Direct Simulation Monte Carlo: A Particle Method for Nonequilibrium Gas Flows

Download Direct Simulation Monte Carlo: A Particle Method for Nonequilibrium Gas Flows

Post on 18-Jan-2016

26 views

Category:

Documents

0 download

Embed Size (px)

DESCRIPTION

Direct Simulation Monte Carlo: A Particle Method for Nonequilibrium Gas Flows. Iain D. Boyd Department of Aerospace Engineering University of Michigan Ann Arbor, MI 48109 Support Provided By: MSI, AFOSR, DARPA, NASA. Physical characteristics of nonequilibrium gas flow. - PowerPoint PPT Presentation

TRANSCRIPT

<ul><li><p>Direct Simulation Monte Carlo:A Particle Method for Nonequilibrium Gas Flows Iain D. BoydDepartment of Aerospace EngineeringUniversity of MichiganAnn Arbor, MI 48109</p><p>Support Provided By:MSI, AFOSR, DARPA, NASA</p></li><li><p> Physical characteristics of nonequilibrium gas flow.</p><p> Direct simulation Monte Carlo (DSMC) method.</p><p> The MONACO DSMC code:data structure;scalar/parallel optimization.</p><p> Illustrative DSMC applications:hypersonic aerothermodynamics;materials processing;spacecraft propulsion.</p><p> Summary and future directions.</p><p>Overview</p></li><li><p> Physical characteristics of nonequilibrium gas systems:low density and/or small length scales;high altitude hypersonics (n=1020 m-3, L=1 m);space propulsion (n=1018 m-3, L=1 cm);micro-fluidics (n=1025 m-3, L=1 m).</p><p> Gas dynamics:rarefied flow (high Knudsen number);collisions still important;continuum equations physically inaccurate.</p><p>Modeling Considerations</p></li><li><p> Characterization ofNonequilibrium Gas FlowsKn0.010.110continuumsliptransitionalfree-molecularEulerNavier-StokesBoltzmann EquationControl equations:Flow Regimes:Collisionless Boltzmann EqnBurnettDSMC</p></li><li><p>Direct Simulation Monte Carlo Particle method for nonequilibrium gas flows:developed by Bird (1960s);particles move/collide in physical space;particles possess microscopic properties, e.g. u (thermal velocity);cell size Dx ~ l, time step Dt ~ t=1/n;collisions handled statistically (not MD);ideal for supersonic/hypersonic flows;may be combined with other methods (CFD, PIC, MD) for complex systems.{u, v, wx, y, zm, erot, evib</p></li><li><p>Direct Simulation Monte Carlo</p></li><li><p>The DSMC Algorithm MOVE:translate particles Dx = u Dt;apply boundary conditions (walls, sources, sinks).</p><p> SORT:generate list of particles in each cell.</p><p> COLLIDE:statistically determine particles that collide in each cell;apply collision dynamics.</p><p> SAMPLE:update sums of various particle properties in each cell.</p></li><li><p> Hypersonics:vehicle aerodynamics (NASA-URETI);hybrid particle-continuum method (AFOSR);TOMEX flight experiment (Aerospace Corp).</p><p> Space propulsion:NEXT ion thruster, FEEP (NASA);Hall thrusters (DOE, NASA);micro-ablation thrusters (AFOSR);two-phase plume flows (AFRL).</p><p> Micro-scale flows:low-speed rarefied flow (DOE).Current DSMC-Related Projects</p></li><li><p>The DSMC Code MONACO MONACO: a general purpose 2D/3D DSMC code.</p><p> Physical models:Variable Soft Sphere (Koura &amp; Matsumoto, 1992);rotational relaxation (Boyd, 1990);vibrational relaxation (Vijayakumar et al., 1999);chemistry (dissociation, recombination, exchange).</p><p> Applications:hypersonic vehicle aerodynamics;spacecraft propulsion systems;micro-scale gas flows, space physics;materials processing (deposition, etching).</p></li><li><p>MONACO: Data Structure Novel DSMC data structure:basic unit of the algorithm is the cell;all data associated with a cell are stored in cache;particles sorted automatically.</p></li><li><p>MONACO: Scalar Optimization Inexpensive cache memory system used on workstations:data localization leads to performance enhancement.</p><p> Optimization for specific processor:e.g. overlap *add*, *multiply* and *logical* instructions.</p></li><li><p>MONACO: Parallel Implementation Grid geometry reflected in the code data structure:domain decomposition employed.</p><p> When a particle crosses a cell edge:particle pointed to new cell;thus, particles sorted-by-cell automatically.</p><p> When a particle crosses a domain edge:communication link employed;linked lists of particles sent as matrix;inter-processor communication minimized;no explicit synchronization required.</p></li><li><p>MONACO: Parallel Implementation</p></li><li><p>MONACO: The Software System Consists of four modular libraries:KERN, GEOM, PHYS, UTIL.</p></li><li><p>MONACO: Code Performance MONACO performance on IBM SP (Cornell, 1996):largest DSMC computation at the time;best performance with many particles/processor;parallel performance ~ 90%;serial performance 30-40%.</p></li><li><p>MONACO: Unstructured GridsHypersonic flow arounda planetary probe3D Surface geometry ofTOMEX flight experiment</p></li><li><p>DSMC Applications:1. Hypersonic Aerothermodynamics</p><p> Hypersonic vehicles encounter a variety of flow regimes: flights/experiments are difficult and expensive; continuum: modeled accurately and efficiently using CFD; rarefied: modeled accurately and efficiently using DSMC.DSMC:particle approachhigh altitudesharp edgesuses kinetic theoryCFD:continuum approachlow altitudelong length scalessolves NS equationsNASAs Hyper-X</p></li><li><p> Flow separation configuration:N2 at M~10 over double cone;data from LENS (Holden).</p><p>Hypersonic Viscous Interaction</p></li><li><p> Cowl lip configuration:N2 at M~14;data from LENS (Holden).</p><p>Shock-Shock Interactions</p></li><li><p> TRIO flight experiment:analysis of pressure gauges;external/internal flows.</p><p>Complex 3D Flows</p></li><li><p>Computations of hypersonic flow around several power-law leading edge configurations performed using MONACO at high altitude.</p><p>Advanced physical modeling:vibrational relaxation and air chemistry;incomplete wall accommodation.</p><p>Effects of sharpening the leading edge:reductions in overall drag coefficient and shock standoff distance;increases in peak heat transfer coefficient.AerothermodynamicsOf Sharp Leading Edges</p></li><li><p> Flow Fields Temperature Ratio (T / T) Cylinder at 7.5 km/s n=0.7 at 7.5 km/s </p></li><li><p> Drag Coefficient Shock Standoff Distance/Heat Transfer Coefficient Aerothermodynamic Assessment</p></li><li><p>DSMC Applications:2. Materials Processing Effect of atomic collisions: between the same species; between different species.Top viewSide view3M experimental chamber for YBCO deposition</p></li><li><p>3D MONACO Modeling 20x60x50 cuboid cells. Non-uniform cell sizes. 2,000,000 particles. Overnight solution time</p></li><li><p>Yttrium Evaporation Source flux: 9.95x10-5 moles/secNumber densityZ-component of velocity</p></li><li><p>Comparison of calculated and measured film deposition thickness.Significant effect of atomic collisions.</p><p>Yttrium Evaporation</p></li><li><p>Calculated and measured atomic absorption spectra: along an aperture close to the substrate symmetry line; at frequencies of 668 nm (left) and 679 nm (right).Yttrium Evaporation</p></li><li><p>Co-evaporation of Yt, Ba, and CuSource fluxes (10-5 moles/cm2/sec) Y : Ba :Cu = 0.84 : 1.68 : 2.52Total Number Density</p></li><li><p>BaCuYtFlux (moles/cm2/s) across the substrateCo-evaporation of Yt, Ba, and Cu</p></li><li><p> Tasks for spacecraft propulsion systems:orbit transfer (e.g. planetary exploration);orbit maintenance (e.g. station-keeping);attitude control.</p><p> Motivations for development of accurate models:simulations less expensive than testing;improve our understanding of existing systems;optimize engine performance and lifetime;assessment of spacecraft integration concerns.</p><p>DSMC Applications:3. Spacecraft Propulsion</p></li><li><p>Spacecraft PropulsionGriddedion thruster(UK-10)Arcjet (Stanford)Hall:stationaryplasma thruster(SPT-100)PulsedPlasmaThruster(EOS-1)</p></li><li><p> Two Russian GEO spacecraft launched in 2000:SPT-100 Hall thrusters used for station-keeping;in-flight characterization program managed by NASA;first in-flight plume data for Hall thrusters.</p><p>Express Spacecraft Diagnostics employed on spacecraft:electric field sensors;Faraday probes (ion current density);retarding potential analyzers, RPAs (ion current density, ion energy distribution function);pressure sensors;disturbance torques (from telemetry data).</p></li><li><p>Express Spacecraft</p></li><li><p>Particle In Cell (PIC){u, v, wx, y, zm, qE3E2E1E4 Particle method for nonequilibrium plasma:developed since the 1960s;charged particles move in physical space;particles possess microscopic properties, e.g. u (thermal velocity);cell size Dx ~ d, time step Dt ~ 1/w;self-consistent electric fields, E;may be combined with DSMC for collisional plasmas.</p></li><li><p>Hybrid DSMC-PIC Particle model for ions, fluid model for electrons.</p><p> Boltzmann relation for electrons provides potential:currentless, isothermal, un-magnetized, collisionless;quasi-neutrality provides potential from ion density: Collision mechanisms:charge exchange;momentum exchange.</p></li><li><p>Number Densities (m-3)Xe+ ionXe atom</p></li><li><p>Ion Current Density</p></li><li><p>Ion Energy DistributionsBeam plasma (15 deg.)CEX plasma (77 deg.)</p></li><li><p> Direct simulation Monte Carlo:now a mature, well-established technique;statistical simulation of particle dynamics;applied in many areas of engineering/physics;use growing due to improved computer power.</p><p> Some advantages of DSMC:accurate simulation of nonequilibrium gas;framework for detailed physical modeling;can handle geometric complexity;can be combined with other methods for multi-scale and multi-process systems.Summary</p></li><li><p> Development of MONACO:unsteady and 3D flows;user help: DSMC for dummies;dynamic domain decomposition;more detailed physical models.</p><p> Extensions of DSMC:hybrid DSMC-CFD (using IP interface);generalized hybrid DSMC-PIC;2-phase DSMC (gas and solid particles);speedup: implicit DSMC, variance reduction.Future Directions</p></li></ul>

Recommended

View more >