Grid generation is the most important step in the CFD process. Without a grid, you cannot even perform a CFD simulation. Even meshless methods require a grid, also known as a lattice. Grid generation sits at a very critical stage in the CFD process. It is the preprocessing step following CAD, prior to running a CFD simulation, and where the majority of your personal time is spent during analysis. In fact, grid generation is where you have the most direct influence over the accuracy and convergence of the simulation.
Not all grid generators are created equal however and it is up to you to determine the best tool for the job. Whether you are setting up a new CFD workflow or looking to improve a preexisting process, here are five questions to ask when shopping around for a preprocessor.
1. How well does this preprocessor fit into my design and analysis process?
What you are looking for here is the ability to import geometries and grids from the tool(s) you use and export native grids and boundary conditions to the solver(s) you use. If you are currently working with two different CAD packages, an in-house design tool, your own code for generating part of the grid, and three different CFD solvers depending on the application, ensure the software works with everything you use. This question lets you know how flexible the software is now and what you can expect in the future. Will the preprocessor easily integrate into your process, or will it try to make you fit into its process?
2. How does it handle dirty geometry?
Not all geometry is created equal. Sometimes you may import geometry from your CAD package or receive geometry from a customer, and it will be dirty. The preprocessor you use should be able to clean analytic CAD and/or allow you to mesh over the problem areas.
3. How automated is the meshing process and is it scriptable?
Let’s face it, meshing is the bottleneck of the CFD process. That being said, automation can go a long way toward reducing your time spent meshing. This question will uncover if the software employs a bottom up or top down meshing process. Each offer automation, but you may find that some approaches come at the cost of robustness and/or control. Frequently, automatic meshing methods fail to produce a mesh or produce a mesh that is not suitable for your needs. What alternatives does the preprocessor offer for completing the mesh or improving the quality of an automatic mesh that is not up to standards? You may also find that some software is extensible through scripting. Scripting can enable you to automate part or all of your meshing process allowing you to get more done in less time without sacrificing control over your grid.
4. How much control does it provide throughout the meshing process?
Every simulation is different and therefore meshing requirements change. Pick a tool that gives you options, such as the ability to generate structured, unstructured, hybrid, and overset grids when necessary. Don’t settle for one or the other or a tool that requires you to pay for each module separately. Who knows when you’ll need to use a different meshing technique to meet your project requirements. Many preprocessors sacrifice control in the quest for push-button meshing. Don’t settle for that either. Look for a preprocessor that gives you control over every step of the process. What does control buy you? You can generate higher quality grids with fewer cells by having control over refinement and grid type when and where you need it.
5. What happens when you call for technical support?
You will probably have questions, run into bugs, or have great ideas that you’d like to see implemented in the software. Having someone to contact directly is as important if not more important than the software itself. Would you prefer your first phone contact to be with a help desk person reading questions from a script, or would you like to be able to call or email your own technical support engineer directly? Dig deep here. Ask them to describe the life of a bug, how it is reported, and how long before you can expect to get a fix. Uncover the process by which feature requests become actual features in the software. Ask if support is limited and how long it typically takes to get a response to a question. And find out if they offer training. You may be surprised to learn that free training is offered once you become a customer to help you get up to speed with the software.
Before committing to an evaluation or a purchase, compare the answers you receive with the requirements of your group. If the two align you may have found the preprocessor you’ve been looking for.