Learning Forum

[LoïcS]

Dear fluent users,

I have an entire model running well. Based on the reference paper, i need to mesh the model with mesh size of 10 µm which means more than 50 000 000 elements. I reduced the model to a quarter to save computational time but i have negative cell volume occuring during the simulation. I tried solutions provide on research gate for the same issue  :

- try PISO method

- Timesteps less than 0.005

- reduce the relaxation energy value from 1.0 to 0.6

but the issue still occurs.

The quarter model

I'm trying to simulate a wall moving normal to a stationary wall with an amplitude ratio of 0.8 of the initial air gap (200µm)
Mesh elements are tetrahedrals, mesh size 100 µm
Dynamic mesh is set to layering and unified remeshing, cell height 20 µm
The model is laminar with air - ideal gas
Time step size =  5e-5 s

[C N]

Hello loics,

Some times left handed faces create some negative volume cells 
To overcome this problem try:
1. Reverse the order in which the surfaces are picked inside the Grid Interfaces panel while creating the non-conformal interface if there is non conformal interface
2. Try the TUI command /mesh/repair-improve/repair (with /mesh/repair-improve/repair-face-handedness yes)
3. If that doesn't work, look carefully at the two surfaces involved especially at corners and turns. Modify the mesh to make sure that the two surfaces are not passing through each other at any point.

4. finally check the dynamic mesh settings.

This should help you solve the problem.

Thanks,

Chaitanya natraj

[LoïcS]

Hello Chaitanya,

Please could you tell me how to go through those steps 1 - 3? I don't have enough experience on fluent.
Thanks

[C N]

Hello Loics,

Is there any interfaces you created for the problem? If not step 1 is not required.

Executing TUI command please go to console and type the command which I have mentioned within the brackets. - step 2 

Go to the spaceclaim and perfrom geometry repair

Check for any issues . Then 

Import it to fluent - check for any mesh overlap at the edges - this can be done going to mesh display option and zoom the object at edges woth the mesh to find out. 

This is how you can perform these steps.

Thanks,

Chaitanya Natraj

[LoïcS]

Hello Chaitanya,

I appreciate your help. I have done following check. There are the results :

no split edges in the model
no extra edges in the model

Quarter model

I have also tried different dynamic settings (smoothing, layering, remeshing) individually and combinate with default setting.

I'm still having negative point cell and sometimes floating point exception. Is there other options that i can try, please?

[LoïcS]

Hello Chaitanya, 
Please, do you have more insights for my issues? The full model take more than 24 hours to run completely. Solving this issue of negative volume cell with the quarter model will help me to save time.