Chapter 1 Introduction to CFD,Introduction to CFX,What is CFD?,Computational fluid dynamics (CFD) is the science of predicting fluid flow, heat and mass transfer, chemical reactions, and related phenomena by solving numerically the set of governing mathematical equations Conservation of mass, momentum, energy, species mass, etc.The results of CFD analyses are relevant in: Conceptual studies of new designs Detailed product development Troubleshooting RedesignCFD analysis complements testing and experimentation by: reducing total effort reducing cost required for experimentation,How Does CFD Work?,ANSYS CFD solvers are based on the finite volume method The fluid region is decomposed into a finite set of control volumes General conservation (transport) equations for mass, momentum, energy, species, etc. are solved on this set of control volumesContinuous partial differential equations (the governing equations) are discretized into a system of linear algebraic equations that can be solved on a computer,* FLUENT control volumes are cell-centered (i.e. they correspond directly with the mesh) while CFX control volumes are node-centered,CFD Modeling Overview,Problem Identification Define goals Identify domain,Pre-Processing Geometry Mesh Physics Solver Settings,Solve Compute solution,Post Processing Examine results,Update Model,Problem Identification Define your modeling goals Identify the domain you will modelPreProcessing and Solver Execution Create a solid model to represent the domain Design and create the mesh (grid) Set up the physics Physical models, domain properties, boundary conditions, Define solver settings numerical schemes, convergence controls, Compute and monitor the solutionPost-Processing Examine the results Consider revisions to the model,1. Define Your Modeling Goals,What results are you looking for (i.e. pressure drop, mass flow rate), and how will they be used? What are your modeling options? What physical models will need to be included in your analysis (i.e. turbulence, compressibility, radiation)? What simplifying assumptions do you have to make? What simplifying assumptions can you make (i.e. symmetry, periodicity)? Do you require a unique modeling capability? User-defined functions (written in C) in FLUENT or User FORTRAN functions in CFXWhat degree of accuracy is required?How quickly do you need the results?Is CFD an appropriate tool?,Problem Identification Define goals Identify domain,2. Identify the Domain You Will Model,How will you isolate a piece of the complete physical system?Where will the computational domain begin and end? Do you have boundary condition information at these boundaries? Can the boundary condition types accommodate that information? Can you extend the domain to a point where reasonable data exists?Can it be simplified or approximated as a 2D or axisymmetric problem?,Cyclone Separator,Problem Identification Define goals Identify domain,3. Create a Solid Model of the Domain,How will you obtain a solid model of the fluid region? Make use of existing CAD models? Create from scratch?Can you simplify the geometry? Remove unnecessary features that would complicate meshing (fillets, bolts)? Make use of symmetry or periodicity?Do you need to split the model so that boundary conditions or domains can be created?,Solid model of a Headlight Assembly,Pre-Processing Geometry Mesh Physics Solver Settings,4. Design and Create the Mesh,What degree of mesh resolution is required in each region of the domain? The mesh must resolve geometric features of interest and capture gradients of concern e.g. velocity, pressure, temperature gradients Can you predict regions of high gradients? Will you use adaption to add resolution?What type of mesh is most appropriate? How complex is the geometry? Can you use a quad/hex mesh or is a tri/tet or hybrid mesh suitable? Are mesh interfaces needed?Do you have sufficient computer resources? How many cells/nodes are required? Which physical models will be used?,Pre-Processing Geometry Meshing Physics Solver Settings,A mesh divides a geometry into many elements. These are used by the CFD solver to construct control volumes,Tri/Tet vs. Quad/Hex Meshes,For flow-aligned geometries, quad/hex meshes can provide higher-quality solutions with fewer cells/nodes than a comparable tri/tet mesh Quad/Hex meshes show reduced false diffusion when the mesh is aligned with the flow. It does require more effort to generate a quad/hex meshMeshing tools designed for a specific application can streamline the process of creating a quad/hex mesh for some geometries. E.g. TurboGrid, IcePak, AirPak,Tri/Tet vs. Quad/Hex Meshes,For complex geometries, quad/hex meshes show no numerical advantage, and you can save meshing effort by using a tri/tet mesh or hybrid mesh Quick to generate Flow is generally not aligned with the mesh Hybrid meshes typically combine tri/tet elements with other elements in selected regions For example, use wedge/prism elements to resolve boundary layers More efficient and accurate than tri/tet alone,