210 research outputs found
A Thermodynamically-Consistent Non-Ideal Stochastic Hard-Sphere Fluid
A grid-free variant of the Direct Simulation Monte Carlo (DSMC) method is
proposed, named the Isotropic DSMC (I-DSMC) method, that is suitable for
simulating dense fluid flows at molecular scales. The I-DSMC algorithm
eliminates all grid artifacts from the traditional DSMC algorithm; it is
Galilean invariant and microscopically isotropic. The stochastic collision
rules in I-DSMC are modified to yield a non-ideal structure factor that gives
consistent compressibility, as first proposed in [Phys. Rev. Lett. 101:075902
(2008)]. The resulting Stochastic Hard Sphere Dynamics (SHSD) fluid is
empirically shown to be thermodynamically identical to a deterministic
Hamiltonian system of penetrable spheres interacting with a linear core pair
potential, well-described by the hypernetted chain (HNC) approximation. We
apply a stochastic Enskog kinetic theory for the SHSD fluid to obtain estimates
for the transport coefficients that are in excellent agreement with particle
simulations over a wide range of densities and collision rates. The fluctuating
hydrodynamic behavior of the SHSD fluid is verified by comparing its dynamic
structure factor against theory based on the Landau-Lifshitz Navier-Stokes
equations. We also study the Brownian motion of a nano-particle suspended in an
SHSD fluid and find a long-time power-law tail in its velocity autocorrelation
function consistent with hydrodynamic theory and molecular dynamics
calculations.Comment: 30 pages, revision adding some clarifications and a new figure. See
also arXiv:0803.035
Lattice Boltzmann simulations of anisotropic particles at liquid interfaces
Complex colloidal fluids, such as emulsions stabilized by complex shaped
particles, play an important role in many industrial applications. However,
understanding their physics requires a study at sufficiently large length
scales while still resolving the microscopic structure of a large number of
particles and of the local hydrodynamics. Due to its high degree of locality,
the lattice Boltzmann method, when combined with a molecular dynamics solver
and parallelized on modern supercomputers, provides a tool that allows such
studies. Still, running simulations on hundreds of thousands of cores is not
trivial. We report on our practical experiences when employing large fractions
of an IBM Blue Gene/P system for our simulations. Then, we extend our model for
spherical particles in multicomponent flows to anisotropic ellipsoidal objects
rendering the shape of e.g. clay particles. The model is applied to a number of
test cases including the adsorption of single particles at fluid interfaces and
the formation and stabilization of Pickering emulsions or bijels.Comment: 10 pages, 5 figures; ParCFD 2011 proceedings contributio
A holistic scalable implementation approach of the lattice Boltzmann method for CPU/GPU heterogeneous clusters
This is the author accepted manuscript. The final version is available from MDPI via the DOI in this record.Heterogeneous clusters are a widely utilized class of supercomputers assembled from
different types of computing devices, for instance CPUs and GPUs, providing a huge computational
potential. Programming them in a scalable way exploiting the maximal performance introduces
numerous challenges such as optimizations for different computing devices, dealing with multiple
levels of parallelism, the application of different programming models, work distribution, and hiding
of communication with computation. We utilize the lattice Boltzmann method for fluid flow as
a representative of a scientific computing application and develop a holistic implementation for
large-scale CPU/GPU heterogeneous clusters. We review and combine a set of best practices and
techniques ranging from optimizations for the particular computing devices to the orchestration
of tens of thousands of CPU cores and thousands of GPUs. Eventually, we come up with
an implementation using all the available computational resources for the lattice Boltzmann
method operators. Our approach shows excellent scalability behavior making it future-proof for
heterogeneous clusters of the upcoming architectures on the exaFLOPS scale. Parallel efficiencies of
more than 90% are achieved leading to 2,604.72 GLUPS utilizing 24,576 CPU cores and 2,048 GPUs of
the CPU/GPU heterogeneous cluster Piz Daint and computing more than 6.8 · 109
lattice cells.This work was supported by the German Research Foundation (DFG) as part of the
Transregional Collaborative Research Centre “Invasive Computing” (SFB/TR 89). In addition, this work was
supported by a grant from the Swiss National Supercomputing Centre (CSCS) under project ID d68. We further
thank the Max Planck Computing & Data Facility (MPCDF) and the Global Scientific Information and Computing
Center (GSIC) for providing computational resources
Modeling Cancer Cell Response to Immunotherapy
Significant work has been done modeling cancerous tumor growth and response to therapy under certain simplifying assumptions, specifically, the assumption of spatial homogeneity. We have chosen a spatially heterogenous model for cancer cell growth using a hybrid Lattice-Gas Cellular Automata method. Cell mitosis, apoptosis, and necrosis are explicitly modeled along with the diffusion of nutrients and a necrotic signal. The model implementation is verified qualitatively and is modified to execute on a parallel computer
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