OpenFOAM: Introduction, Capabilities and HPC Needs - LinkSCEEM

Implementing Continuum ModelsEquation Mimicking• Natural language of continuum mechanics: partial differential equations• Example: turbulence kinetic energy equation[ ]∂k1 2∂t +∇•(uk)−∇•[(ν +ν t)∇k] = ν t2 (∇u+∇uT ) − ǫ okk o• Objective: represent differential equations in their natural languagesolve(fvm::ddt(k)+ fvm::div(phi, k)- fvm::laplacian(nu() + nut, k)== nut*magSqr(symm(fvc::grad(U)))- fvm::Sp(epsilon/k, k));• Correspondence between the implementation and the original equation is clear• Library-based implementation of custom physical models with full freedom insolution algorithms. New: support for multi-equation implicit couplingCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 4

OpenFOAM: Executive OverviewPhysical Modelling Capability Highlights• Basic: Laplace, potential flow, passive scalar/vector/tensor transport• Incompressible and compressible flow: segregated pressure-based algorithms• Heat transfer: buoyancy-driven flows, conjugate heat transfer• Multiphase: Euler-Euler, VOF free surface capturing and surface tracking• RANS for turbulent flows: 2-equation, RSTM; full LES capability• Pre-mixed and Diesel combustion, spray and in-cylinder flows• Stress analysis, fluid-structure interaction, electromagnetics, MHD, etc.Cyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 5

Examples of SimulationExample of Capabilities of OpenFOAM Relevant to Your CFD Needs• This is only a part of the OpenFOAM capabilities!• Chosen for relevance and illustration of the range of capabilities rather thanexhaustive illustration of range of capabilities• In some cases, simplified geometry is used due to confidentiality• Regularly, the work resulted in a new solver; in many cases, it is developed as anextension or combination of existing capabilitiesDescription of Simulation and Setup• Physics and numerical method setup• Standard or customised solver; details of mesh resolution and customisationCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 9

Complex Mesh MotionDynamic Mesh Examples of Complex Combination of Motion and SlidingCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 15

Geometric Shape OptimisationGeometric Shape Optimisation with Parametrised Geometry• Specify a desired object of optimisation and use the parametrisation of geometryto explore the allowed solution space in order to find the minimum of theoptimisation objectiveobjective = f(shape)1. Parametrisation of Geometry• Computational geometry is complex and usually available as thecomputational mesh: a large amount of data• Parametrisation tool: RBF mesh morphing, defining deformation at a smallnumber of mesh-independent points in space2. CFD Flow Solver is used to provide the flow solution on the current geometry, inpreparation for objective evaluation3. Evaluation of Objective: usually a derived property of the flow solution4. Optimiser Algorithm: explores the solution space by providing sets of shapecoordinates and receiving the value of objective. The search algorithm iterativelylimits the space of solutions in search of a minimum value of objectiveCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 16

Darwin’s Inspirations: Geology –current features are result ofbillions of years of gradual change

OpenFOAM Turbo ToolsGeneral Grid Interface• Turbomachinery CFD requires additional features: implemented in library form• General Grid Interface (GGI) and its derived forms◦ Cyclic GGI◦ Partial overlap GGI◦ Mixing plane interface (under testing)• Implementation and parallelisation is complete: currently running validation casesin collaboration with commercial clients and Turbomachinery Working Group• Other turbo-related components in pipeline: harmonic balance solver solver• Library-level implementation allows re-use of GGI beyond turbomachineryCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 18

Lagrangian Phase ModelLagrangian Particle Modelling• Particles are modelled in Lagrangian framework: tracked through fluid domain• Equation of motion for a Lagrangian particle:m pdv pdt= 1 2 ρ|v −v P|(v −v P ) d2 pπ4 C D +m p g ρ l −ρρ l+F ∇pwhere◦ m p is the particle mass◦ v p is the particle velocity◦ v is fluid velocity◦ d p is particle diameter◦ C D is the drag coefficient◦ g is the gravitational acceleration◦ ρ l is the particle (liquid) density◦ ρ is the fluid density◦ F ∇p is the force on the particle due to the fluid pressure gradientCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 21

Lagrangian Phase ModelIntegration of Motion Equation• Motion equation for a particle is an Ordinary Differential Equation (ODE)• During integration, particle traverses a number of cells and continuous phaseproperties are updated based on its position• For accurate integration, motion equation is formulated as an ODE and solverusing an ODE solverParticle Tracking• Cell-to-face-to-cell tracking: no inverted Jacobians, no lost particles• If more accurate integration is necessary, sub-cycling is usedIntegration stencilUCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 22

Lagrangian Phase ModelSolid Particles with Wall Interaction• Using particle tracking code with physics equations attached to a particles• Particle-to-particle interaction: proximity model with correction• Wall interaction model: bounce with energy lossDiesel Spray Model• Consists of a parcel, injector, spray and a set of sub-models• parcel class: a particle represents a population of droplets: mean diameter andstatistical distribution of particle size and other properties• Cloud of parcels = Diesel spray: spray• Example: Junwei Su, Imperial College and Xi’an Jiaotong University, ChinaCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 23

Finite Area MethodFinite Area Discretisation• Finite Area Method discretised equations on a curved surface in 3-D• Surface is discretised using polygonal faces. Discretisation takes into accountsurface curvature. A level of smoothness is assumed in calculation of curvatureterms• Surface motion is allowed: decomposed into normal and tangential motion• Nomenclature for a surface element P and its neighbour Nn fjn PeL enNee’mPid eNS POpenFOAM: Introduction, Capabilities and HPC Needs – p. 24Cyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012

Thin Liquid Film ModelModelling Assumptions: 2-D Approximation of a Free Surface Flow• Isothermal incompressible laminar flow• Boundary layer approximation:◦ Tangential derivatives are negligible compared to normal◦ Normal velocity component is negligible compared to tangential◦ Pressure is constant across the film depth• Similitude of the flow variables in the direction normal to the substrate◦ Prescribed cubic velocity profilehS fsp gv gm d,iDependent variables: h and ¯vS w¯vσvnv fsv d,i¯v = 1 hh∫0vdhv(η) = v fs •diag(aη +bη 2 +cη 3 )η = n h , 0 ≤ n ≤ hCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 25

Thin Liquid Film ModelValidation: Droplet Spreading Under Surface Tension• Liquid film equations governing the flow; self-similar velocity profile• Droplet spread driven by gravity and counteracted by surface tension• Equation set possesses an (axi-symmetric) analytical solution: validation ofimplementation and numerics4e-053.5e-053e-05t = 0.000 st = 0.015 sAnalytical solution2.5e-05h, m2e-051.5e-051e-055e-0600 0.0005 0.001 0.0015 0.002r, mImplementation of Liquid Film Model Volume-Surface-Lagrangian Coupling• Full second order in space and time on curved moving surfaces• Parallelisation bound to volumetric FVM• Shared matrix and discretisation: possibility of close coupling with FVMCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 26

Coupling Wall Film with SprayVolume-Surface-Lagrangian Coupling• Coupling a volumetric flow model with Lagrangian particles for a dispersed phaseto a thin liquid film model on the solid wall Custom solver• In terms of numerics, coupling of volumetric, surface and Lagrangian models iseasy to handle: different modelling paradigms• Main coupling challenge is to implement all components side-by-side and controltheir interaction: volumetric-to-particle-to-surface data exchange• Close coupling is achieved by sub-cycling or iterations over the system for eachtime-stepCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 27

Close Coupling: Matrix LevelAssembling a Matrix for Conjugate Heat Transfer Problems• OpenFOAM supports multi-region simulations, with possibility of separateaddressing and physics for each mesh: multiple meshes, with local fields• Some equations present only locally, while others span multiple meshescoupledFvScalarMatrix TEqns(2);TEqns.hook(fvm::ddt(T) + fvm::div(phi, T)- fvm::laplacian(DT, T));TEqns.hook(fvm::ddt(Tsolid) - fvm::laplacian(DTsolid, Tsolid));TEqns.solve();• Coupled solver handles multiple matrices together in internal solver sweeps:arbitrary matrix-to-matrix and domain-to-domain couplingCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 28

Linear Algebra and Solver SupportExample: Conjugate Heat Transfer• Coupling may be established geometrically: adjacent surface pairs• Each variable is stored only on a mesh where it is active: (U, p, T)• Choice of conjugate variables is completely arbitrary: e.g. catalytic reactions• Coupling is established only per-variable: handling a general coupled complexphysics problem rather than conjugate heat transfer problem specifically• Allows additional models to be solved on each region without overhead: structuralstress analysis, turbulence or LESCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 29

Fluid-Structure InteractionFluid-Structure Coupling Capabilities in OpenFOAM• As a Continuum Mechanics solver, OpenFOAM can deal with both fluid andstructure components: easier setup of coupling• (Parallelised) surface coupling tools implemented in library form: facilitate couplingto external solvers without “coupling libraries” using proxy surface mesh• Structural mechanics in OpenFOAM targeted to non-linear phenomena: considerbest combination of tools◦ Large deformation formulation in absolute Lagrangian formulation◦ Independent parallelisation in the fluid and solid domain◦ Parallelised data transfer in FSI coupling• Dynamic mesh tools and boundary handling used to manipulate the fluid meshCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 30

Deployment and SupportDeployment and Support• OpenFOAM developed mainly on Linux/Unix: Windows and Mac OS X ports alsoexist but are not widely used. This needs to be improved: binary distribution forMac OS X and native Microsoft Windows port• Data input/output is in file form: there is no global graphical user interface that canbe tailored to sufficient level of usability (work in progress)• No built-in 3-D graphics; 2-D, graphing and sampling tools present• External post-processing with integrated readers: paraFoam (open source)Cyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 31

SummaryProject Status Summary• OpenFOAM is a free software, available to all at no charge: GNU Public License• Object-oriented approach facilitates model implementation• Equation mimicking opens new grounds in Computational Continuum Mechanics• Extensive capabilities already implemented; open design for easy customisation• Solvers are validated in detail and match the efficiency of commercial codes• Open Source model dramatically drops the cost of industrial CFDOpenFOAM in Research and Industry• Technical development driven by Special Interest Groups & Birds-of-a-FeatherTurbomachinery Ship Hydrodynamics Simulation of EnginesTurbulence Fluid-Structure Interaction Multiphase FlowsAeroacoustics Combustion and explosions Solid MechanicsDocumentation High Performance Computing OpenFOAM in Teaching• Sixth OpenFOAM Workshop: Penn State University 13-16 June 2011http://www.openfoamworkshop.orgCyprus Advanced HPC Workshop Winter 2012. Tracking: FH6190763 Feb/2012OpenFOAM: Introduction, Capabilities and HPC Needs – p. 32