SimScale CAE Forum

Ducting Boundary conditions for known inlet duct gage pressure and an outlet that sends air into the atmosphere at 0 Pa gage


I have an inlet duct the I pressurize with a variable speed fan that draws in ‘outside’ air by increasing the rpm until the inlet duct pressure reaches 961 Pa.

The inlet duct goes into a large plenum that has an outlet hole (approximately 100x the area of the inlet duct) in it that vents to ‘outside’ air.

I would like to know the inlet duct air velocity and the average pressure in the plenum.

I have a test project setup for this, but I can not figure out the correct Inlet and Outlet boundary conditions to use.

Everything I have tried so far seems to result in area averages on all the plenum walls of 961 Pa which is not logically possible or the results diverge.

Any ideas on this one?


DalesStuff: My Topics and Projects that I need quick access to :wink:

Hi @DaleKramer
Just perform a pressure driven analysis, Just apply three B.C pressure inlet and pressure outlet and walls condition. For more go through the link.


I have tried again the simple and obvious ‘Pressure Driven Analysis’ setup (this sim run is still there), with Inlet face set to 961Pa and the Outlet face set to Mean pressure of 0 Pa and the sim run looked to be converging (albeit at much higher residuals than I like) but the Area averages on the plenum Faces is just not right and it eventually diverged on all faces (including the Inlet and Outlet faces)

I am using ‘New Mesh Preview’ for my meshes but maybe I will go back to Hex Parametric and see if that helps…


HI @DaleKramer
Have a look at the run 5, unfortunately the run stopped due to max runtinme.


Darn, I left it run while I got some zzzz’s. I always forget to change that on new projects…

Anyway, restarted now.

I am trying all fineness and both algo’s.

Things are just strange here, such a simple project to have go awry.

I am used to incompressible sims where I expect to have mostly attached flow and which needs a boundary layer.

Strange to see this convergence/result behavior here where flow in almost never attached.

Not sure where to go next…


Do you have any set of result you are expecting from simulation.


No, just what my brain expects and that is not often correct :wink:

I expect that since the exit is 100x the area of the inlet, that all the wall area averages will have an average pressure significantly lower than 961 Pa… (in the area of ~10 Pa which is 100x less than inlet pressure) …


and what about velocity inlet?


I do not have that info…

I have a known diameter and free air thrust that a fan can produce (hence known Pa)…

With a 100x increase in exit area, I do not expect that to change the fan Pa data much…


Can you share the fan data used to calculate Pressure.



@power_users Please help and have a look at the project, I am now stuck


I have a round axial flow fan of 50 mm diameter that produces 770 g of static thrust (961 Pa)

I know that it is not the inlet area that I have in this project, but I can get a fan that would match what I am showing…


Have a try running Run 5 for full step, i think it will get the better results.


What is full step?


Means the simulation end time specified.


Iteration ‘End Time’ or max real time?

Sim run ‘TestDuct2 659814v mesh …’ is running now to 2000s interation time and 300000s real time… and results are already diverging…

‘Average area Top Wall’ p is now at 3e+5 at 1730s …


Iteration end time. RUN 5 of this link.


That just takes me to my dashboard…


Oh sorry, it was set as private project.


If you just open my link in the first post again, you should see the current status of my runs in real time…