CFD Workflow Guide: How to Set up a Fluid Dynamics Analysis
Computational Science is the use of computers to solve complex problems. It is becoming more and more popular in the scientific world due to its potential to solve complicated problems in very cost-effective ways. With computers becoming more powerful, simulations are getting better and better. In fact, the future of all scientific experimentation and engineering analysis lay with computation and simulation. Computational Science is now widely accepted as the third pillar of scientific research complementing both experimentation and theory.
Computational Fluid Dynamics (CFD) is the major branch of Computational Science and it refers to the usage of computers to solve fluid mechanics problems. In CFD, Navier–Stokes equations are solved at different points on the grid by using different numerical methods and schemes.
The CFD Workflow:
Every CFD problem has the same workflow which can be divided into three basic steps.
Pre-processing in the CFD workflow involves the preparation of geometry, meshing the geometry, setting up the material properties, initial conditions and boundary conditions.
Having a good CAD geometry, without defects, is very crucial for the setup of a CFD analysis. Common geometry defects are:
- Missing faces
- Overlapping faces
- Free faces, edges, nodes
- Sharp angles
Preparing the geometry
- Fill the gaps
- Split surfaces with high curvature
- Remove unnecessary details
At the end, the geometry should be clean, watertight solid. It can be uploaded in different formats such as .stp/.step, .stl, .iges or .brep, but the recommended one is .stp/.step because it is more stable than other formats.
Mesh generation is a crucial step in the CFD workflow. It divides the physical domain into a finite number of discrete regions called control volumes or cells. The numerical solution is calculated in these individual cells by solving the governing fluid flow equations inside. The mesh should be refined depending on the problem. The areas with large changes in fluid properties should be adequately refined. In this respect, the physics of the problem helps. Make an intuitive guess of the flow profile and then refine the areas of the large gradients. Always start with a coarse mesh and then refine the specific areas. Keep the mesh size as small as possible within accuracy limits.
Selection of mode and flow properties
The simulation model is selected based on the physics of the problem. First select the simulation type (incompressible, compressible, natural convection etc). Once the simulation type is selected, set the other physical problem parameters such as turbulence (if you want to calculate the effects of turbulence on the fluid flow) or laminar (if you want to neglect the effects of turbulence on the fluid flow). If the fluid flow is time-dependent, then choose the transient and if you need the solution which is independent of time, then select the steady state.
Specification of initial and boundary conditions
Now we have to give the material properties, initial conditions, boundary conditions. Assign the material properties to the fluid. One of the most crucial parts of setting up a numerical simulation is giving the initial and boundary condition. Numerical simulations are based on solving the partial differential equations on the given domain and these equations require the initial and boundary conditions (depending on the problem type) for Well-Posedness.
Next step is specifying the numerical parameters i.e setting up solver parameters, discretization schemes etc. Depending on the simulation type, every problem has a unique structure. Usually, one problem can be solved by multiples solvers and with different solver parameters but to efficiently solve the problem, it is very important to give the correct solver parameters and numerical schemes.
One of the major parts of any numerical simulation is interpreting the results. This is done in post-processing where the flow fields are analyzed by different filters like streamlines, contour plots etc. The post-processing is the last step of the CFD workflow and with it, you can visualize your simulation results and make decisions for design optimization.
Here it is, a quick guide to get you through your CFD analysis. Follow these steps to set up your simulation and get the results you want. And if you need more in-depth explanations, here are a few articles worth reading: