# Fan questions in Caedium

Hello.

I am trying to model a fan entirely within my flow volume.
I have been doing this by using a fan inlet on one end of a very short cylinder and a fan outlet on the other end (of the same cylinder). I did this so as to approximate a fan whose blades occupy the space of that cylinder. This "fan" sucks air from one area inside the flow volume and blows directly into a plenum with vents. The area outside the plenum vents is contiguous with the flow volume space that feeds the fan's intake. Pretty simple.
At the far boundaries of the flow volume space are Inlet-outlet boundaries set on their default setting.

Then simulation converges beautifully without any finessing of initial conditions, relaxation factors, etc. All residuals are under 0.0001 except for Ux, which is lateral to the main flow direction and expected to be highest, though it is well under 0.001.
Lift and drag monitors of walls impacting the flow reach uniform levels and remain there for the many thousands of iterations I have run the simulation.
However, the pressure generated in the plenum is roughly twice that which the fan can generate at zero flow (P0). Considering that the plenum is "leaky" due to numerous vents, I am even more skeptical of the apparent pressure generated there. The pressure outside the plenum vents and elsewhere in the flow volume seems to be very realistic, as do the flow velocities.

I was curious what might be causing this apparently unrealistic pressure.

Should I be using the internal fan condition instead?
If so, do I need to use a double sided face for its geometry, or can I simply use the same short cylinder I was using and then somehow link the opposing boundary faces by making them both internal fans? I'm a bit unclear on how to model an internal fan. Again, I am trying to model something the actual thickness of the fan in question, though that is not critical.

Thanks in advance for any help!

### Use default outlet and set total pressure

The fan boundary conditions are intended for internal flows, e.g., computer case, HVAC ducting, with a standard (default) and well positioned outlet.

You could try replacing your outlet fan BC with a standard Outlet BC which will fix the outlet pressure. Then you need to fix the inlet velocity, by setting the Total Pressure (p0) = Reference Pressure (gauge pressure for incompressible flow = 0) + Dynamic Pressure, where Dynamic Pressure = 1/2.rho.v^2, where v = inlet speed.

### I will try this. Thank you.

I will try this. Thank you.