# Modelling a fan in a wind tunnel...help

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 August 2, 2017, 09:53 Modelling a fan in a wind tunnel...help #1 New Member   Join Date: Aug 2017 Posts: 5 Rep Power: 6 More specifically the fan is in a duct, said duct is moving at 13ms. The way I've set it up so far is that there's an inlet velocity of 13ms. Outlet pressure of 0. However I can't successfully setup to have the fan or a face to state there's an extra pressure region. The fan has a dynamic pressure of 10.4Pa. Whenever I try to implement the mass flow rate caused from the fan, the setup fails with return code of zero.

 August 2, 2017, 19:03 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,042 Rep Power: 134 You need to think about your system. You have defined a 13m/s inlet, which over the inlet area defines a flow rate. But then you try to define the flow rate at the fan and it is bound to be a different number. The software crashes as you are telling it to use two different flow rates simultaneously. You need to define your system in a consistent fashion.

 August 3, 2017, 08:24 #3 New Member   Join Date: Aug 2017 Posts: 5 Rep Power: 6 How would I go about setting it up properly then? Any tutorials to follow?

 August 4, 2017, 09:43 #5 New Member   Join Date: Aug 2017 Posts: 5 Rep Power: 6 So as a summary, I will no longer be modelling the entire duct moving at 13ms; I will mathematically solve for the point at which the duct pressure and fan pressure meet, and model the duct under this set of parameters? No longer implementing the fact the duct itself is moving.

 August 5, 2017, 06:13 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,042 Rep Power: 134 I missed the motion. The motion can be accounted for a few ways: * You can increase the inlet pressure to include the dynamic head. * You can have a boundary upstream of the duct with your 13m/s velocity. You then model your duct and the bypass path around the duct. You have to decide which is appropriate - it depends on whether the bypass flow is important.

 August 5, 2017, 07:56 #7 New Member   Join Date: Aug 2017 Posts: 5 Rep Power: 6 No worries. The bypass flow is not important in this case. The flow inside the duct is most important. Please do talk me through the dynamic pressure setup You see when we model the duct without any fan we end up with a high pressure bubble at the duct inlet. And flowlines show us flow reversal is taking place. So we hoped modelling a fan or at least what the fan does could move the pressure bubble back into the duct creating a pressure gradient rather than a bubble.

 August 5, 2017, 08:14 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,042 Rep Power: 134 If you use an opening for the entrance, set to opening pressure it will set the total pressure. The pressure to set it to is ambient plus the dynamic pressure.

 Tags ansys, cfx, ducting, fan