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Absolute or Gauge Pressure for Total Pressure?

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Old   February 6, 2013, 12:27
Default Absolute or Gauge Pressure for Total Pressure?
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For turbomachinery applications I see the convention is to look at Total Pressure ratio. However, it is not stated anywhere I can see whether the static pressure component is absolute or gauge. This can make a massive difference and I would imagine it would have to be absolute to have meaning. But no terms I search for reveal the convention on this.
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Old   February 6, 2013, 13:34
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Good question, I also find that it is often not clearly stated what values to use, gage or absolute. When dealing with ratios, whatever reference being used for pressure needs to be consistent with temperature also. If you are using an absolute temperature scale (which we often do) then you also need to be working in absolute pressures.
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Old   February 6, 2013, 13:37
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It appears that in CFX when total pressure is calculated it is done so off of relative pressure. That makes me think it is a standard, but is by no means conclusive. I agree with you as well, consistency in physical quantities must be there, which is why it seems using absolute would be the better choice, but does not seem to be.
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Old   February 6, 2013, 15:01
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Quote:
Originally Posted by Torque_Converter View Post
For turbomachinery applications I see the convention is to look at Total Pressure ratio. However, it is not stated anywhere I can see whether the static pressure component is absolute or gauge. This can make a massive difference and I would imagine it would have to be absolute to have meaning. But no terms I search for reveal the convention on this.
Check your software manual. In Ansys Fluent, Static Pressure = Gauge Pressure
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Old   February 6, 2013, 15:02
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I imagine it is the same in CFX, which means Total Pressure is based on Gauge Static, but does that mean that is what is the common practice in turbomachinery performance maps when they list total pressure ratio?
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Old   February 6, 2013, 17:58
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Originally Posted by Torque_Converter View Post
I imagine it is the same in CFX, which means Total Pressure is based on Gauge Static, but does that mean that is what is the common practice in turbomachinery performance maps when they list total pressure ratio?
Hi guys, some thoughts that might help:

I know and have verified that Fluent reports the "total pressure" as a gage pressure. To get the absolute total pressure, you have to manually add the reference pressure.

Given that inlet and outlet pressures specified in boundary conditions in CFX are all inputted as relative pressures, I imagine the total pressure reported by CFX is also a relative pressure.

Pressure ratios and temperature ratios really only make sense if they are absolute quantities. In turbomachinery we always deal with thermodynamic pressure and temperature (which are absolute quantities). Only in CFD are these quantities handled in gage/relative pressure (for numerical accuracy). So I'm pretty willing to bet that the absolute pressure ratios are correctly plotted in at least "most good maps."
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Old   February 6, 2013, 18:26
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Thanks Lucky, I agree, CFX is relative pressure. And I also thought that only absolute pressures were useful for ratios.
It appears that to automate things, especially as I watch the convergence of important CEL quantities like Total Pressure Ratio I can either: 1. set the reference pressure to 0, but this of course creates numerical inaccuracy even potentially with double precision, or 2. add 101,325 to the massFlowAve total pressure at inlet and outlet in my CEL expression.

Then this ratio is, as you say, what would be relevant for performance mapping of PR vs. m_dot.
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