Learning Forum

[xgteml]

Hello, I am going through Raef's exact pump simulation but I keep getting the message:

Warning: convergence tolerance of 1.000000e-06 not reached during Hybrid Initialization. 

and

Error: floating point exception. Error Object: #f

 

When I attempt to run the calculation.

Any ideas what to try?

Link to video: https://www.youtube.com/watch?v=OJ_WjAlKQ3M&t=306s

Thanks

[Keyur Kanade]

Are you using student version? What is cell count you created?

What is mesh quality? Please check quality in Fluent. The min orthogonal quality should be more than 0.1. 

Please check your inlet conditions. 

Please cross check other settings. 

 

[xgteml]

Hello, it is the commercial version. Here is the current mesh, I started with a finer mesh, but then went to this just to see if I could get a solution.

 

 

I have an inlet velocity of 0.3 m/s and an outlet pressure of 20,000 Pa just like the tutorial suggests.

Thanks

[Keyur Kanade]

can you please change Mesh Metric to orthogonal quality and post the image. 

[xgteml]

Yes, thanks.

[Keyur Kanade]

Please improve mesh so that your min. ortho quality is more than 0.1. 

[xgteml]

What is the best way to do this? Mesh element size?

[Keyur Kanade]

Check following videos.

First find where are bad elements. You may want to modify geometry at those locations. You may use different mesh sizing as well. 

https://www.youtube.com/watch?v=AzvPxjNyr3w

https://www.youtube.com/watch?v=e_irQcWnTTs

https://www.youtube.com/watch?v=jSV7Pd7qAFU

 

[xgteml]

Thanks for the videos. Even with refinement, I am not able to get the min. ortho quality to be more than 0.1.

[Keyur Kanade]

First of all I think your geometry decomposition for inner rotating cell zone is not correct. Please check video correctly. 

Please correct geometry decomposition first. 

Then check the mesh quality video and follow steps to locate the bad element positions. Then take corrective action at these locations. 

If you correct the geometry decomposition, I believe your mesh quality would be good. 

[xgteml]

Hello, I am pretty certain the geometry composition using the Boolean commands is correct.

 

I am confused though. When I take corrective action to correct the mesh, the minimum orthogonal quality gets even lower.

[Keyur Kanade]

Then check the mesh quality video and follow steps to locate the bad element positions. 

Post image for the same. 

[xgteml]

I did notice the inlet gets imported into cell zone conditions while in the tutorial it is not. Could this be an issue?

[xgteml]

After increasing mesh quality, I now get this message:

 

 

Decreasing time step does not aid in generating solution.

[xgteml]

Current mesh settings:

[xgteml]

Still get the same error message with further improving the mesh: Negative cell volume detected.

[xgteml]

Is it most likely due to the mesh or something else?

[xgteml]

It must be in the mesh. I am uncertain how to proceed.

[xgteml]

I also tried this tutorial using steady state with an inlet pressure of 0 Pa and a mass flow outlet rate of 80 kg/s. I am getting incorrect values.

 

[xgteml]

Going back to transient, now this happens?

 

[Keyur Kanade]

It must be mesh. 
Can you please insert more images of the mesh. Some sectional views, some zoomed in views. 

Also please reduce the time step. Make sure that in one time step mesh is not moved more than a cell length. 

[xgteml]

Thank you for the help. Just one last question, since this is internal flow, with a velocity inlet and pressure outlet specified, if the pump was scaled larger, would the results change or is it all relative?

[Keyur Kanade]

if geometry is scaled then other input parameters will also need to be changed. 

if this helps you, please mark this as 'is solution' to help others on forum.