Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V8: Focus Session: Simulations Using Particles |
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Sponsoring Units: DCOMP DFD Chair: Lorena Barba, University of Bristol Room: Baltimore Convention Center 314 |
Thursday, March 16, 2006 11:15AM - 11:51AM |
V8.00001: Nanoscale flows on open chemical channels Invited Speaker: We investigate the nano-scale flows of low-volatility liquids along ``chemical channels'': patterns of completely-wetting solid embedded in a planar substrate, and sandwiched between less wetting solid regions. In the case of a long, straight wetting stripe, we use molecular dynamics simulations as basic computational tool, comparing the results to a simple long-wavelength approximation and a full stability analysis based on the Stokes equations. The different approaches are qualitatively consistent, and we find that while thin liquid ridges are stable both statically and during flow, a (linear) pearling instability develops if the thickness of the ridge exceeds half of the width of the channel. In the flowing case periodic bulges propagate along the channel and subsequently merge due to nonlinear effects. However, the ridge does not break up even in the unstable case, and the qualitative behavior is unchanged when the fluid can spill over onto a partially wetting exterior solid region. For more complicated patterns involving the splitting or merger of wetting stripes, we again find that liquid flows continuously along the wetting region despite the pearling instability. In this case, intriguing switching dynamics is found for moving pearls at junctions. [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:27PM |
V8.00002: Immersed boundaries and particles Invited Speaker: This talk will present ongoing works in our group dealing with particle simulation of complex flows. We will show some non conventional particle methods to simulate incompressible elasticity and fluid-structure interactions. We will also show how particle methods in these fields, as well as for more conventional advection dominated physics, can lead to new ideas for Eulerian schemes. [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V8.00003: Particle Methods in Numerical Cosmology Hugh Couchman Particle methods play a central role in numerical simulations of cosmic structure. These methods are particularly important for simulations of two-component universes that include both baryonic and ``dark matter.'' Particles are used to model both the collisionless dark matter---using a classical inverse square law gravitational attraction---and, with Smoothed Particle Hydrodynamics (SPH), the baryonic component. Although Eulerian methods are also now widely used to model cosmological hydrodynamics, SPH exhibits many useful and important properties for cosmology: it is robust and simple to code, meshes well with the necessary particle representation of the collisionless dark matter and is able to model large density contrasts and irregular geometries easily and reliably. These methods have been used to model purely collisionless cosmic fluids with up to 10$^{10}$ particles and to model both baryonic and dark matter universes with approximately 10$^{8.5}$ particles. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V8.00004: Fast Parallel Particle Methods: Angstroms to Gigaparsecs Michael Warren Fast multipole methods have become an ubiquitous tool for the simulation of physical systems with long-range interactions. Since their introduction they have been applied to a vast range of problems. Our own parallel hashed oct-tree code (HOT) has been applied to a number of physical systems with long-range interactions, including gravitational and smoothed particle hydrodynamic interactions in astrophysical systems, fluid flows with vortex-particle methods, electromagnetic scattering and aerodynamics. Several these simulations were recognized with Gordon Bell prizes for significant achievement in parallel processing. We will discuss some recent work which used a series of 1-billion particle dark matter simulations to accurately determine the mass function of galaxy halos. These simulations required over $4\times10^{18}$ floating point operations (4 exaflops). Another focus of our current research is extending the HOT framework to biological systems, with the goal of simulating systems using over ten times as many atoms as the current state-of-the-art. This requires addressing several issues with current multipole algorithms, such as spatially-correlated errors and the ability to handle disparate time scales efficiently. [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V8.00005: Efficient particle simulations based on combining the Vortex-In-Cell and the Parallel Fast Multipole methods Gregoire Winckelmans, Roger Cocle, Goeric Daeninck, Francois Thirifay Particle methods are quality methods for simulating unsteady, convection dominated, flows, as they have negligible dissipation and dispersion. The vortex particle method is used for incompressible flows; also for buoyancy-driven flows by adding the temperature. The method can also be used for combustion, by using variable volume particles with vorticity, velocity divergence, temperature and species mass fractions. Quality particle methods also require interpolation/redistribution schemes. We here consider the Vortex-In-Cell (VIC) approach, where all operations, except convection, are done using a grid: Poisson solver, stretching, diffusion, etc. The vorticity field is also maintained divergence free by projection when required (which also requires solving a Poisson equation). In our implementation, we use the Fast Multipole method to obtain the boundary condition for solving the Poisson equation: this allows for a grid that tightly contains the particles. The method is also parallelized: the Parallel Fast Multipole (PFM) code provides the proper boundary condition on each subdomain, without iteration. Illustrative results will be presented in DNS and LES (also using multiscale models): vortex rings, wake vortices (also with ground effects), combustion. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V8.00006: A Particle/Panel Method for Vortex Sheet Roll-Up Robert Krasny Vortex sheets are commonly used in fluid dynamics to model thin shear layers in slightly viscous flow. Some of the first Lagrangian particle simulations in fluid dynamics used the point vortex approximation to study vortex sheet roll-up. We will review the early fundamental contributions on this topic by Rosenhead, Birkhoff, and Moore, and then discuss more recent developments. In particular, a method is described for computing vortex sheet roll-up in 3D flow in which the sheet surface is represented by a set of quadrilateral panels with Lagrangian particles at the vertices. The particles are advected by a regularized Biot-Savart integral and the induced velocity is evaluated by a particle-cluster treecode. The panels are adaptively subdivided to maintain resolution as the sheet deforms. The method is applied to simulate the collision of two vortex rings. The results shed light on the dynamics of vortex filaments in fully 3D flow. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V8.00007: Particle dynamics-based hybrid simulation of vibrated gas-fluidized beds of cohesive fine powders Sung Joon Moon, Yannis Kevrekidis, Sankaran Sundaresan We use three-dimensional molecular dynamics simulations of macroscopic particles, coupled with volume-averaged gas phase hydrodynamics, to study vertically vibrated gas-fluidized beds of fine, cohesive powders. The interstitial gas flow is restricted to be effectively one-dimensional (1D) in the beds of narrow cross-sectional areas we consider. This model captures the spontaneous development of 1D traveling voidage waves, which corresponds to bubble formation in real fluidized beds. We use this model to probe the manner in which vibration and gas flow combine to influence the dynamics of cohesive particles. We find that as the gas flow rate increases, cyclic pressure pulsation produced by vibration becomes more and more significant than direct impact, and in a fully fluidized bed this pulsation is virtually the only relevant mechanism. We demonstrate that vibration assists fluidization by creating large tensile stresses during transient periods, which helps break up the cohesive assembly into agglomerates. We also study spontaneous demixing in beds of a mixture of particles of different densities, so-called the ``phase separation,'' using an equation-free multiscale approach. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V8.00008: Dissipative Particle Dynamics Simulations of Two-Phase Flows Anupam Tiwari, John Abraham Dissipative particle dynamics (DPD) is a coarse-grained particle method that includes thermal fluctuations. A mean-field theory based model is developed for two-phase flows. Surface tension arises in the model due to terms that account for long-range attractive forces. The model is validated through static simulations carried out to reproduce the Laplace law relationship, and dynamic simulations of liquid cylinder and drop oscillations. It is shown that in both cases analytical and computed results agree within 8{\%}. We will also present results from simulations of capillary waves and Rayleigh-Taylor instability. In the case of capillary waves, comparisons will be shown with analytical results, and in the case of Rayleigh-Taylor instability, comparisons will be shown with analytical and other computed results. As an application of the model, results from simulations of thermally induced jet breakup will also be presented. [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V8.00009: Polymer chain simulations in microchannels with Dissipative Particle Dynamics Vasileios Symeonidis, George Karniadakis, Bruce Caswell In this work we employ Dissipative Particle Dynamics (\textsc{dpd}) for simulations of dilute polymer solutions using bead-spring representations. We present comparison of two time-marching schemes: the popular velocity-Verlet and Lowe's scheme. Schmidt number effects are investigated for a series of cases, including $\lambda$-\textsc{dna} molecules under shear (using the Marko-Siggia wormlike chain spring law) and Poiseuille flow in microchannels. Effects on the polymer depletion layer, power-law profiles and apparent viscosities are presented as a function of the number of beads per polymer chain. [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V8.00010: High order viscous vortex methods with deforming elliptical Gaussians Louis Rossi, Rodrigo Platte Vortex methods are numerical schemes for approximating solutions to the Navier-Stokes equations using a linear combination of moving basis functions to approximate the vorticity field of a fluid. Typically, the basis function velocity is determined through a Biot-Savart integral applied at the basis function centroid. Since vortex methods are naturally adaptive, they are advantageous in flows dominated by localized regions of vorticity such as jets, wakes and boundary layers. A semi-discrete convergence formulation leads to a new viscous vortex method based on deforming elliptical Gaussian basis functions that achieves fourth order spatial convergence. One odd thing about the new method is that basis functions do not move with the physical flow velocity at the basis function centroid as is usually specified in vortex methods. Rather, high order accuracy is obtained when one adds a consistently small flow field curvature correction. We will present two distinct approaches to the evaluation of the Biot-Savart integral for elliptical Gaussian basis functions. Non-trivial flow field calculations will demonstrate the efficacy of the method for both convection-diffusion problems and Navier-Stokes flows in 2D. [Preview Abstract] |
Thursday, March 16, 2006 2:03PM - 2:15PM |
V8.00011: Accelerating Atomistic Molecular Dynamics Simulation in Entropic Systems Xin Zhou The time scale of the traditional atomistic molecular dynamics simulations is too short to study wide slow dynamics of complex systems. Hyperdynamics method developed by A. F. Voter in studying of solids can not use directly in entropic systems such as fluids, biopolymers etc. By applying suitable order parameters with the symmetry of the studied systems and the characteristics of short trajectory, we build the condition of extending the hyperdynamics into fluids and algorithms. We test our results in a few modeling systems and expect the methods is used generally in simulating atomistically slow dynamics of complex fluids and biopolymers. [Preview Abstract] |
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