--- title: "Steps of a CFD analysis" subject: "Applied Computational Fluid Dynamics" chapter: 1 key: "semester_7/applied_computational_fluid_dynamics:cfd-solution-procedure" source: https://www.bctengineer.com/q/semester_7/applied_computational_fluid_dynamics/cfd-solution-procedure site: BCT Engineering --- # Steps of a CFD analysis Applied Computational Fluid Dynamics · Chapter 1: Introduction to CFD and Fluid Mechanics Review - Asked in: year not given, 6 marks - Video: [Introduction to Computational Fluid Dynamics (CFD) - Part 1](https://www.youtube.com/watch?v=oZWGgid2Cbg) — csystudio - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/applied_computational_fluid_dynamics/cfd-solution-procedure - Whole chapter: https://www.bctengineer.com/md/semester_7/applied_computational_fluid_dynamics/chapter-1 ## Question Explain the three main stages of a CFD simulation with the activities carried out in each stage. Draw a flowchart of the procedure. ## Short answer Pre-processing defines geometry, generates mesh, selects physics and boundary conditions. The solver discretises the governing equations, applies pressure-velocity coupling such as SIMPLE, and iterates until residuals fall to tolerance. Post-processing plots contours, extracts forces and pressure drop, and checks mesh independence and validation. ## Answer A CFD analysis has three stages: **pre-processing**, **solving** and **post-processing**. ```text Problem definition | v +---------------------+ | 1. PRE-PROCESSING | geometry, mesh, | | physics, BCs +---------------------+ | v +---------------------+ | 2. SOLVER |<--+ not converged | discretise, iterate| | +---------------------+---+ | converged v +---------------------+ | 3. POST-PROCESSING | plots, forces, | | validation +---------------------+ ``` #### 1. Pre-processing 1. Define the problem and the aim (e.g. pressure drop, lift). 2. Create or import the geometry (CAD) and clean it; choose the computational domain size. 3. Generate the mesh (structured/unstructured) and refine near walls and gradients. 4. Select the physical models: laminar/turbulent, steady/transient, compressible/incompressible, heat transfer. 5. Define fluid properties and boundary and initial conditions (inlet velocity, outlet pressure, wall, symmetry). #### 2. Solver 1. The governing equations are integrated over control volumes (FVM) or approximated by differences (FDM) to give algebraic equations. 2. A pressure-velocity coupling method (e.g. SIMPLE) and linear solvers (Gauss-Seidel, multigrid) solve them iteratively. 3. Iteration continues until the residuals fall to a set tolerance (e.g. $10^{-4}$ to $10^{-6}$) and monitored quantities stop changing. #### 3. Post-processing - Contour, vector and streamline plots, line plots, animations. - Integrated results: forces, pressure drop, heat transfer rate, mass-flow balance. - Check mesh independence and compare with experimental or analytical data (verification and validation). - If results are unsatisfactory, return to the mesh or the model and repeat. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the three main stages of a CFD simulation? A. Meshing, drawing, printing B. Pre-processing, solving, post-processing C. Discretisation, validation, optimisation D. Geometry, experiment, comparison Answer: B. A CFD analysis consists of pre-processing, the solver stage and post-processing. (Key point: Three stages) 2. Which activity belongs to pre-processing? A. Plotting streamlines B. Checking residuals for convergence C. Computing drag force from the solution D. Generating the mesh and setting boundary conditions Answer: D. Geometry, mesh, physical models, fluid properties and boundary conditions are all set before solving. (Key point: Pre-processing activities) 3. When does the solver stop iterating? A. When residuals fall to the set tolerance and monitored quantities stop changing B. After exactly one iteration C. When the mesh has been refined once D. When the plots look colourful Answer: A. Iteration continues until residuals reach a tolerance (e.g. 10^-4 to 10^-6) and monitored quantities are steady. (Key point: Convergence criterion) 4. What should be done in post-processing if results are unsatisfactory? A. Accept them since the solver converged B. Return to the mesh or model and repeat C. Delete the geometry D. Increase the residual tolerance Answer: B. Post-processing includes mesh-independence checks and validation; poor results send you back to the mesh or the model. 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