# density in Simplefoam internal flow

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 April 15, 2012, 11:39 density in Simplefoam internal flow #1 Senior Member   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 7 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? __________________ Mihai Pruna's Bio

 April 15, 2012, 13:06 #2 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 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   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 7 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? __________________ Mihai Pruna's Bio

 April 15, 2012, 15:21 #4 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 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   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 7 so the pressure values you get in post processing will have to be multiplied by the density? __________________ Mihai Pruna's Bio

 April 15, 2012, 15:35 #6 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 Yes, correct.

April 15, 2012, 15:46
#7
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Mihai Pruna
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Quote:
 Originally Posted by lovecraft22 Yes, correct.
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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 April 15, 2012, 15:51 #8 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 Yes, correct!

April 15, 2012, 15:52
#9
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Mihai Pruna
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Quote:
 Originally Posted by lovecraft22 Yes, correct!
and there's no way to input my rho and be able to give OF the actual numbers for pressure?
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 April 15, 2012, 16:46 #10 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 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   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 7 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! __________________ 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   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 7 yes, this is turbulent. nu is found in the transport properties. __________________ Mihai Pruna's Bio

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