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density in Simplefoam internal flow

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Old   April 15, 2012, 11:39
Default density in Simplefoam internal flow
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Mihai Pruna
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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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Old   April 15, 2012, 13:06
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Could you explain why you think that the density should be a real value? I'm not following you…
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Old   April 15, 2012, 15:14
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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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Old   April 15, 2012, 15:21
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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.
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Old   April 15, 2012, 15:33
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so the pressure values you get in post processing will have to be multiplied by the density?
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Old   April 15, 2012, 15:35
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Yes, correct.
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Old   April 15, 2012, 15:46
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Quote:
Originally Posted by lovecraft22 View Post
Yes, correct.
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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Old   April 15, 2012, 15:51
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Yes, correct!
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Old   April 15, 2012, 15:52
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Quote:
Originally Posted by lovecraft22 View Post
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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Old   April 15, 2012, 16:46
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constant/transportProperties: you can set the kinematic viscosity (nu) that is the dynamic viscosity over the density.
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Old   April 16, 2012, 09:10
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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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Old   April 16, 2012, 12:39
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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..??
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Old   April 16, 2012, 12:44
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yes, this is turbulent. nu is found in the transport properties.
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