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April 15, 2012, 11:39 
density in Simplefoam internal flow

#1 
Senior Member

If the goal is to obtain pressure losses, I assume that even for incompressible flow the density should be a real value rather than 1.
Am I correct and if so where do I set that?
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April 15, 2012, 13:06 

#2 
Senior Member

Could you explain why you think that the density should be a real value? I'm not following you…


April 15, 2012, 15:14 

#3 
Senior Member

Assuming you want to know the actual total pressure losses, for instance, as well as your actual flow velocities, don't you have to specify the rho in order to get meaningful values for both quantities?
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Mihai Pruna's Bio 

April 15, 2012, 15:21 

#4 
Senior Member

Nope. OpenFoam gives you the pressure over the density as an output, not the pressure itself..
So, if you divide every term of your total pressure expression by the density you get: P/rho+0.5V^2=P0/rho and the total pressure you get is, again, divided by the density in openFoam so everything is consistent. 

April 15, 2012, 15:33 

#5 
Senior Member

so the pressure values you get in post processing will have to be multiplied by the density?
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Mihai Pruna's Bio 

April 15, 2012, 15:35 

#6 
Senior Member

Yes, correct.


April 15, 2012, 15:46 

#7 
Senior Member

thanks for the answers.
Just to be sure, if I wanted to generate a pressure difference of 101,000N/m2,for air, I would input 101000/1.3 (roughly) in my BCs and then multiply the results by 1.3 as I'm looking at them in paraView, right?
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Mihai Pruna's Bio 

April 15, 2012, 15:51 

#8 
Senior Member

Yes, correct!


April 15, 2012, 15:52 

#9 
Senior Member

and there's no way to input my rho and be able to give OF the actual numbers for pressure?
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Mihai Pruna's Bio 

April 15, 2012, 16:46 

#10 
Senior Member

constant/transportProperties: you can set the kinematic viscosity (nu) that is the dynamic viscosity over the density.


April 16, 2012, 09:10 

#11 
Senior Member

aha! thanks! how does it then determine the rho, if you input the kinematic viscosity?
is it possible it factors in the nu for the pressure input? I'm getting some huge velocities with a pressure at inlet of 100000. With just 1000 I'm getting a respectable 50m/s. However, I'm not sure this would make sense, because of the nature of the equation... See my post here: time step huge continuity errors, please take a look at my BCs Thanks for all the help so far!
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Mihai Pruna's Bio Last edited by mihaipruna; April 16, 2012 at 09:35. 

April 16, 2012, 12:39 

#12 
New Member
Nikhil
Join Date: Sep 2011
Posts: 9
Rep Power: 5 
In SimpleFOAM, the P/rho is being solved instead of p (divide the momentum equation by rho both sides). Also if you check your input file for P you will notice that the dimensions of p are m2/s2 instead of N/m2. The code doesn't need to calculate rho to solve the equations. All it needs is nu which it calculates from the singleTransportModel.H and viscosityModel.H. One just need to make sure that p/rho values are given in p dictionary.
No, it doesn't factor in nu. Is this a turbulent flow..?? 

April 16, 2012, 12:44 

#13 
Senior Member

yes, this is turbulent. nu is found in the transport properties.
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