417 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
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
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
The statistical properties of turbulence in the presence of a smart small-scale control
By means of high-resolution numerical simulations, we compare the statistical
properties of homogeneous and isotropic turbulence to those of the
Navier-Stokes equation where small-scale vortex filaments are strongly
depleted, thanks to a non-linear extra viscosity acting preferentially on high
vorticity regions. We show that the presence of such smart small-scale drag can
strongly reduce intermittency and non-Gaussian fluctuations. Our results pave
the way towards a deeper understanding on the fundamental role of degrees of
freedom in turbulence as well as on the impact of (pseudo)coherent structures
on the statistical small-scale properties. Our work can be seen as a first
attempt to develop smart-Lagrangian forcing/drag mechanisms to control
turbulence.Comment: 5 pages, 5 figure
Heat flux scaling in turbulent Rayleigh-B\'enard convection with an imposed longitudinal wind
We present a numerical study of Rayleigh-B\'enard convection disturbed by a
longitudinal wind. Our results show that under the action of the wind, the
vertical heat flux through the cell initially decreases, due to the mechanism
of plumes-sweeping, and then increases again when turbulent forced convection
dominates over the buoyancy. As a result, the Nusselt number is a non-monotonic
function of the shear Reynolds number. We provide a simple model that captures
with good accuracy all the dynamical regimes observed. We expect that our
findings can lead the way to a more fundamental understanding of the of the
complex interplay between mean-wind and plumes ejection in the
Rayleigh-B\'enard phenomenology.Comment: 5 pages, 4 figure
Earthquake statistics inferred from plastic events in soft-glassy materials
We propose a new approach for generating synthetic earthquake catalogues
based on the physics of soft glasses. The continuum approach produces
yield-stress materials based on Lattice-Boltzmann simulations. We show that, if
the material is stimulated below yield stress, plastic events occur, which have
strong similarities with seismic events. Based on a suitable definition of
displacement in the continuum, we show that the plastic events obey a
Gutenberg-Richter law with exponents similar to those for real earthquakes. We
further find that average acceleration, energy release, stress drop and
recurrence times scale with the same exponent. The approach is fully
self-consistent and all quantities can be calculated at all scales without the
need of ad hoc friction or statistical laws. We therefore suggest that our
approach may lead to new insight into understanding of the physics connecting
the micro and macro scale of earthquakes.Comment: 13 pages, 7 figure
Statistics of small scale vortex filaments in turbulence
We study the statistical properties of coherent, small-scales,
filamentary-like structures in Turbulence. In order to follow in time such
complex spatial structures, we integrate Lagrangian and Eulerian measurements
by seeding the flow with light particles. We show that light particles
preferentially concentrate in small filamentary regions of high persistent
vorticity (vortex filaments). We measure the fractal dimension of the
attracting set and the probability that two particles do not separate for long
time lapses. We fortify the signal-to-noise ratio by exploiting multi-particles
correlations on the dynamics of bunches of particles. In doing that, we are
able to give a first quantitative estimation of the vortex-filaments
life-times, showing the presence of events as long as the integral correlation
time. The same technique introduced here could be used in experiments as long
as one is capable to track clouds of bubbles in turbulence for a relatively
long period of time, at high Reynolds numbers; shading light on the dynamics of
small-scale vorticity in realistic turbulent flows.Comment: 5 pages, 5 figure
Decaying and kicked turbulence in a shell model
Decaying and periodically kicked turbulence are analyzed within the GOY shell
model, to allow for sufficiently large scaling regimes. Energy is transfered
towards the small scales in intermittent bursts. Nevertheless, mean field
arguments are sufficient to account for the ensemble averaged energy decay E(t)
\~t^{-2} or the parameter dependences for the ensemble averaged total energy in
the kicked case. Within numerical precision, the inertial subrange
intermittency remains the same, whether the system is forced or decaying.Comment: 14 pages, 8 figure
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