Confusing CFX-Post Terminology in Turbomachinery Applications
 

If you are simulating turbomachine CFD using CFX, and if you have not realized yet what I am going to say, this needs to be addressed not to confuse people.

In the CFX-Post, by default, there are flow variables of,
- Mach number (that is the relative Mach we say)
- Mach number in Stn Frame (that is the absolute Mach we say).

In lieu of the above, as for total temperature we can easily regard that,
- Total temperature = Relative total temperature
- Total temperature in Stn Frame = Absolute total temperature.

However, that is NOT true !

For pressure, temperature and density, CFX has 3 kinds :

1. No extensions
2. in Stn Frame
3. in Rel Frame

which mean,

1. Based on the rothalpy, I = h + (W^2-U^2)/2
2. Based on the absolute total enthalpy, ho = h + C^2/2
3. Based on the relative total enthalpy, ho,rel = h + W^2/2

where h = static enthalpy, W = relative velocity and C = absolute velocity.

Accordingly, the 3 kinds of total temperatures are (for a perfect gas),

1. “Total Temperature” = To,rothalpy = T + (W^2 – U^2)/2Cp
2. “Total Temperature in Stn Frame” = To,abs = T + C^2/2Cp
3. “Total Temperature in Rel Frame” = To,rel = T + W^2/2Cp

and also the 3 kinds of total pressures (and total densities) are in the same way.

In the Post, a plot of the streamwise averaged total temperature shows an almost constant line in the rotor domain, because of the conservation of the rothalpy. But our common sense says it is wrong because "total temperature" that we know should increase in compressors and decrease in turbine rotors.

Ansys should know that the kind 1 above is never used among turbomachinery people, leading to useless terms only confusing by its name. We say two kinds only like “absolute” and “relative” which are the kind 2 and 3 above.

I have asked Ansys support to educate people who would be easily confused by the CFX unusual terms. In CFX-Post, you should not call “Total Temperature/Pressure/Density” as they are, but should instead call them “Total Temperature/Pressure/Density based on Rothalpy” !

Interesting comment; however, I am really surprised by your confusion.

ANSYS CFX have been around quite sometime using such terminology. I recall the rothalpy based quantities started to show later, and I do not recall such vocabulary creating that much confusion in this forum.

Regarding which ones are used by turbomachinery people, your mileage may be different than others, and you seem to be forcing your views on what is useful and what it is not.

I have similar views on other areas of physics modeling, and I have learned to agree to disagree.

Hi, Opaque. Thanks for your reply.
The reason why you were surprised by my confusion looks not to be familiar to turbomachinery area. As stated in the title, this info is oriented to turbomachinery aero community, because "Mach" terminology is not consistent with "Pressure/Temperature/Density" terminology in CFX-Post, leading to an easy confusion among turbomachine people.