# Inclusion of density while calculating power

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 May 29, 2014, 08:27 Inclusion of density while calculating power #1 Member   Join Date: Nov 2012 Posts: 62 Rep Power: 6 Hello, I am having a little confusion right now. In the source codes of incompressible flow solvers of OpenFOAM I have seen that, pressure quantity is divided with density rho beforehand in order to get dynamic pressure. Now my question is that, as rho is already used to get dynamic pressure- do I need to use density while calculating power co-efficient of turbo machinaries? I mean by the definition of Cp = Power_Produced/ (0.5* projected area * density * velocity^3). Now do I need to add density in this calculation? __________________ Happy Foaming

May 29, 2014, 11:49
#2
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Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
Posts: 398
Rep Power: 14
Quote:
 Originally Posted by Naruto Hello, I am having a little confusion right now. In the source codes of incompressible flow solvers of OpenFOAM I have seen that, pressure quantity is divided with density rho beforehand in order to get dynamic pressure. Now my question is that, as rho is already used to get dynamic pressure- do I need to use density while calculating power co-efficient of turbo machinaries? I mean by the definition of Cp = Power_Produced/ (0.5* projected area * density * velocity^3). Now do I need to add density in this calculation?
Yes as you need it to make Cp dimensionless and pressure is not used in the definition. However, because the flow is incompressible, it's simply a constant.

 May 29, 2014, 11:56 #3 Member   Join Date: Nov 2012 Posts: 62 Rep Power: 6 Thanks for your reply. But I think I have solved the problem. It is little bit puzzling. I would try to give a short explanation. cfd.jpg The attached is the force file from propeller tutorial. As you may see the rhoinfo has been kept 1. But if you give rhoinfo as 1000 or any other quantity the output would change. However the best practice is to keep rhoinfo 1. Thank you all. __________________ Happy Foaming

May 29, 2014, 12:57
#4
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Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
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Quote:
 Originally Posted by Naruto Thanks for your reply. But I think I have solved the problem. It is little bit puzzling. I would try to give a short explanation. Attachment 31291 The attached is the force file from propeller tutorial. As you may see the rhoinfo has been kept 1. But if you give rhoinfo as 1000 or any other quantity the output would change. However the best practice is to keep rhoinfo 1. Thank you all.
No, you choose density, rhoInf, based on the fluid properties you're simulating and, if available, comparing to experiment. If you're working fluid is air at standard temperature and pressure, rhoInf ~ 1.2. If you're working fluid is water, rhoInf = 997. Your performance metrics should change with working fluids properties.

 June 9, 2014, 07:57 #5 Member   Join Date: Nov 2012 Posts: 62 Rep Power: 6 I am sorry to write again. Assume I am calculating which needs density say power co-efficeint. If the input of rhoinf is 1. Then the calculation would be simply like: Cp= Power produced/(0.5*Area*v^3). But if the rhoinf is 1.25, then the Cp would be like: Cp = Power produced/(0.5*1.25*Area*v^3). Same thing could go for pressure co-efficient too. Are my assumptions correct? __________________ Happy Foaming

 June 10, 2014, 16:18 #6 Senior Member   Chris Sideroff Join Date: Mar 2009 Location: Ottawa, ON, CAN Posts: 398 Rep Power: 14 Yes, that's correct

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