# Replacing Jet Fan with a Momentum Source Term

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 September 2, 2016, 04:57 Replacing Jet Fan with a Momentum Source Term #1 Senior Member     Amin Join Date: Oct 2013 Location: Germany Posts: 396 Rep Power: 11 Hi, I have a couple of jet fans in my domain and wanna replace them with momentum source terms in related direction. The unit for these kind of source terms is (N/m^3). It doesn't make any sense to me, what does this unit mean? how should we calculate this source term? For each jet fan and generally other similar turbo-machinery devices, we know the thrust, mass flow rate, angular velocity etc. I'm not sure if the (m^3) is related to the mass flow rate, since the thrust applied to the flow is not a function of the mass flow rate, I suppose. Any reply is highly appreciated

 September 2, 2016, 05:20 #2 Member   CT Join Date: May 2016 Location: Turkey Posts: 32 Rep Power: 6 Hello Amin, A jet fan generates thrust. When the air particules in the releated volume enter a jet fan, it quits with high speed. Jet fan throws it to the axis of rotation. N/m^3 means the force per meter cubic, which is thrust. You can calculate this value to divide force value of jet fan into volume of jet fan. You can obtain the force value(thrust) of jet fan in the catalogue of it.

 September 2, 2016, 09:09 #3 Senior Member     Amin Join Date: Oct 2013 Location: Germany Posts: 396 Rep Power: 11 Thanks for your response, That's the point, as far as I know thrust is a sort of Force (N) and is independent from the volume of fluid. I'm not sure if we could relate the volume of fluid to thrust, but if we assume that's true, what's the volume of fluid? is it the volume it blows during a full rotation? if so, the thrust is a function of mass flow rate.

September 3, 2016, 00:52
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Amin,
You're providing the the quantity of Newtons per cell volume in the case of a UDF or Newtons on a per cubic meter basis over the whole fluid zone volume you define as your fan (which fluent will then simply convert to Newtons per cell internally)

https://www.sharcnet.ca/Software/Ans...bc_source.html

ComputerGuy

p.s. Interesting idea. I've never done what you're trying to do.

With a loop over all cells in the domain I assume with your force balance for the fan
F= ((m*v2)-(m*v1))/(dt)
source=F/C_VOLUME(c,t)

Are you simply planning to take an estimated volumetric rate for your fan, turn it into mass flow rate using the density of the fluid, assume a ~0 m/s v1 inlet condition, and use an average dt across the fan to calculate your force term? Hope you'll share what you learn.

Quote:
 Originally Posted by amin.z Thanks for your response, That's the point, as far as I know thrust is a sort of Force (N) and is independent from the volume of fluid. I'm not sure if we could relate the volume of fluid to thrust, but if we assume that's true, what's the volume of fluid? is it the volume it blows during a full rotation? if so, the thrust is a function of mass flow rate.