32 research outputs found
Aerosol cluster impact and break-up : model and implementation.
In this report a model for simulating aerosol cluster impact with rigid walls is presented. The model is based on JKR adhesion theory and is implemented as an enhancement to the granular (DEM) package within the LAMMPS code. The theory behind the model is outlined and preliminary results are shown. Modeling the interactions of small particles is relevant to a number of applications (e.g., soils, powders, colloidal suspensions, etc.). Modeling the behavior of aerosol particles during agglomeration and cluster dynamics upon impact with a wall is of particular interest. In this report we describe preliminary efforts to develop and implement physical models for aerosol particle interactions. Future work will consist of deploying these models to simulate aerosol cluster behavior upon impact with a rigid wall for the purpose of developing relationships for impact speed and probability of stick/bounce/break-up as well as to assess the distribution of cluster sizes if break-up occurs. These relationships will be developed consistent with the need for inputs into system-level codes. Section 2 gives background and details on the physical model as well as implementations issues. Section 3 presents some preliminary results which lead to discussion in Section 4 of future plans
A soft departure from jamming: the compaction of deformable granular matter under high pressures
The high-pressure compaction of three dimensional granular packings is
simulated using a bonded particle model (BPM) to capture linear elastic
deformation. In the model, grains are represented by a collection of point
particles connected by bonds. A simple multibody interaction is introduced to
control Poisson's ratio and the arrangement of particles on the surface of a
grain is varied to model both high- and low-frictional grains. At low
pressures, the growth in packing fraction and coordination number follow the
expected behavior near jamming and exhibit friction dependence. As the pressure
increases, deviations from the low-pressure power-law scaling emerge after the
packing fraction grows by approximately 0.1 and results from simulations with
different friction coefficients converge. These results are compared to
predictions from traditional discrete element method simulations which,
depending on the definition of packing fraction and coordination number, may
only differ by a factor of two. As grains deform under compaction, the average
volumetric strain and asphericity, a measure of the change in the shape of
grains, are found to grow as power laws and depend heavily on the Poisson's
ratio of the constituent solid. Larger Poisson's ratios are associated with
less volumetric strain and more asphericity and the apparent power-law exponent
of the asphericity may vary. The elastic properties of the packed grains are
also calculated as a function of packing fraction. In particular, we find the
Poisson's ratio near jamming is 1/2 but decreases to 1/4 before rising again as
systems densify
Velocity Correlations in Dense Gravity Driven Granular Chute Flow
We report numerical results for velocity correlations in dense,
gravity-driven granular flow down an inclined plane. For the grains on the
surface layer, our results are consistent with experimental measurements
reported by Pouliquen. We show that the correlation structure within planes
parallel to the surface persists in the bulk. The two-point velocity
correlation function exhibits exponential decay for small to intermediate
values of the separation between spheres. The correlation lengths identified by
exponential fits to the data show nontrivial dependence on the averaging time
\dt used to determine grain velocities. We discuss the correlation length
dependence on averaging time, incline angle, pile height, depth of the layer,
system size and grain stiffness, and relate the results to other length scales
associated with the rheology of the system. We find that correlation lengths
are typically quite small, of the order of a particle diameter, and increase
approximately logarithmically with a minimum pile height for which flow is
possible, \hstop, contrary to the theoretical expectation of a proportional
relationship between the two length scales.Comment: 21 pages, 16 figure
Evaporation of Lennard-Jones Fluids
Evaporation and condensation at a liquid/vapor interface are ubiquitous
interphase mass and energy transfer phenomena that are still not well
understood. We have carried out large scale molecular dynamics simulations of
Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to
investigate these processes with molecular detail. For LJ monomers in contact
with a vacuum, the evaporation rate is found to be very high with significant
evaporative cooling and an accompanying density gradient in the liquid domain
near the liquid/vapor interface. Increasing the chain length to just dimers
significantly reduces the evaporation rate. We confirm that mechanical
equilibrium plays a key role in determining the evaporation rate and the
density and temperature profiles across the liquid/vapor interface. The
velocity distributions of evaporated molecules and the evaporation and
condensation coefficients are measured and compared to the predictions of an
existing model based on kinetic theory of gases. Our results indicate that for
both monatomic and polyatomic molecules, the evaporation and condensation
coefficients are equal when systems are not far from equilibrium and smaller
than one, and decrease with increasing temperature. For the same reduced
temperature , where is the critical temperature, these two
coefficients are higher for LJ dimers and trimers than for monomers, in
contrast to the traditional viewpoint that they are close to unity for
monatomic molecules and decrease for polyatomic molecules. Furthermore, data
for the two coefficients collapse onto a master curve when plotted against a
translational length ratio between the liquid and vapor phase.Comment: revised version, 15 pages, 15 figures, to appear in J. Chem. Phy
Granular packing simulation protocols: tap, press and relax
Granular matter takes many paths to pack. Gentle compression, compaction or
repetitive tapping can happen in natural and industrial processes. The path
influences the packing microstructure, and thus macroscale properties,
particularly for frictional grains. We perform discrete element modeling
simulations to construct packings of frictional spheres implementing a range of
stress-controlled protocols with 3D periodic boundary conditions. A
volume-controlled over-compression method is compared to four stress-controlled
methods, including over-compression and release, gentle under-compression and
cyclical compression and release. The packing volume fraction of each method
depends on the pressure, initial kinetic energy and protocol parameters. A
non-monotonic pressure dependence in the volume fraction, but not the
coordination number occurs when dilute particles initialized with a non-zero
kinetic energy are compressed, but can be reduced with the inclusion of drag.
The fraction of frictional contacts correlates with the volume fraction
minimum. Packings were cyclically compressed 1000 times. Response to
compression depends on pressure; low pressure packings have a constant volume
fraction regime, while high pressure packings continue to get dense with number
of cycles. The capability of stress-controlled, bulk-like particle simulations
to capture different protocols is showcased, and the ability to pack at low
pressures demonstrates unexpected behavior