My goal: calculate the volume flow rate through a gap with a predefined pressure difference. This gap is currently just a hole in the wall, but later on I want to put in elements which will limit the flow, and I want to see the difference in percentage volume flow rate with restrictions or not.
I started with modelling a wind tunnel with a gap of a certain area. I choose an internal analysis, air, and input 2 static pressures (difference of 10 Pa), 101315 at one side and 101325 Pa at the other side. You can see 2 gaps in the middle (I have excluded one in the analysis, I am now just focussing on one gap). I made sure that there is a high local mesh at the gap section. I have checked this with a cut plot.
I calculate the volume flow rate by putting an extruded boss in the gap and select the surface to calculate the volume flow rate, but excluding this boss in the simulation so it doesn't block the flow. After calculation the flow looks OK visually:
As you can see, the air only goes through one gap as the other gap is excluded from the analysis.
The interesting part is for me the result. It now gives me a volume flow rate of 2066 m3/h. I was pretty confident about this number, till I started playing with the length of the tunnel; the numbers changed!
I don't understand. Our company hired an external specialist who checked my model and input and said it was OK. He even imported my model in his own CFD software with the same input parameters but he got another value. However, his value is consistent. He afterwards calculated it by hand and his hand calculation was pretty close to the solution he got from his software.
So what am I doing wrong? The volume flow rate of a surface area with prescribed pressure difference shouldn't be that difficult, and shouldn't change if I change the length of the wind tunnel.
Can someone help me in the right direction? Thank you.