## Fluids

#### Fluent: Convergence problem

• Lambert Maus Lazlus
Subscriber

Hi, I am new to Ansys. I'm using Fluent to study the molten flow in die casting cavity. I try to copy similar model in youtube.. As attached, my calculation stopped when the molten started enters the cavity which has small cross-section area. I also did reduce the timestep size from 1e-4 to 1e-5 to 1e-6 to 1e-7 and the same error occured as attached. How can I solve this problem? Thank you.

• Supreetha J
Subscriber
Hello,

This could be related to the mesh size and quality in the area where the molten enters the cavity with a small cross-sectional area which can cause issues with the stability of the simulation and lead to numerical errors or convergence issues.
Kindly check the mesh quality and make sure that the mesh is properly refined in the regions where there are flow gradients and sharp changes in geometry.

Thank you.
• NickFL
Subscriber

Keep in mind what the CFL number is. It is U*Dt/h, where U is the velocity in a certain direction, Dt is the time step and h is the grid cell size in the direction of U. So in fact, there would be 3 different CFL, one for each direction. Basically, the CFL will tell you how many cells a fluid particle would jump through in one time step (assuming consistent mesh size). To keep the CFL number low, we must stretch the grid cells, meaning make h bigger, but the key is only to do it in the streamwise direction. As the flow moves into the smaller channels, it will speed up, and we need to make sure we have high quality elements that are stretched in the streamwise direction.

What I would do is go back to DesignModeler and slice the geometry, creating a multi-bodied part. When you bring this into meshing, you can then sweep the long constant diameter sections and allow these to have a high aspect ratio. At the moment, I assume that you just have created a Tet-bomb mesh. A sliced model will allow you to create higher quality Hexas that can be stretched to aspect ratios of 1000 without losing accuracy (assuming double precision solve). These stretched cells will reduce the CFL in these cells and hopefully allow your solution to converge better.