238 research outputs found
Influence of asperities on fluid and thermal flow in a fracture: a coupled Lattice Boltzmann study
The characteristics of the hydro-thermal flow which occurs when a cold fluid
is injected into a hot fractured bedrock depend on the morphology of the
fracture. We consider a sharp triangular asperity, invariant in one direction,
perturbing an otherwise flat fracture. We investigate its influence on the
macroscopic hydraulic transmissivity and heat transfer efficiency, at fixed low
Reynolds number. In this study, numerical simulations are done with a coupled
lattice Boltzmann method that solves both the complete Navier-Stokes and
advection-diffusion equations in three dimensions. The results are compared
with those obtained under lubrication approximations which rely on many
hypotheses and neglect the three-dimensional (3D) effects. The lubrication
results are obtained by analytically solving the Stokes equation and a
two-dimensional (integrated over the thickness) advection-diffusion equation.
We use a lattice Boltzmann method with a double distribution (for mass and
energy transport) on hypercubic and cubic lattices. Beyond some critical slope
for the boundaries, the velocity profile is observed to be far from a quadratic
profile in the vicinity of the sharp asperity: the fluid within the triangular
asperity is quasi-static. We find that taking account of both the 3D effects
and the cooling of the rock, are important for the thermal exchange. Neglecting
these effects with lubrication approximations results in overestimating the
heat exchange efficiency. The evolution of the temperature over time, towards
steady state, also shows complex behavior: some sites alternately reheat and
cool down several times, making it difficult to forecast the extracted heat.Comment: In Journal of Geophysical Research B (2013) online firs
Lattice Boltzmann Models for Complex Fluids
We present various Lattice Boltzmann Models which reproduce the effects of
rough walls, shear thinning and granular flow. We examine the boundary layers
generated by the roughness of the walls. Shear thinning produces plug flow with
a sharp density contrast at the boundaries. Density waves are spontaneously
generated when the viscosity has a nonlinear dependence on density which
characterizes granular flow.Comment: 11 pages, plain TeX, preprint HLRZ 23/9
Hyperballistic superdiffusion and explosive solutions to the non-linear diffusion equation
By means of a particle model that includes interactions only via the local
particle concentration, we show that hyperballistic diffusion may result. This
is done by findng the exact solution of the corresponding non-linear diffusion
equation, as well as by particle simulations. The connection between these
levels of description is provided by the Fokker-Planck equation describing the
particle dynamics.Comment: 5 pages, 5 figure
The escape problem for active particles confined to a disc
We study the escape problem for interacting, self-propelled particles
confined to a disc, where particles can exit through one open slot on the
circumference. Within a minimal 2D Vicsek model, we numerically study the
statistics of escape events when the self-propelled particles can be in a
flocking state. We show that while an exponential survival probability is
characteristic for non-interaction self-propelled particles at all times, the
interacting particles have an initial exponential phase crossing over to a
sub-exponential late-time behavior. We propose a new phenomenological model
based on non-stationary Poisson processes which includes the Allee effect to
explain this sub-exponential trend and perform numerical simulations for
various noise intensities
Dynamic aerofracture of dense granular packings
International audienceA transition in hydraulically induced granular displacement patterns is studied by means of discrete numerical molecular dynamics simulations. During this transition the patterns change from fractures and fingers to finely dispersed bubbles. The dynamics of the displacement patterns are studied in a rectangular Hele-Shaw cell filled with a dense but permeable two-dimensional granular layer. At one side of the cell the pressure of the compressible interstitial gas is increased. At the opposite side from the inlet of the cell a semipermeable boundary is located. This boundary is only permeable towards the gas phase while preventing grains from leaving the cell. The imposed pressure gradient compacts the grains. In the process we can identify and describe a mechanism that controls the transition of the emerging displacement patterns from fractures and fingers to finely dispersed bubbles as a function of the interstitial gas's properties and the characteristics of the granular phase
How would you integrate the equations of motion in dissipative particle dynamics simulations?
In this work we assess the quality and performance of several novel
dissipative particle dynamics integration schemes that have not previously been
tested independently. Based on a thorough comparison we identify the respective
methods of Lowe and Shardlow as particularly promising candidates for future
studies of large-scale properties of soft matter systems
Numerical approach to frictional fingers
Experiments on confined multiphase flow systems, involving air and a dense suspension, have revealed a diverse set of flow morphologies. As the air displaces the suspension, the beads that make up the suspension can accumulate along the interface. The dynamics can generate “frictional fingers” of air coated by densely packed grains. We present here a simplified model for the dynamics together with a new numerical strategy for simulating the frictional finger behavior. We further make theoretical predictions for the characteristic width associated with the frictional fingers, based on the yield stress criterion, and compare these to experimental results. The agreement between theory and experiments validates our model and allows us to estimate the unknown parameter in the yield stress criterion, which we use in the simulations
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