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Old   January 28, 2008, 17:58
Default Pressure & Density functions
  #1
CFD newbie
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Hi

How do we make the pressure function of density. What is the expression for it. I need to find pressure fluctuations on the trailing edge of an airfoil. Thank you very much.
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Old   January 28, 2008, 18:33
Default Re: Pressure & Density functions
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S. Gatzka
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Is it a compressible or incompressible flow?
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Old   January 28, 2008, 18:48
Default Re: Pressure & Density functions
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Compressible indeed!
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Old   January 28, 2008, 18:58
Default Re: Pressure & Density functions
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S. Gatzka
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And isentropic?
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Old   January 28, 2008, 19:11
Default Re: Pressure & Density functions
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Well i will be using smagorinsky's model to model the effects of isentropic turbulence on my large eddies. What this has to do with the expression for the function of pressure.

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Old   January 28, 2008, 20:10
Default Re: Pressure & Density functions
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S. Gatzka
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On isentropic flows the pressure and density varies with the mach-number. Is that what you mean?
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Old   January 28, 2008, 22:18
Default Re: Pressure & Density functions
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otd
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To get the pressure, you need the density and the temperature - AND an equation of state for the fluid.

For air, the ideal gas law is often used.

If you're doing a CFD flow for a compressible fluid, you'll usually be solving the energy conservation law in addition to 1, 2, or 3 momentum equations and the mass conservation law.

And of course appropriate turbulence models.

Your question appears to be pretty fundamental. What have I missed?
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Old   January 29, 2008, 07:21
Default Re: Pressure & Density functions
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I need to obtain pressure fluctuations at the trailing edge of my airfoil. In order to do that i will have to run a compressible simulation. Compressible flow is activated in CFX using one of the following:

1) By using the total energy model in conjunction with an ideal gas or real fluid.

2) BY using the total energy model in conjunction with general fluid whose density is a function of pressure.

Now i would like to opt for second option. Then how do i make my density function of pressure. You guys have confused me even more with your answers. I am new CFD user, please keep it simple. Thanks
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Old   January 29, 2008, 07:51
Default Re: Pressure & Density functions
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sega
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I don't get it right.

Maybe you can use the isentropic relation between pressure and density for an ideal gas?

p=C*rho^gamma

C is a constant at a reference value. C=p0*rho0^-gamma.

But I think this is not what you are thinking about ...
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Old   January 29, 2008, 11:12
Default Re: Pressure & Density functions
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otd
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Ask the CFX technical support guys (or gals?).
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Old   January 29, 2008, 11:24
Default Re: Pressure & Density functions
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Thanks for your reply otd. I was not expecting it to be that technical especially when i have access to experts CFD users like yourself.

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Old   January 29, 2008, 12:56
Default Re: Pressure & Density functions
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Ananda Himansu
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You should use Option 1 (the total energy model in conjunction with an ideal gas or real fluid). Pick an ideal gas unless your Mach number is very very high or Reynolds number is very very low.

Option 2 is probably meant for special situations, such as barotropic flow (http://en.wikipedia.org/wiki/Barotropic), and you should avoid it unless you are dealing with that sort of situation where the isobars coincide with the isopycnics.
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Old   January 29, 2008, 13:47
Default Re: Pressure & Density functions
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Thank you very much Himansu. So in the first case i do not need to make my density function of pressure. In an ideal gas they should be proportional, if density changes so will pressure. Is that right!

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Old   January 29, 2008, 14:31
Default Re: Pressure & Density functions
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Ananda Himansu
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In an ideal gas, the density is a function of both pressure and temperature, as stipulated by the equation of state. By picking Option 1 and using an ideal gas, CFX will calculate two out of three (density, temperature, pressure) using the conservation laws of mass and energy. CFX will use the ideal gas equation of state to automatically calculate the third of the three, so that you do not need to worry about specifying the dependence of density on pressure.
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Old   January 29, 2008, 15:13
Default Re: Pressure & Density functions
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Thank you so much Himansu. This should solve my problem.

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