Learning Forum

[chunhan]

Hey everyone I am pretty new to this software and I have read and tried most of the forum discussion and youtube tutorials but I am still unable to solve this error. I hope someone is able to help me here.

So my problem is simplified to moving a small box (train) into the tunnel (rectangle) with an inlet velocity. I am using a UDF for this and dynamic meshing. I have used the Boolean method to subtract my box out and used the smoothing and remeshing method. The attached photo is my set up. My mesh element size is 0.3m and I have calculated my timestep using the CFL condition.

Any help would be very much appreciated thank you guys in advance!

[DrAmine]

If you are getting negative volume mesh, your mesh has corrupted because of two reasons. One is higher time step size. By reducing the time step size, you can eliminate the negative volume. Second one, your mesh might be bad already. That are usually the main reasons for that.n

[YasserSelima]

The geometry is huge to have good mesh, given the limitation on the student version.nWhy don't you scale the problem ... divide all real lengths by 1000 ... then scale your inputs to keep dynamic similarity?n

[Rob]

I'd not scale the domain as you still finish up with loads of cells. However, if you can take advantage of symmetry it could help. I'd also review domain decomposition and layering as I suspect we can reduce the cell count by using hex cells and simplify the remeshing with some simple geometry reworking. n

[chunhan]

Hi DrAmine,nI have tried to reduce my time step from 0.5 to 0.001s but it still doesn't work. I used the auto meshing function in fluent by just adjusting my cell size. I'm not sure if that is correct. Looking at the orthogonal quality and the rest of the mesh qualities, it looks fine. But if the mesh quality is the problem I am not sure how to increase the quality of my mesh. n

[chunhan]

Hi YasserSelima,nWill definitely try to solution out and see how it goes. Thank you for the suggestion!nn

[chunhan]

Hi Rob,nI am not very sure on how I can take advantage of the symmetry. However, I do understand why it can be an advantage. So I should try to change my mesh to hex cells to reduce cell count?n

[Rob]

It's a suggestion, without a detailed look at the case (which I won't be doing) I can't say for sure. As a comparison a cube with 10 cells per edge gives 1000 cells with hex and around 7500 with tet. Which is better? nLayering is an alternative moving mesh remeshing approach. We've used this several times for trains in tunnels and pigs in pipes. This avoids the issue you'll see later in that Fluent may be remeshing (or at least checking) the entire model to correct a few cells. n

[DrAmine]

What is moving in your case? Which boundary? Screenshots?n