CFD: Running the Black Box

1. CFD: Running the Black Box Setting up and Running a CFD Simulation

2. CFD Workflow: Steps 1-2 Define Geometry • CAD • Flow Domain Mesh the Domain • Discretize space Images courtesy of ESI-Group.

3. CFD Workflow: Steps 3-5 Define Physics • Material Properties • Physical Phenomena • Flow Behavior Define Boundary Conditions • Inlets/Outlets/Walls • Porous media, fans Set Solver Parameters • Iterations • Relaxation • Convergence criteria

4. CFD Workflow: Steps 6-7 Solve • Convergence of Residuals Post-process • Analyze results Right image courtesy of Mentor Graphics.

5. Before you begin…

6. What is my goal for using CFD? • Much of the setup for a CFD problem depends on your goal • Achieve final validation of a prototype • Identify feasible designs for a new product • Understand complex physical phenomena • Optimize product geometry

7. Considerations in CFD:Model Quality vs. Simulation Time • Five components: • Geometry: 2d vs 3d, complex vs simple • Mesh: cell count, skewness, structured vs. unstructured • Physics: physical phenomena (chemical rxn’s, free-surface), turbulence model • Solver: numerical methodology, parameters • Hardware: memory, CPU, # of processors

8. What is my goal for using CFD? • Much of the setup for a CFD problem depends on your goal • Achieve final validation of a prototype • Highly accurate physics and geometry • Identify feasible designs for a new product • Simple physics and geometry • Capture complex physical phenomena • Highly accurate physics and simple geometry • Optimize product geometry • Simple physics and highly accurate geometry

9. Setting up a CFD Model

10. Geometry • Import CAD Model or Create Geometry • CAD Programs: Solidworks, Creo, Siemens NX, CATIA, Autodesk, etc. • CFD programs usually allow for some simplified geometry creation capability Left image courtesy of ESI-GROUP, right image courtesy of Exa Corporation

11. Geometry Considerations in CFD • 2D or 3D Model • Symmetry • Symmetric • Axisymmetric • None • Highly detailed or simplified • Identify flow domain (interior or exterior of the geometry) • All flow takes place IN a geometry, thus for exterior flows, a “bounding box” geometry must be created to encapsulate the area of interest

12. Geometry Considerations in CFD: 2D vs 3D Modelling • Using 2D models (as opposed to 3D) is one of the biggest savers of computational time! • Cell count • Degrees of Freedom (no z axis) • 2D axisymmetric model captures most of what a 3D model will • Even 2D non-axisymmetric models do quite well if 3D geometry doesn’t vary in z direction • However if geometry has complex 3D shape then 2D models are not an option • Even partial 3D geometry can save time compared to full 3D model

13. Mesh the Geometry • Discretize the geometry into smaller and (ideally) identical components F-15E CFD solution from Cobalt Solutions, mesh from Pointwise. URL: www.pointwise.com/apps/f15e.shtml Right image courtesy of ESI-GROUP

14. Mesh Considerations in CFD • Meshing is one of the most crucial steps in CFD analysis • It can be considered as an application in itself, with extensive research, mathematics, and software programs dedicated to it • Meshing can have a significant affect on the accuracy of CFD results and how quickly a solution is reached • Poor mesh quality can lead to incorrect results! • Many meshing programs and tools exist to automatically mesh a geometry based on user specified parameters (type of mesh, mesh size, maximum skew, etc.) and the type of solver to be used • Luckily meshing programs contain many checks and warnings to the user to ensure the mesh is of good enough quality, but user expertise is often required for complex cases

15. Define Physics • Define the physics that govern the behavior of your model • The definition of the physics of the CFD analysis determines many of the equations and mathematical models that will be used when solving.

16. Physics Considerations in CFD • Fluid dynamics (flow) • Turbulence • Heat Transfer • Chemistry (incl. electrochemistry and biochemistry) • Particles • Free surface (Volume of Fluid) • Cavitation • Electromagnetism • Plasma • Structural Dynamics

17. Physics Considerations in CFD: Fluid Flow • Laminar or Turbulent • Large eddy simulation (LES) • Detached eddy simulation (DES) • Direct Numerical Simulation (DNS) • RANS-based • K-epsilon • K-omega • Gravitational, thermal, or buoyancy effects?

18. Define Boundary Conditions (B.C.’s) • Define your boundary conditions • The boundary conditions not only determine how the boundaries of your model behave, but as they are the only input/output to the model, they affect how the entire model behaves.

19. B.C. Considerations in CFD • Proper boundary condition definition is crucial to ensuring accurate CFD modelling. • Poor boundary condition definition can cause unrealistic model behavior and thus incorrect results • Many types of boundary conditions • Inlet • Velocity, Pressure, Mass Flow • Outlet • Conservative, Pressure, Mass Flow • Wall • Slip, non-slip

20. Set Solver Parameters • Select your solver and define the parameters • The solver parameters determine how the solver behaves and the equations used to interpolate and manipulate the numerical schemes

21. Solver Considerations in CFD • The solver settings and parameters can have a huge impact on simulation time and the accuracy of results • Many different parameters/settings • Type of Solver (i.e. Numerical Methodology) • FVM, FEM, FDM, BEM • Differential Schemes • First-order upwind, Central differencing, QUICK • Convergence Criteria • # of iterations, residual value

22. Solve and Post-Process • Click “Solve” or “Start” or “Run” and analyze your convergence criteria. • BEWARE: Convergence of the simulation does not mean that your solution is correct! Right image courtesy of Mentor Graphics.