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Object-Oriented CFDSolver Design

Hrvoje Jasak

[email protected]

Wikki Ltd. United Kingdom

10/Mar2005

Object-Oriented CFD Solver Design – p.1/29

Outline

Objective

• 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


Background

State 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


Software Design

“Physics-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


Software Design

“Application-based” approach

• A collection of components involving variousphysics phenomena (beyond CFD)

• Complex model-to-model interaction

• Complex simulation and optimisationobjectives, unexpected conclusions

• Made-to-measure modelling

Requires more flexible software design:

can object orientation help?


FOAM: CCM in C++

FOAM: Field Operation and Manipulation

• Natural language of continuum mechanics:partial differential equations

∂k

∂t+ ∇•(uk) −∇•[(ν + νt)∇k] =

νt

[

1

2(∇u + ∇u

T )

]2

−εo

ko

k


FOAM: CCM in C++

Objective: Represent equations in software inthe 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)

);


FOAM: CCM in C++

Object Software representation C++ Class

Space and time Mesh + time (database) polyMesh, time

Tensor (List of) numbers + algebra vector, tensor

Field List of values Field

Boundary condition Values + condition patchField

Geometric field Field + boundary conditions geometricField

Field algebra + − ∗ / tr(), sin(), exp() . . . field operators

Interpolation Differencing schemes interpolation

Differentiation ddt, div, grad, curl fvc, fec

Matrix Matrix coefficients lduMatrix

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

Model library Library turbulenceModel

Application main() –


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{};


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


Geometry Handling

Complex geometry requirements

• Complex geometry is a rule, not exception

• Polyhedral cell support◦ Cell described as a polyhedron bounded

by polygons◦ Consistent handling of all cell types◦ More freedom in mesh generation

• Recent developments: polyhedral FVMprovides equivalent accuracy at lower cost


Geometry Handling


Geometry Handling

Time-varying geometry cases

• Automatic mesh motion

• Topological mesh changes with poly support


Speed of Execution

Handling large-scale computations

• Efficient numerics◦ Best discretisation practice for a given

problem◦ Iterative solvers almost inevitable◦ Careful analysis of non-linearity and

inter-equation coupling

• Massive parallelism: domain decomposition


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!


Examples of Application

Objective: illustrate examples of FOAM library inuse

• 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


Diesel Combustion

Diesel Combustion in Scania D-12 Engine

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

• RANS, k − ε turbulence model, simplified5-species chemistry and 1 reaction,Chalmers PaSR combustion model

• Temperature on a cutting planes

• Spray droplets coloured with temperature


Diesel Combustion

Diesel Combustion in Scania D-12 Engine


Capillary Jet

Ink-jet printer nozzle, 20µm diameter

• Pulsating flow, umean = 20m/s

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


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 surface

correction◦ Fully developed pipe flow inlet


Diesel Injector

• Mesh size: 1.2 to 8 million CVs, aggressivelocal refinement, 50k time-steps

• 6µs initiation time, 20µs averaging time


Surface tracking

rF

vF

vb = −vF

y

x

y′

x′

aF

o′SA

SB

o

Free

surface

Free surface tracking

• 2 phases = 2 meshes

• Mesh adjusted forinterface motion

• Coupled b.c.

Air-water system

• 2-D: rb = 0.75 mm

• 3-D: rb = 1 mm


Surfactant Effect

Clean surface

Pollution by surfactant chemicals


3-D Rising Bubble

Complex coupling problem: FVM flow solver +

FEM mesh motion + FAM surfactantsObject-Oriented CFD Solver Design – p.24/29

Fluid-Solid Coupling

Pipeline failure: crack propagation and leakage


Fluid-Solid Coupling

Enlarged deformation of the pipe


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


Summary

• Object-oriented approach facilitates modelimplementation: 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 Tropša, prof. A Ivankovic, UC Dublin

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


FOAM: CCM in C++

Main characteristics

• Wide area of applications: all of CCM!

• Shared tools and code re-use

Versatility

• Unstructured meshes, automatic meshmotion + topological changes

• Finite Volume, Finite Element, Lagrangiantracking and Finite Area methods

• Efficiency through massive parallelism


Documents

Object-Oriented CFD Solver Design - POWERLAB - OnLinepowerlab.fsb.hr/ped/kturbo/OpenFOAM/slides/BAESystems_10Mar200… · Object-Oriented CFD Solver Design – p.8/29. ... Free surface