Velocity field started diverging


For CHT V2.0 with External flow volume, I keep getting an Error with the below message.

“Velocity field started diverging. Please check the mesh quality near the reported location and try refining the mesh. If the problem occured near a boundary, please check the boundary conditions. In case of doubt, please ask for assistance via our support chat.”

It seems the initial simulation is going okay in every test, but at some point, “Zero Kelvin” air backflows into the pressure outlet where air should be going out, and creates a massive tornado. And the simulation fails. Please check below images. This is what happens all the time some moments before the simulation failing.

I checked the forum and see if I had ISO volumes bigger than 100 (which weren’t), if there was any bad mesh (I didn’t find any), tested increasing mesh fineness to 7, tested changing boundary condition of external flow volume to slip to non-slip (didn’t work), tested changing pressure outlet to mean flow to fixed flow (didn’t work), increased the external flow volume size (didn’t work), etc, etc… all didn’t work.

Any of your insight would be greatly appreciated!

Hi there,
Thank you for posting on the forum
Can you please share your project link?


I just shared the project with the support team and added your username.

I also added your username to the file of my previous question at the below link.

Looking forward to hearing back from you!


Hi @sungwoo_jang,

Thanks for posting your question. As it seems a decent mesh, I would recommend you to manually set the relaxation factors under Numerics tab as follows:

Please give it a go and make a new run, then let us know how it goes.


Hi Goncalves,

Thanks for your input. Unfortunately reducing the relaxation factor helped a bit, but didn’t really solve the issue, and feel it just delays it. With the super-low relaxation factor (0.01), the simulation does run towards the end, but the residual graph is not ideal as you see from below image. I also noticed abnormal velocity near the pressure outlet. Please check below images

One thing I am very confused is that I set the external flow volume boundary (except ground) as “Slip” and “Transparent” to the solar load. Still, I do notice the speed of wind dropping around the slip wall and the temperature rising at the transparent wall. Please check the below image. I triple-checked if I got any settings wrong, but I was not able to pinpoint.

I also tried unchecking “compressible” to see its behavior, and below is what happens. I just don’t understand why “-272C” air is creeping “inwards” from the pressure “outlet” when I set far-field temperature as OC.

Lastly, I am also curious if forcing the relaxation factor below 0.15 is typically a good practice? I haven’t seen any simulation program using a relaxation factor this low, and I am concerned that I am forcing the simulation and getting inaccurate results.

Please let me know how to resolve this. I have been putting a lot of simulation time (and my work hours) into this, and I don’t think I am getting anywhere…