SimScale CAE Forum

Convergence problem in incompressible analysis around a FSAE car

Hello,

I am doing a CFD analysis on one of our FSAE models. The problem I am facing is the convergence of pressure. We have tried almost everything from finer mesh to running till 1500 iterations but it doesn’t seem to converge. My residuals don’t go below 1000. Also, due to this, I am getting some absurd results like 4N drag - which of course is not possible for any vehicle.

I checked my mesh quality and stuff but it is fine. I also checked the convergence plot of the inlet, outlet, etc but they all have steady values in the last iterations.

Hope you guys can help me!!

Link to my project: https://www.simscale.com/workbench/?pid=6334428506248639852&mi=spec%3A5d4ab225-1cec-45e1-9a12-f4815327e9ba%2Cservice%3ASIMULATION%2Cstrategy%3A8

P.S. - Refer to run2 in my project

Tagging @tsite and @jousefm as I haven’t got any response yet…

Cheers!

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Hi there, thank you for using the forum!

I see that after run 2, you created another one named ‘With potential flow on’, that actually seems to gradually converge, right?

By the way, would you be interested in using the Standard meshing algorithm instead?

Best regards,
Fillia

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Hi @tsite,

Thanks for replying.

If you check my results with Potential Flow on, the pressure and other quantities arent very stable. For instance, my pressure value in the domain isn’t stable in that simulation. Hence, we cant really say that that sim is correct. Right? (Please correct me if I am wrong).

Talking about standard mesher, see the simulation named New 1 v7 Standard. I tried doing standard mesh but I am getting a 40M cells huge mesh with a volume ratio of 130 which again cant be put up for simulation.

Hope you can help further…

Cheers!!

Hello again…

I am trying the standard mesher. But along with that, I am also trying Hex Dom Parametric.
I am facing problems in the mesh that I have never faced. I am attaching screenshots for better understanding.

  1. The mesh seems to make a hole in my geometry which doesn’t even exist in reality. I tried finding some possible solutions. I tried with Explicit Feature and Implicit Feature both alternatively turned off and turned on but the results are not much different

  1. I have Mesh coming out of the body and making unidentified shapes due to some reason.

  1. Also, I am trying to put my own Boundary Layer Conditions but for some reason, boundary layers doesnt even show up. Here is what i want my boundary layer to be

Y+ = 30
Thickness = 0.001654 m
at 25 Kmph

For this, I have put up my settings as

Layers: 5
Expansion ratio: 1.3
Min thickness: 0.01 (Changed it in recent sim and quite lesser than actually required (0.0150))
Final layer thickness: 4.7e-3
Layer Size: Off

Hope you guys can help with it.

Also, I am tagging @DaleKramer @jousefm @Ricardopg and @dschroeder so that they can help too.

Cheers!!

Hi,

One question about the overall settings. I know you mentioned that you’d like to run this simulation at 25 kph. Is this really intended? And if so, are you taking this into account when evaluating whether the results look physical or not?

Just elaborating a little bit: aerodynamic drag has a quadratic relationship with velocity (it’s not a linear correlation). Since the speeds for this sim are far lower than usual FSAE simulations (roughly 5x~) lower, the forces are also going to be much smaller than you are used to.

Cheers

1 Like

Hello @RicardoParis,

Thanks for the insight. Yes, we are willing to do sims at 25 kmph as it is our calculated turning speed at the tightest corner.

Talking about simulation, my main problem is poor convergence and improper meshing. If you check the project, I don’t have proper convergence. Due to this, I am focusing on better meshing. But in that also, I am not able to achieve good mesh as you can see in my later updates here:

Hope you can help out with that problem or perhaps I should make another post for it as the problem is a little off track? Let me know…

Cheers!!

Hi there, any updates on project with the Standard mesher? I would love to take a look (The hex dominant does not seem to be a good fit for this case)

Hi there @tsite ,

We are trying to mesh in standard mesher but we are getting too high volume ratio i.e. 107. If you check our simulation named New 1 Standard 2, the run failed due to diverging velocity which of course we are trying to solve. But here are the two questions

  1. We are already at a mesh of 13 M which is pretty fine for half car simulations. So how can i tackle the divergence problem without further refinement?

  2. Can you please address the problems I have stated in my earlier reply

that I was facing in hex dominant because I am facing the same kind of problem in my sim name Diff 8 4 21 in the same project (with different geometry then earlier)

Hope you can help…

Cheers!!