parking garage ventilation help

[montyrivers]

Hi, I'm trying to model a parking garage ventilation system, and am having trouble getting results.  I have setup flow rates for each exhaust and supply duct where a diffuser would be, but when I run the solution, I just get that everything is at zero velocity.  I checked and the volume in the garage is set to Variable and the material type is Air.  I must be doing something wrong, but I have tried many different things to get some results and can't seem to make it work correctly.  I tried to attach my .cfz file, but it's too large.  I am hoping that someone can give me some advice on how to proceed.  Thanks!

[nmuoio]

You mentioned that flow rates are applied to each supply and exhaust duct.  That may be the problem.  The flow rates should only be applied to the supply ducts.  The exhaust ducts should have a pressure=0 boundary condition.  The software will calculate the flow through the exhaust ducts.

The software labels any boundary with a prescribed flow rate as an "inlet" and any boundary with a P=0 as an "outlet".  Useful information (volume flow, mass flow, pressure, ...) for each inlet and outlet can be found in the "summary history".  This would be a good place to find the calculated exhaust flow rates.

You also mentioned that you set the garage air volume to "Variable."  The assumes that buoyancy is significant.  If the airflow is dominated by the prescribed flow rates through the supply ducts, then you can leave it as "Fixed."  But whether it is "variable" or 'fixed" won't cause your particular problem.  Your exhaust boundary conditions, as stated above, is the likely cause.

[Royce_adsk]

Just switch 1 of your returns to a 0 pressure.

[montyrivers]

Thanks for the detailed advice.  I spent a good amount of time this week trying to solve the issue I was having and have learned more about what is required by the software.  I also created a smaller model instead of my big parking garage to simplify things and save time while I troubleshoot the simulation.  In this model, I created two ducts, one for supply and one for return.  I use air as the material for the ducts, and of course the filled volume of the room, and left it fixed as you suggested.  For boundary conditions, I applied a volume flow rate on the outside surface of the supply duct, and applied zero pressure to the outside surface of the return duct.  With this, I get some results, but not correct results.  It seems something is 'blocking' the flow from entering the room.  I have set all of the cross-sectional surface throughout the ducts to zero pressure as well, but it seems there is still blockage at the wall.  There is velocity of the air inside the room, however after adding some traces to the inlet, it seems that most traces do not enter the room.  I think there is a computational error that is allowing some flow into the room.  I've attached a cfz file of my simplified room.  Some boundary conditions I'm sure are wrong as I was trying lots of things to get it to work.  Any advice is much appreciated, and thanks already for the two replies I got earlier.  They were very helpful.

Monty

[Royce_adsk]

You are still overconstraining your model setup.

Lets take this step by step:

1) The use of 0 velocity is awkward.  Instead of using the 0 velocity I would prefer to see you just model the duct with a 3D volume.  

2) Pressure boundary conditions should not be applied at every face between air volumes.  You just need a flow rate at one end and a 0 pressure at the other to represent the inlet and outlet.  The software assumes that neighboring fluid volumes are continuous.

3) Results: