topic Re: Boundary Conditions- Air Supply and extract System in CFD Forum

Boundary Conditions- Air Supply and extract System

Anonymous — Sat, 28 Apr 2018 19:51:03 GMT

Hi,

I'm studying mechanical engineering and I'm really new with the fantastic CFD world.
I'm modelling my first CFD model and I have a question (which may be silly but it's something that I want to understand).
I'm modelling a ventilation system for a room where there is a supply system and an extract system.
Each system has a dedicate fan. (please see abelow a sketch).

Ventilation System
With Autodesk CFD I'm modelling only the room with two openings (which represent the grilles).
For the supply opening I have set up the flow rate= 100l/s
For the return I would like to do the same (because at the end what I know is that the flow rate for the inlet is = to the flow rate at the outlet. The same should be the velocity right?)

Unfortunately I can't run this model because it seems that I can't define the inlet and outlet flow at the same time.
What I understand is that I should define the Inlet flow and the Outlet pressure if I want to run this model.
Can you explain me why?
If I have to set the Outlet pressure what should be the value? I can't use the environment pressure because the extract fan is creating a negative pressure and I don't know what is the value.
Can someone explain me how can I calculate the pressure at the extract grille?

Thanks for your answers.
I really appreciate your help!

Re: Boundary Conditions- Air Supply and extract System

Anonymous — Mon, 30 Apr 2018 05:03:14 GMT

to avoid over-constraining the numerical model, look for conservation of mass and boundary conditions. user's guide might be enough to help.
for the pressure, look for the definitions of static, absolute, gauge pressure, etc

Re: Boundary Conditions- Air Supply and extract System

Jon.Wilde — Mon, 30 Apr 2018 06:45:49 GMT

Welcome to the best of the engineering worlds 🙂

As this is incompressible, the pressure drop/rise will be the same no matter what pressure you assign to the outlet.

So a zero is OK here 🙂

As we use a mathematical model, you need to leave room for some error. This is why you cannot assign exact conditions everywhere, as @Anonymous rightly said, you would over constrain the model.

Does that help?

Jon