Object-Oriented CFD Solver Design

Transcription

1 Object-Oriented CFD Solver Design Hrvoje Jasak Wikki Ltd. United Kingdom 10/Mar2005 Object-Oriented CFD Solver Design p.1/29

2 Outline Objective Present new approach to software design in Computational Continuum Mechanics Topics A new approach to model representation Object-orientation in numerical simulation software: code re-use and layered design Examples of complex model implementation Object-Oriented CFD Solver Design p.2/29

3 Background State of the Art Numerical modelling is becoming a part of product design Improved computing performance Improved modelling Sufficient validation and experience Two-fold requirements Ease of use and process integration Quick and reliable model implementation Object-Oriented CFD Solver Design p.3/29

4 Software Design Physics-based approach Traditional view of the problem Well defined physics, studied in isolation Clearly defined objective Numerical software sometimes limiting design: e.g. Lockheed Martin F-117 Stealth Typically handled by monolithic software with well-defined capabilities Object-Oriented CFD Solver Design p.4/29

5 Software Design Application-based approach A collection of components involving various physics 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? Object-Oriented CFD Solver Design p.5/29

6 FOAM: CCM in C++ FOAM: Field Operation and Manipulation Natural language of continuum mechanics: partial differential equations k t + (uk) [(ν + ν t) k] = [ ] 2 1 ν t 2 ( u + ut ) ɛ o k k o Object-Oriented CFD Solver Design p.6/29

7 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) ); Object-Oriented CFD Solver Design p.7/29

8 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() Object-Oriented CFD Solver Design p.8/29

9 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{}; Object-Oriented CFD Solver Design p.9/29

10 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 Object-Oriented CFD Solver Design p.10/29

11 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 FVM provides equivalent accuracy at lower cost Object-Oriented CFD Solver Design p.11/29

12 Geometry Handling Object-Oriented CFD Solver Design p.12/29

13 Geometry Handling Time-varying geometry cases Automatic mesh motion Topological mesh changes with poly support Object-Oriented CFD Solver Design p.13/29

14 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 Object-Oriented CFD Solver Design p.14/29

15 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! Object-Oriented CFD Solver Design p.15/29

16 Examples of Application Objective: 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 Object-Oriented CFD Solver Design p.16/29

17 Diesel Combustion Diesel Combustion in Scania D-12 Engine 1/8 sector with 75 % load and n-heptane fuel RANS, k ɛ turbulence model, simplified 5-species chemistry and 1 reaction, Chalmers PaSR combustion model Temperature on a cutting planes Spray droplets coloured with temperature Object-Oriented CFD Solver Design p.17/29

18 Diesel Combustion Diesel Combustion in Scania D-12 Engine Object-Oriented CFD Solver Design p.18/29

19 Capillary Jet Ink-jet printer nozzle, 20µm diameter Pulsating flow, u mean = 20m/s Tuning frequency (50kHz) and amplitude (5%) Object-Oriented CFD Solver Design p.19/29

20 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 Object-Oriented CFD Solver Design p.20/29

21 Diesel Injector Mesh size: 1.2 to 8 million CVs, aggressive local refinement, 50k time-steps 6µs initiation time, 20µs averaging time Object-Oriented CFD Solver Design p.21/29

22 Surface tracking v b = v F y o v F S B S A r F x y o a F x Free surface Free surface tracking 2 phases = 2 meshes Mesh adjusted for interface motion Coupled b.c. Air-water system 2-D: r b = 0.75 mm 3-D: r b = 1 mm Object-Oriented CFD Solver Design p.22/29

23 Surfactant Effect Clean surface Pollution by surfactant chemicals Object-Oriented CFD Solver Design p.23/29

24 3-D Rising Bubble Complex coupling problem: FVM flow solver + FEM mesh motion + FAM surfactants Object-Oriented CFD Solver Design p.24/29

25 Fluid-Solid Coupling Pipeline failure: crack propagation and leakage Object-Oriented CFD Solver Design p.25/29

26 Fluid-Solid Coupling Enlarged deformation of the pipe Object-Oriented CFD Solver Design p.26/29

27 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 research: excellent numerics and pre-implemented models Object-Oriented CFD Solver Design p.27/29

28 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 Tropša, prof. A Ivanković, UC Dublin Foam and OpenFOAM are released under GPL: Object-Oriented CFD Solver Design p.28/29

29 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, Lagrangian tracking and Finite Area methods Efficiency through massive parallelism Object-Oriented CFD Solver Design p.29/29