Finite differences: structured Finite volume or finite element: structured or unstructured • Application Thin boundary layers best resolved with highly-stretched structured grids Unstructured grids useful for complex geometries Unstructured grids permit automatic adaptive refinement based on the pressure gradient, or regions interested (FLUENT) structured unstructured
25 Numerical methods (grid transformation) y x o o Physical domain Computational domain x x f f f f f x x x y y f f f f f y y y • Transformation between physical (x,y,z) and computational ( ) domains, important for body-fitted grids. The partial derivatives at these two domains have the relationship (2D as an example) Transform
26 High performance computing and post- processing • CFD computations (e.g. 3D unsteady flows) are usually very expensive which requires parallel high performance supercomputers (e.g. IBM 690) with the use of multi-block technique . • As required by the multi-block technique, CFD codes need to be developed using the Massage Passing Interface (MPI) Standard to transfer data between different blocks. • Post-processing: 1. Visualize the CFD results (contour, velocity vectors, streamlines, pathlines, streak lines, and iso-surface in 3D, etc.), and 2. CFD UA : verification and validation using EFD data (more details later) • Post-processing usually through using commercial software
27 Types of CFD codes • Commercial CFD code : FLUENT, Star- CD, CFDRC, CFX/AEA, etc. • Research CFD code : CFDSHIP-IOWA • Public domain software (PHI3D, HYDRO, and WinpipeD, etc.) • Other CFD software includes the Grid generation software (e.g. Gridgen, Gambit) and flow visualization software (e.g. Tecplot, FieldView) CFDSHIPIOWA
28 CFD Educational Interface Lab1: Pipe Flow Lab 2: Airfoil Flow Lab3: Diffuser Lab4: Ahmed car 1. Definition of “CFD Process” 2. Boundary conditions 3. Iterative error 4. Grid error 5. Developing length of laminar and turbulent pipe flows. 6. Verification using AFD 7. Validation using EFD 1. Boundary conditions 2. Effect of order of accuracy on verification results 3. Effect of grid generation topology, “C” and “O” Meshes 4. Effect of angle of attack/turbulent models on flow field 5. Verification and Validation using EFD 1. Meshing and iterative convergence 2. Boundary layer separation 3. Axial velocity profile 4. Streamlines 5. Effect of turbulence models 6. Effect of expansion angle and comparison with LES, EFD, and RANS. 1. Meshing and iterative convergence 2. Boundary layer separation 3. Axial velocity profile 4. Streamlines 5. Effect of slant angle and comparison with LES, EFD, and RANS.
29 CFD process • Purposes of CFD codes will be different for different applications: investigation of bubble-fluid interactions for bubbly flows, study of wave induced massively separated flows for free-surface, etc.
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- Summer '18