Lecture 01 Introduction to the CFD,Lecture Theme:All CFD simulations follow the same key stages. This lecture will explain how to go from the original planning stage to analyzing the end resultsLearning Aims: You will learn:The basics of what CFD is and how it worksThe different steps involved in a successful CFD projectLearning Objectives:When you begin your own CFD project, you will know what each of the steps requires and be able to plan accordingly,Introduction,What is CFD?,Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat and mass transfer, chemical reactions, and related phenomena.To predict these phenomena, CFD solves equations for conservation of mass, momentum, energy etcCFD is used in all stages of the engineering process: Conceptual studies of new designs Detailed product development Optimization Troubleshooting RedesignCFD analysis complements testing and experimentation by reducing total effort and cost required for experimentation and data acquisition,How Does CFD Work?,ANSYS CFD solvers are based on the finite volume methodDomain is discretized into a finite set of control volumesGeneral conservation (transport) equations for mass, momentum, energy, species, etc. are solved on this set of control volumesPartial differential equations are discretized into a system of algebraic equationsAll algebraic equations are then solved numerically to render the solution field,Equation fContinuity 1X momentum uY momentum vZ momentum wEnergy h,FLUENT control volumes are cell-centered (i.e. they correspond directly with the mesh) while CFX control volumes are node-centered,Step 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 simplifying assumptions can you make (i.e. symmetry, periodicity)? What simplifying assumptions do you have to make? What physical models will need to be included in your analysisWhat degree of accuracy is required?How quickly do you need the results?Is CFD an appropriate tool?,Step 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 axi-symmetric problem?,Domain of Interest as Part of a Larger System (not modeled),Domain of interest isolated and meshed for CFD simulation.,Step 3. Create a Solid Model of the Domain,How will you obtain a model of the fluid region? Make use of existing CAD models? Extract the fluid region from a solid part? Create from scratch?Can you simplify the geometry? Remove unnecessary features that would complicate meshing (fillets, bolts)? Make use of symmetry or periodicity? Are both the flow and boundary conditions symmetric / periodic?Do you need to split the model so that boundary conditions or domains can be created?,Original CAD Part,Extracted Fluid Region,Step 4. Design and Create the Mesh,What degree of mesh resolution is required in each region of the domain? Can you predict regions of high gradients? The mesh must resolve geometric features of interest and capture gradients of concern, e.g. velocity, pressure, temperature 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 non-conformal interfaces needed?Do you have sufficient computer resources? How many cells/nodes are required? How many physical models will be used?,Step 5: Set Up the Solver,For a given problem, you will need to:Define material properties Fluid Solid Mixture 燃烧 ，气体Select appropriate physical models Turbulence, combustion, multiphase, etc.Prescribe operating conditionsPrescribe boundary conditions at all boundary zonesProvide initial values or a previous solutionSet up solver controlsSet up convergence monitors,For complex problems solving a simplified or 2D problem will provide valuable experience with the models and solver settings for your problem in a short amount of time,Step 6: Compute the Solution,The discretized conservation equations are solved iteratively until convergence Convergence is reached when: Changes in solution variables from one iteration to the next are negligible Residuals provide a mechanism to help monitor this trend Overall property conservation is achieved Imbalances measure global conservation Quantities of interest (e.g. drag, pressure drop) have reached steady values Monitor points track quantities of interestThe accuracy of a converged solution is dependent upon: Appropriateness and accuracy of physical models Assumptions made Mesh resolution and independence Numerical errors,A converged and mesh-independent solution on a well-posed problem will provide useful engineering results!,