152 research outputs found
A lattice mesoscopic model of dynamically heterogeneous fluids
We introduce a mesoscopic three-dimensional Lattice Boltzmann Model which
attempts to mimick the physical features associated with cage effects in
dynamically heterogeneous fluids. To this purpose, we extend the standard
Lattice Boltzmann dynamics with self-consistent constraints based on the
non-local density of the surrounding fluid. The resulting dynamics exhibits
typical features of dynamic heterogeneous fluids, such as non-Gaussian density
distributions and long-time relaxation. Due to its intrinsically parallel
dynamics, and absence of statistical noise, the method is expected to compute
significantly faster than molecular dynamics, Monte Carlo and lattice glass
models.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Volumetric formulation of lattice Boltzmann models with energy conservation
We analyze a volumetric formulation of lattice Boltzmann for compressible
thermal fluid flows. The velocity set is chosen with the desired accuracy,
based on the Gauss-Hermite quadrature procedure, and tested against controlled
problems in bounded and unbounded fluids. The method allows the simulation of
thermohydrodyamical problems without the need to preserve the exact
space-filling nature of the velocity set, but still ensuring the exact
conservation laws for density, momentum and energy. Issues related to boundary
condition problems and improvements based on grid refinement are also
investigated.Comment: 8 figure
Diel variations in cell division and biomass production of Emiliania huxleyi — Consequences for the calculation of physiological cell parameters
Cell division of the coccolithophore Emiliania huxleyi and other phytoplankton typically becomes entrained
to diel light/dark cycles under laboratory conditions, with division occurring primarily during dark phases and
production occurring during light phases. Under these conditions, increases in cell and biomass concentrations
deviate from exponential functions on time scales < 24 h. These deviations lead to significant diel variations in
common measurements of phytoplankton physiology such as cellular quotas of particulate organic and inorganic carbon (POC, PIC) and their production rates. Being time-dependent, only the temporal mean of the various values during the day are comparable between experiments. Deviations from exponential growth furthermore imply that increases in cell and biomass concentrations cannot be expressed by the daily growth rate ÎĽ24 h (typically determined from daily increments in cell concentrations). Consequently, conventional calculations of production as the product of a cellular quota (e.g., POC quota) and ÎĽ24 h are mathematically incorrect. To account for this, we here describe short-term changes in cell and biomass concentrations of fast
-dividing, dilute-batch cultures of E. huxleyi grown under a diel light/dark cycle using linear regression. Based on the derived models, we present calculations for daily means of cellular quotas and production rates. Conventional (time-specific) measurements of cellular quotas and production differ from daily means by up to 65% in our example and, under some circumstances, cause false “effects” of treatments. Intending to reduce errors in ecophysiological studies, we recommend determining daily means—mathematically or by adjusting the experimental setup or sampling times appropriately
Herschel-Bulkley rheology from lattice kinetic theory of soft-glassy materials
We provide a clear evidence that a two species mesoscopic Lattice Boltzmann
(LB) model with competing short-range attractive and mid-range repulsive
interactions supports emergent Herschel-Bulkley (HB) rheology, i.e. a power-law
dependence of the shear-stress as a function of the strain rate, beyond a given
yield-stress threshold. This kinetic formulation supports a seamless transition
from flowing to non-flowing behaviour, through a smooth tuning of the
parameters governing the mesoscopic interactions between the two species. The
present model may become a valuable computational tool for the investigation of
the rheology of soft-glassy materials on scales of experimental interest.Comment: 5 figure
Lattice Boltzmann models for non-ideal fluids with arrested phase-separation
The effects of mid-range repulsion in Lattice Boltzmann models on the
coalescence/breakup behaviour of single-component, non-ideal fluids are
investigated. It is found that mid-range repulsive interactions allow the
formation of spray-like, multi-droplet configurations, with droplet size
directly related to the strength of the repulsive interaction. The simulations
show that just a tiny ten-percent of mid-range repulsive pseudo-energy can
boost the surface/volume ratio of the phase- separated fluid by nearly two
orders of magnitude. Drawing upon a formal analogy with magnetic Ising systems,
a pseudo-potential energy is defined, which is found to behave like a
quasi-conserved quantity for most of the time-evolution. This offers a useful
quantitative indicator of the stability of the various configurations, thus
helping the task of their interpretation and classification. The present
approach appears to be a promising tool for the computational modelling of
complex flow phenomena, such as atomization, spray formation and
micro-emulsions, break-up phenomena and possibly glassy-like systems as well.Comment: 12 pages, 9 figure
Impalement transitions in droplets impacting microstructured superhydrophobic surfaces
Liquid droplets impacting a superhydrophobic surface decorated with
micro-scale posts often bounce off the surface. However, by decreasing the
impact velocity droplets may land on the surface in a fakir state, and by
increasing it posts may impale droplets that are then stuck on the surface. We
use a two-phase lattice-Boltzmann model to simulate droplet impact on
superhydrophobic surfaces, and show that it may result in a fakir state also
for reasonable high impact velocities. This happens more easily if the surface
is made more hydrophobic or the post height is increased, thereby making the
impaled state energetically less favourable.Comment: 8 pages, 4 figures, to appear in Europhysics Letter
Numerical simulations of compressible Rayleigh-Taylor turbulence in stratified fluids
We present results from numerical simulations of Rayleigh-Taylor turbulence,
performed using a recently proposed lattice Boltzmann method able to describe
consistently a thermal compressible flow subject to an external forcing. The
method allowed us to study the system both in the nearly-Boussinesq and
strongly compressible regimes. Moreover, we show that when the stratification
is important, the presence of the adiabatic gradient causes the arrest of the
mixing process.Comment: 15 pages, 11 figures. Proceedings of II Conference on Turbulent
Mixing and Beyond (TMB-2009
Investigation of a lattice Boltzmann model with a variable speed of sound
A lattice Boltzmann model is considered in which the speed of sound can be
varied independently of the other parameters. The range over which the speed of
sound can be varied is investigated and good agreement is found between
simulations and theory. The onset of nonlinear effects due to variations in the
speed of sound is also investigated and good agreement is again found with
theory. It is also shown that the fluid viscosity is not altered by changing
the speed of sound
Modelling thermal flow in a transition regime using a lattice Boltzmann approach
Lattice Boltzmann models are already able to capture important rarefied flow phenomena, such as velocity-slip and temperature jump, provided the effects of the Knudsen layer are minimal. However, both conventional hydrodynamics, as exemplified by the Navier-Stokes-Fourier equations, and the lattice Boltzmann method fail to predict the nonlinear velocity and temperature variations in the Knudsen layer that have been observed in kinetic theory. In the present paper, we propose an extension to the lattice Boltzmann method that will enable the simulation of thermal flows in the transition regime where Knudsen layer effects are significant. A correction function is introduced that accounts for the reduction in the mean free path near a wall. This new approach is compared with direct simulation Monte Carlo data for Fourier flow and good qualitative agreement is obtained for Knudsen numbers up to 1.58
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