BAESystems_10Mar2005

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Object-Oriented CFDSolver Design

Hrvoje Jasak

[email protected]

Wikki Ltd. United Kingdom

10/Mar2005

Object-Oriented CFD Solver Design p.1/2


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OutlineObjective

Present new approach to software design inComputational Continuum Mechanics

Topics A new approach to model representation

Object-orientation in numerical simulationsoftware: code re-use and layered design

Examples of complex model implementation



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BackgroundState of the Art

Numerical modelling is becoming a part ofproduct design

Improved computing performance Improved modelling

Sufficient validation and experience

Two-fold requirements

Ease of use and process integration

Quick and reliable model implementation



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Software DesignPhysics-based approach

Traditional view of the problem

Well defined physics, studied in isolation

Clearly defined objective

Numerical software sometimes limitingdesign: e.g. Lockheed Martin F-117 Stealth

Typically handled by monolithic software with

well-defined capabilities



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Software DesignApplication-based approach

A collection of components involving variousphysics phenomena (beyond CFD)

Complex model-to-model interaction Complex simulation and optimisation

objectives, unexpected conclusions

Made-to-measure modelling

Requires more flexible software design:

can object orientation help?



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FOAM: CCM in C++FOAM: Field Operation and Manipulation

Natural language of continuum mechanics:partial differential equations

kt

+ (uk) [(+ t)k] =

t1

2(u + uT

)2

oko k



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FOAM: CCM in C++Objective: Represent equations in software in

the natural language

solve

(

fvm::ddt(k)

+ fvm::div(phi, k)

- fvm::laplacian(nu() + nut, k)

== nut*magSqr(symm(fvc::grad(U)))

- fvm::Sp(epsilon/k, k)

);



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FOAM: CCM in C++Object Software representation C++ Class

Space and time Mesh + time (database) polyMesh, timeTensor (List of) numbers + algebra vector, tensor

Field List of values Field

Boundary condition Values + condition patchField

Geometric fi eld Field + boundary conditions geometricField

Field algebra + / tr(),sin(),exp() . . . fi eld operators

Interpolation Differencing schemes interpolation

Differentiation ddt, div, grad, curl fvc, fec

Matrix Matrix coeffi cients lduMatrix

Discretisation ddt, d2dt2, div, laplacian fvm, fem, fam

Model library Library turbulenceModel

Application main()



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FOAM: CCM in C++Common interface for related models

class turbulenceModel

{

virtual volTensorField R() const = 0;

virtual fvVectorMatrix divR

(

volVectorField& U

) const = 0;

virtual void correct() = 0;

};

class SpalartAllmaras : public turbulenceModel{};



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FOAM: CCM in C++Model-to-model interaction

fvVectorMatrix UEqn

(

fvm::ddt(rho, U)

+ fvm::div(phi, U)

+ turbulence->divR(U)

==

- fvc::grad(p)

);

New components do not disturb existing code



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Geometry HandlingComplex geometry requirements

Complex geometry is a rule, not exception

Polyhedral cell support

Cell described as a polyhedron boundedby polygons

Consistent handling of all cell types

More freedom in mesh generation

Recent developments: polyhedral FVM

provides equivalent accuracy at lower cost



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Geometry Handling



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Geometry HandlingTime-varying geometry cases

Automatic mesh motion

Topological mesh changes with poly support

http://./movies/mixer2D-anim.mpghttp://./movies/movingConeTopologicalSmall-anim.mpg


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Speed of ExecutionHandling large-scale computations

Efficient numerics

Best discretisation practice for a givenproblem

Iterative solvers almost inevitable

Careful analysis of non-linearity and

inter-equation coupling Massive parallelism: domain decomposition



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Layered Development

Design encourages code re-use: shared tools

Code developed and tested in isolation

Vectors, tensors and field algebra

Mesh handling, refinement, topo changes

Discretisation, boundary conditions

Matrices and solver technology

Physics by segment Custom applications

Ultimate user-coding capabilities!



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Examples of ApplicationObjective: illustrate examples of FOAM library in

use Diesel Combustion: Scania D-12 Engine

Free surface flow modelling Capillary jets, LES + free surface

Surface tracking: rising bubble

Solid-fluid interaction: plastic pipeline failure



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Diesel CombustionDiesel Combustion in Scania D-12 Engine

1/8 sector with 75 % load and n-heptane fuel

RANS, k turbulence model, simplified

5-species chemistry and 1 reaction,Chalmers PaSR combustion model

Temperature on a cutting planes

Spray droplets coloured with temperature



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Diesel CombustionDiesel Combustion in Scania D-12 Engine

http://./movies/scaniaEng.mpg


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Capillary Jet

Ink-jet printer nozzle, 20m diameter

Pulsating flow, umean = 20m/s

Tuning frequency (50kHz) and amplitude (5%)

http://./movies/highFreq.mpghttp://./movies/midFreq.mpghttp://./movies/lowFreq.mpg


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Diesel Injector

LES of a Diesel Injector

d = 0.2mm, high velocity and surface tension

Mean injection velocity: 460m/s

Diesel fuel injected into air, 5.2MPa, 900K Turbulent and subsonic flow, no cavitation

1-equation LES model with no free surfacecorrection

Fully developed pipe flow inlet



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Diesel Injector

Mesh size: 1.2 to 8 million CVs, aggressive

local refinement, 50k time-steps 6s initiation time, 20s averaging time

http://./movies/gammaIsoU.mpg


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Surface tracking

rF

vF

vb = vF

y

x

y

x

aF

oSASB

o

Free

surface

Free surface tracking

2 phases = 2 meshes

Mesh adjusted for

interface motion Coupled b.c.

Air-water system

2-D: rb = 0.75 mm

3-D: rb = 1 mm



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Surfactant Effect

Clean surface

Pollution by surfactant chemicals

http://./movies/2dBubble-surfactant-vectors.mpghttp://./movies/2dBubble-surfactant-vectors.mpghttp://./movies/2dBubble-surfactant-vectors.mpghttp://./movies/2dBubble-clean-mesh.mpghttp://./movies/2dBubble-clean-pressure.mpghttp://./movies/2dBubble-clean-vectors.mpg


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3-D Rising Bubble

Complex coupling problem: FVM flow solver +

FEM mesh motion + FAM surfactantsObject-Oriented CFD Solver Design p.24/2
http://./movies/3dBubble-surfactant-vectors-colored.mpg


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Fluid-Solid Coupling

Pipeline failure: crack propagation and leakage

http://./movies/pipe_solid_fluid640x384.mpg


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Fluid-Solid Coupling

Enlarged deformation of the pipe

http://./movies/pipe_solid_only640x384.mpg


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Other Capabilities

FOAM contains other capabilities as well

RANS and LES turbulence modelling

Thermophysical and transport model libraries

A-posteriori error estimation Adaptive mesh refinement

Software used for algorithm and numerics re-search: excellent numerics and pre-implemented

models



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Summary

Object-oriented approach facilitates model

implementation: layered design + re-use Equation mimicking opens new CCM grounds

Extensive capabilities already implemented Open design for easy user customisation

Acknowledgements

Scania engine: Dr. Niklas Nordin, Chalmers University Sweden

Spray breakup: Eugene de Villiers, Imperial College

Cracking pipe: Dr. Vlado Tropa, prof. A Ivankovic, UC Dublin

Foam and OpenFOAM are released under GPL: http://www.openfoam.org



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FOAM: CCM in C++

Main characteristics

Wide area of applications: all of CCM!

Shared tools and code re-use

Versatility Unstructured meshes, automatic mesh

motion + topological changes

Finite Volume, Finite Element, Lagrangiantracking and Finite Area methods

Efficiency through massive parallelismObject-Oriented CFD Solver Design p.29/2

Documents

BAESystems_10Mar2005