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Difference between total pressure, total pressure in Stn and in Rel frames

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Old   January 21, 2016, 08:33
Default Difference between total pressure, total pressure in Stn and in Rel frames
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Hi all

I have been doing a bit of work with ANSYS CFX to model a radial flow turbine and I've noticed that when post processing the results I am given three options for the total pressure;

1) Total pressure
2) Total pressure in relative frame
3) Total pressure in stationary frame

I understand the difference between pressure and total pressure and why the total pressure changes as the reference frame changes. My question is what is the 'total pressure' referring to? Surely it should be either in stationary or relative frame, I don't understand how there can be three differenet values for it.

I have also noticed that for a lot of my simulations the 'total pressure' (but not the values for stationary or relative frames) is lower than the (static) pressure, I'm not sure how this is possible.
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Old   January 21, 2016, 11:54
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Taken from a previous post (by user "turbo"):
Quote:
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.
You can extrapolate the formulas for Total Pressure from the Total Temperature ones. Note that Total Temperature can also be lower than the static temperature if U >>> W, similar interpretation for Total Pressure and Static Pressure.

Some textbooks suggests that Rothalpy is short for Rotating Total Enthalpy, the variables without extension are equivalent (and perhaps should have been named) Rotating Total Temperature, and Rotating Total Pressure which are not the same that their "in Rel Frame" versions.

Here is a link to what "Rotary Stagnation Pressure" definition is http://downloads.hindawi.com/journal...001/424583.pdf. I must admit I have never heard of such name.

Hope the above helps
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Old   January 21, 2016, 13:20
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Here comes a confusion in turbomachinery community people !
As I posted before, CFX terminology is really unusual and weird to us. Just ignore "Total Pressure/Temperature/Density" names (without frame extensions) which are NEVER used in turbomachinery analysis. Please go to them with frame extensions in the Pre and Post.
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Old   January 21, 2016, 14:12
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For your knowledge, the names may be confusing to you and others (not an argument); however, their usefulness seems to be dependent on your turbomachinery background.

I would leave these articles from well respected turbomachinery sources:

http://ntrs.nasa.gov/archive/nasa/ca...9900014415.pdf

http://asmedigitalcollection.asme.or...?resultClick=1

http://gasturbinespower.asmedigitalc...icleid=1419991

http://task.gda.pl/files/quart/TQ2001/04/TQ0405C7.PDF <<Interestingly by Prof C Hirsch

If you read them carefully, you may learn how those quantities are used by some turbomachinery experts in the industry.
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Old   January 21, 2016, 15:51
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Hi Opaque,
Please tell directly us where you find the importance of using "Total temperature/pressure/density based on Rothalpy" in the presence of those cited references. I have read most of them already. My point is that nobody is interested in looking at those variables because, for example, the total temperature based on rothalpy is always constant in the rotor. CFX needs to rename them.

The first reference is using the rothalpy term once inside, because it is used to describe the governing equations for CFD which is of course because we need to use the rothalpy in the equations. In the CFX users' point of view, not the developer's, in evaluating CFD results those terms are NOT sought in turbomachinery people.

I hope you could understand the area of turbomachinery in this chance.
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Old   January 21, 2016, 16:38
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Here is my last contribution to this argument since I am not proclaiming myself to know better than anyone else about a specific subject.


I will just indicated a piece from the last citation (by Prof Hirsch) :
Quote:
Secondary Flows Components

It is known from the basic studies [27, 11, 15], as also confirmed by the analysis of Hirsch et al. [23], that the viscous effects progressively dominate the flow behavior when moving downstream in the impeller. An appropriate way of understanding the various flow contributions, due to viscous effects, is to follow the rate of increase of streamwise vorticity, describing the secondary flows generated by the rotary stagnation pressure gradients in the boundary layers, see e.g. [28]. This relation was derived initially by Hawthorne in 1974 in an unpublished report, and can also be found in [29].
...
Similarly, the other references also make use of both Rotary Total Temperature/Total Pressure to explain their points.

I leave it with you to learn from the citations, or dismiss them as you feel is appropriate.

Done!!
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Old   January 21, 2016, 17:05
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Hi Opaque,
Please do not stop discussion, which would be helping this forum anyways.
Thanks for the citation on the rotary stagnation pressure gradients to be mentioned in the development of secondary flows in the blade passage. Yes, it could be sought by such a specific need, but in our almost design practice, when the CFD is done, we look for (real) total pressure/temperature from the solution, not the rothalpy-based ones. I am not saying they should be gone, but CFX needs to rename them as "Total temperature/pressure/density Based on Rothalpy", instead of just what they are now. In lieu of Mach number terminology in CFX, they are NOT following the same convention, which is confusing !
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