How to calculate energy dissipation of a CFD result?

jealor · January 4, 2019, 05:06

Hi everyone
I have conducted a 3D CFD simulation of a propeller. I have read from some paper that the volume integral of density multiply with turbulent dissipation rate(rou*e),will results in the power loss of the fluid field(assuming the friction of the wall is small). But my calculation showed that the power loss calculated by integration of (rou*e) is much smaller than the actual power loss,calculated by the torque and rotate speed and efficiency. So I was wondering, is any power loss is overlooked if just the integral of (rou*e) is taken into account? If so, is there any method that I can calculate the power loss of the fluid field from CFD result?

Thanks very much and best regards!

AtoHM · January 4, 2019, 06:44

Silly question: if you model only a segment of the whole thing, did you consider multiplying by number of blades?

jealor · January 4, 2019, 07:39

Quote:

Originally Posted by AtoHM

Silly question: if you model only a segment of the whole thing, did you consider multiplying by number of blades?

Thanks for your reply. I did considered the number of fluid passages and I also tried to model and simulate the whole field, and the result sames to be similar, so I think this may be a wrong way to calculate the power loss. However I just can't figure the right method.......

LuckyTran · January 23, 2019, 13:08

That is only the turbulent dissipation, there is also a laminar/molecular dissipation (which is related to the laplacian term in the navier-stokes).

There's also just dissipation coming from numerics.

jealor · January 25, 2019, 04:48

Quote:

Originally Posted by LuckyTran

That is only the turbulent dissipation, there is also a laminar/molecular dissipation (which is related to the laplacian term in the navier-stokes).

There's also just dissipation coming from numerics.

Thanks for your reply!

I find that there is dissipation loss for laminar flow and calculated it as the equation implies. However the laminar loss is much smaller than the turbulent loss. And the sum of the two loss is also smaller than the power loss calculated by the torque and power calculated on the shaft. So I was thinking there might be some power that can not be transferred to the flow field so it can not be calculated……

January 4, 2019, 05:06	How to calculate energy dissipation of a CFD result?	#1
jealor New Member Klxing Join Date: Oct 2016 Posts: 15 Rep Power: 10	Hi everyone I have conducted a 3D CFD simulation of a propeller. I have read from some paper that the volume integral of density multiply with turbulent dissipation rate(roue),will results in the power loss of the fluid field(assuming the friction of the wall is small). But my calculation showed that the power loss calculated by integration of (roue) is much smaller than the actual power loss,calculated by the torque and rotate speed and efficiency. So I was wondering, is any power loss is overlooked if just the integral of (rou*e) is taken into account? If so, is there any method that I can calculate the power loss of the fluid field from CFD result? Thanks very much and best regards!

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January 4, 2019, 06:44		#2
AtoHM Senior Member M Join Date: Dec 2017 Posts: 703 Rep Power: 13	Silly question: if you model only a segment of the whole thing, did you consider multiplying by number of blades?

January 23, 2019, 13:08		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	That is only the turbulent dissipation, there is also a laminar/molecular dissipation (which is related to the laplacian term in the navier-stokes). There's also just dissipation coming from numerics.