SimScale CAE Forum

Convergence mixed buoyant/forced convection


#1

Hi all,

I’m evaluating if simscale is suited for my needs so I’m running some simple analysis (link at bottom). I’m looking at a a simple room with displacement ventilation, in steady state. I seem to have trouble with convergence (residuals remain high). I’m new at CFD. Any help would be appreciated.

https://www.simscale.com/workbench?publiclink=22a5bd9a-d838-457f-a89f-bed789eee0f6


#2

@sbrideau,

To confirm that the solution is converged / not converged, in addition to residual plots, it is usually a good idea to monitor the solution field at some location in your domain. I would suggest that you add a Result Control Item for Area Averages on a face where the temperature does not have a fixed value boundary condition.

For example, in your case, you can add an Area Average result control item on the Occupants, and at the Outlet, so you can monitor the average temperature there.


#3

I just tried that and there is no additional line on the residual plot. How do you get the results?


#4

You can see the plots under “Area averages” in the “Simulations” drop-down in the Post-Processor tab.


#5

Great thanks. Definitely not converged. I’ll keep working. Maybe my inlet and outlet BCs are off.


#6

I’ve been playing around with BCs all day and can’t figure this out. Any tips on where to look?


#7

One reason could be that the zero-gradient boundary condition is not valid at the outlet - could you try modifying the geometry such that the outlet extends outwards a bit? A length of 4 meters would be a good starting point.


#8

In this simulation, on your old geometry, I tried runs with higher relaxation factors. In the simulation run titled “relax-1.0” you can see converged results in the postprocessor. The fluctuation at the outlet is probably because of the reason I mentioned in the comment above. Please let me know if extending the outlet outward solves that issue.