• By
  • November 10, 2021
  • AutoCAD


In the last blog from AutoCAD Course in Pune, we had seen what is CFD and how does it work?, now in this from AutoCAD Classes in Pune, we will see what is the actual CFD process, what is the workflow of CFD project?

CFD process from start to finish is as follows:

1. Validation Runs –

First part of the any simulation is a validation run. This is completely unrelate to the project, it is actually a comparison between a CFD analysis and an experiment of exact same thing. Validation runs are not actually meant to real world results. They are actually meant to test the accuracy of the CFD process and the CFD operator. They are meant to prove that the operator and the software are accurate and reliable. Once we actually proven that the CFD operator and the software can be trusted, we get into the actual project.

First thing happens is geometry preparation. The client is going to give the CFD operator some set of geometry. This might be a 3D CAD file or a 2D drawing, whatever it is in all likelihood, it is not likely for CFD. So, very first thing we do is build a 3D model or clean up a 3d model. The whole process takes about 15% of the project cost.

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2. Diagnostic Runs –

This is where the CFD operator is setting up the model, trying out different settings and showing themselves that the model is accurate and reliable. That actually is no small fit, that take around 45% of project cost just to set up that get done. 

3. Production Runs –

Now we actually get into the production Runs. These are the things we invest for writing the results. This is 40% of product cost.

Let’s see the process parameters in details.

4. Geometry –

First thing that we have to do is to form a closed solid. This is going to be a 3D object that is completely closed off, we are removing an internal structure as well, we are going to do a lot of de-featuring of small fillets, small cavities. We are going to remove lot of things that are not necessary. Ex.  Handrails in ships does not add any significance to the final answer. We are also creating the boundaries of the domain. Ship is a solid object but we are not modelling a solid object. We are modelling the fluid around the ship. So, we have to create some boundaries around there because we don’t want to model the fluid on entire earth, so we have to set the stopping point. And then that creates the fluids area around the ship, so we set the section of the area around the ship, puts the whole ship sitting right in the middle. That is our fluent domain.

5. Meshing –

We create a mesh size which is based on the validation studies, then we are going to add the refinements around the important parts. Refinement areas vary as per the size of the mesh gets smaller. We are also going to look for areas where we can make the mesh larger, there is areas where we need actual space but they don’t actually need concentrated mesh. And the last thing is about putting a boundary layer mesh.

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6. Physics of the meshing –

Now we are actually getting into the physics of the system, so start with the bare minimum, get its stable then add in extra equations as you go until you reach your requirements. We are writing fluid properties as well, anything like mesh motions those necessary for sea keeping or motions of ship. As part of all these physics setup we pick a turbulence model and from the setting of all of these equations one of the questions in all of the backs come in head is how much accuracy is needed? there are quite few settings going on here and the question of accuracy so, lets move to the next.

7. Run Simulation (Diagnostic Run) –

Diagnostic run is isn’t about production, this is meant to the internal quality checks. We are checking for the convergence, making sure the simulation actually finishes. That’s what the convergence means. And make sure that we get enough data to actually get stable results. And residuals are checking stability, bugs and bad performance.

8. Post process (Diagnostic) –

The direct output from CFD simulation is just a database follow up millions and millions of numbers. We can’t read it directly, so we are going to post processing. We use the tool to turn that into human readable information, check for further errors. Plots we are going to create are Y-plus plots, courant number plots. You can do just one diagnostic run. You have to actually repeat this process with several diagnostic runs at different mesh settings and we can combine all the information into a mesh independence study.

9. Mesh Independence Study –

CFD accuracy highly depends on mesh setting. Hence the only way to get the best answer is to try different mesh settings and then we have to compare the different answers. There is a form of method of comparison called mesh independence study.

10. Production Runs – 

This is where the money gets spent a lot, so double check the settings one more time. Plan sufficient time in your schedule for the project. Something is going to wrong, so monitor the runs.

11. Production post processing – 

Production post processing is actually going to focus on two main things. Most projects are trying to answer one specific question, but also why was the answer, generally we want more specific answer to go with that.

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12. Report –

Everything is going to be wrap up into the report. There is going to be lots of results, lots of visualization, explanation for the results. There should be enough detail that a third party could reproduce results if possible.

We start with the geometry and right after the geometry stage, AutoCAD Training in Pune have to think ahead 3 to 4 steps which is going to impact the simulation quality right after the meshing quality. Most of the simulation quality gets set right after the meshing stage. The diagnostic runs are whether really satisfying themselves that the model is setup correctly and combine in mesh independence study. After that we run the production runs and production post processing and all things are wrapped up into final thick report. That is the CFD process in Ansys Fluent.


Vaishnavi Shinde
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