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Old   January 7, 2004, 12:19
Default Confusion about pressures in CFX5.6
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Hi, everyone

I am modelling natural convection flow in a simple rectangular box with openings connected to external. Buoyancy model with Boussinesq assumption is used. I am getting confused about the different types of pressures in CFX5.6.

It says on page 10 of Solver Modelling menu: "Note that the Results file 'pressure' values do not contain the hydrostatic contribution to 'pressrue', and this should be added to obtain actual values of pressure relative to the 'Reference Pressure' in these cases. If the flow is buoyant and the 'Reference Pressure' is set to 0 Pa, then the hydrostatic pressure contribution will be included".

1. is this true if you set 'reference pressure = 0' then the dydrostatic pressure contribution is automatically included?

2. What is the 'pressure' in the results file? Is it the 'Modified Pressure' defined as: P_stat + 2*rho*k/3 ?

3. Is the 'total pressure' in the results file defined as: P_tot =P_stat + 0.5*(U*U)? then what is the difference between the 'total pressure' and the 'modified pressure'?

4. Which pressure value is representing the natural driving force?

5. I understand that in CFX4 the 'modified pressure' was further modified to P_modified = P_stat + 2*rho*k/3 –rho_ref*g*xi. when using eddy viscosity models. Then the scalar 'Real Pressure' will represent its value in output file. Am I right?

Thank you very much for your help

Regards Forrest
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Old   January 12, 2004, 05:19
Default Re: Confusion about pressures in CFX5.6
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Hi, guys

Your comments about my questions are welcome. You don't bother to answer them all, just give me some points to help me understand what i was reading.

Thank you for your time in advance

Regards, Forrest
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Old   January 20, 2004, 10:41
Default Re: Confusion about pressures in CFX5.6
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The 'Pressure' field written to CFX-Post is: - relative to the reference pressure. This is a constant offset. - does not include the hydrostatic contribution due to the buoyancy reference density if the flow is buoyant. This is a linear variation added to the pressure field. If your buoyancy reference density is set to 0, then the pressure does have the full hydrostatic contribution. - does not include dynamic pressure (this is the difference between static and total pressure)

So to answer your questions: 1. Reference pressure is completely independent of the hydrostatic pressure. 2. Yes. 3. Yes. Actually, this equation is true only for incompressible flow. It is more involved for ideal gases, real gases, rotating domains, etc... 4. The 'modified pressure', which is the variable called Pressure, is the driving force in momentum. However, the 'true' or 'absolute' pressure is required to evaluate properties which depend on pressure (eg, density for compressible fluids). In CFX-5.7, the absolute pressure will also be written to the results file so you can visualize this as well (including the effects of reference pressure and hydrostatic pressure).
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Old   January 20, 2004, 13:11
Default Re: Confusion about pressures in CFX5.6
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Hi, Phil

Thank you for your helpful comments, need sometime to get my head around.

Regards, Forrest
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