of a suspension and that this effect has
thus to be accounted for in macroscopic continuum models. The present paper
aims at developing a non-particulate model that reproduces not only the
rheology but also the cell-induced velocity fluctuations, responsible for
enhanced diffusivity. The results are obtained from a coarse-grained blood
model based on the lattice Boltzmann method. The benchmark system comprises a
flow between two parallel plates with one of them featuring a smooth obstacle
imitating a stenosis. Appropriate boundary conditions are developed for the
particulate model to generate equilibrated cell configurations mimicking an
infinite channel in front of the stenosis. The averaged flow field in the bulk
of the channel can be described well by a non-particulate simulation with a
matched viscosity. We show that our proposed phenomenological model is capable
to reproduce many features of the velocity fluctuations.Comment: 6 pages, 6 figure
- …