# Physics of an incompressible fluid

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 March 23, 2015, 10:14 #81 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19 I gather Jonas might not have understood this "in you room the closed domain does not permit flow to go in/out from walls. Therefore, the fan must be introduced as an external force in the momentum equation to prevent continuity be unsatisfied. This force, if is not divergence-free acts as source term in the elliptic pressure equation." statement. Denaro is correct.

 March 23, 2015, 10:26 #82 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19 Here is the point that people have been trying to make, the fan provides a delta p, it DOES NOT provide a given velocity. So, in actuality you have assumed a velocity, then calculate the delta p, adjust the velocity, calculate the delta p, and iterate till you are satisfied. anon_h and H0T_S0UP like this.

 March 23, 2015, 14:16 #83 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,793 Rep Power: 71 A fan can be introduced by means of a dp/dtheta gradients in the tangential momentum equation. You can decompose this forcing term along the cartesian direction. See F. Nygard, H. I. Andersson DNS of swirling turbulent pipe flow, Int. J. Numer. Meth. Fluids 2010; 64:945–972

 March 23, 2015, 14:52 #84 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19 Do you mean dp/dr? Yes, the torque from the fan must show up in the flow.

March 23, 2015, 16:09
#85
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Jonas T. Holdeman, Jr.
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Quote:
 Originally Posted by Martin Hegedus Do you mean dp/dr? Yes, the torque from the fan must show up in the flow.
I don't think torque is relevant. One could imagine a fan with counter-rotating blades, or a grille that straightened the flow.

 March 23, 2015, 16:41 #86 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19 Yes, put I don't get the impression that is what is being talked about. We're talking about ceiling fans and disk actuator theory.

 March 23, 2015, 16:45 #87 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19 And yes, if we are modeling the individual blades there will be a dp/dtheta.

April 22, 2019, 10:40
#88
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Filippo Maria Denaro
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Quote:
 Originally Posted by Louise Charlton Sorry, a little lost here but this sounds like a discussion about potential flow (Panel methods, etc.)

No, potential flows share the divergence-free constraint but you can generilize the topic for more general viscous flows

 Tags action-at-a-distance, body force, incompressible fluid, physics