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Modelling a fan in a wind tunnel...help

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Old   August 2, 2017, 09:53
Default Modelling a fan in a wind tunnel...help
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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.
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Old   August 2, 2017, 19:03
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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.
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Old   August 3, 2017, 08:24
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How would I go about setting it up properly then? Any tutorials to follow?
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Old   August 3, 2017, 18:24
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No, your problem is not a CFD question. Your problem is understanding your system. You need to think about your system and when you understand how your system works then you can implement a CFD simulation modelling the correct system and it should work.

Here are some comments which may be applicable in your case:
* Fan performance is usually described using a fan curve. This is a curve of pressure rise versus flow rate.
* Your duct will have a flow resistance. The flow resistance will be a pressure drop from inlet to outlet, and will be the sum of entry and exit losses, friction losses along the duct and any other losses. The the flow resistance pressure drop will also vary with flow rate.
* If you draw the fan performance curve and duct resistance curve on the same graph the lines should intersect somewhere. This intersection point is where the flow rate has the same pressure gain in the fan as the duct.
* This point will be the steady state operating point of the system.

If this description is applicable to your model, I would model this by:
* opening (pressure) inlet
* source term to generate the pressure rise in the fan
* opening (pressure) outlet
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Old   August 4, 2017, 09:43
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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.
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Old   August 5, 2017, 06:13
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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.
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Old   August 5, 2017, 07:56
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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.
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Old   August 5, 2017, 08:14
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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.
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