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Error occurs when I set the Opening Temperature using an expression

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Old   April 1, 2016, 02:48
Default Error occurs when I set the Opening Temperature using an expression
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Tony
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Hi, everyone,
I chose "Opening" as boundary type of the outlet of a compressor diffuser. For the section of "Heat Transfer", I chose "Opening Temperature" and want it to change with Mach number and pressure at the outlet. So I fill an expression in the blank of "Opening Temperature".(see option.jpg)

The expression named tTotalNext is shown below (see expression.jpg):
(massFlowAve(Total Temperature )@Diffuser Outlet)*(pOutNext /(massFlowAve(Total Pressure )@Diffuser Outlet))^(2/7)

Then an error immediately occurs after starting the run (see error.jpg).
A paragraph in the report:
Error processing expression 'Opening Temperature'.
The expression is invalid because:
Mach Number is not available for use in this term
Total Pressure is not available for use in this term
Total Temperature is not available for use in this term
mOld is not available for use in this term
pOld is not available for use in this term
Error processing expression: Opening Temperature = tTotalNext
+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| Z:\Program Files (x86)\ANSYS |
| Inc\v161\CFX\bin\winnt-amd64\ccl2flow.exe was interrupted by |
| signal QUIT (3) |
+--------------------------------------------------------------------+
This run of the ANSYS CFX Solver has finished.

It looks like CFX cannot get Mach number, Total Pressure etc. to calculate this expression. But the initial solution must already have those info. I just start learning CFX.

Why this error happens and can anyone tell me other ways to set opening temperature as an equation? Thanks in advance.
Attached Images
File Type: jpg error.JPG (22.7 KB, 13 views)
File Type: jpg expression.JPG (18.7 KB, 13 views)
File Type: jpg option.JPG (52.5 KB, 16 views)
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Old   April 1, 2016, 05:06
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Hmm
not sure if it helps, but I think I had a similar problem. Wanted to use "Absolute Pressure" and the solver crashed saying the variable is not available.
Speaking with the support revealed I had to use "p" instead of "Absolute Pressure".
A List of variables available is to be found in the reference guide of the documentation.
There you will find for example that "Total Temperature" is represented by a variable named "Ttot"
Chapter is named "Variables in Ansys CFX". Chapter number seems to be different depending on the Ansys Version. In Ansys 17 it is Chapter 16 of the Reference Guide.
Maybe this might help.
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Old   April 1, 2016, 05:45
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Thanks for your reply.
I substituted Total Temperature and Total Pressure with Ttot and ptot respectively. The solver crashed again. However, it's the first time I know Total Temperature can be written as Ttot. Thank you.

Here is the message from report:
Error processing expression 'Opening Temperature'.
The expression is invalid because:
Mach Number is not available for use in this term
Ttot is not available for use in this term
mOld is not available for use in this term
pOld is not available for use in this term
ptot is not available for use in this term
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Old   April 1, 2016, 06:03
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I find a thread (http://www.cfd-online.com/Forums/cfx...ple-solve.html) suggesting to modify RULES file. I got that file in CFX/etc/. I looked for the descriptions about Relative Pressure, which must can be imposed by expression, and Opening Temperature.
PARAMETER: Relative Pressure
Parameter Type = Real
Quantity Type = Pressure
Dependency List = ANY
Profile Variables List = p,Pressure, ptot,Total Pressure, \
ptotstn,Total Pressure in Stn Frame
Solver Name = PRES
Dynamic Reread Item = Yes
END


PARAMETER: Opening Temperature
Parameter Type = Real
Solver Name = TTOT
Quantity Type = Temperature
Dependency List = XYZT,rNoDim,aitern,citern,atstep,ctstep, \
acplgstep,cstagger,T
Profile Variables List = T, Temperature
Lower Bound = 0 [K]
Dynamic Reread Item = Yes
END

Now I don't know what is the proper way to modify the description of Opening Temperature. And can I update Opening Temperature by other ways like Fortran?
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Old   April 1, 2016, 06:16
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I add ANY in the Denpendency List of Opening Temperature. Then it works. Is there any risk doing this? It seems to remove all the restrictions for Opening Temperature.

PARAMETER: Opening Temperature
Parameter Type = Real
Solver Name = TTOT
Quantity Type = Temperature
Dependency List = ANY,XYZT,rNoDim,aitern,citern,atstep,ctstep, \
acplgstep,cstagger,T
Profile Variables List = T, Temperature
Lower Bound = 0 [K]
Dynamic Reread Item = Yes
END
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Old   April 1, 2016, 07:10
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Quote:
Is there any risk doing this?
Sure is. Modifying the RULES file is a really bad idea in most cases. The restrictions are put on things for a reason. I strongly recommend you do not modify the RULES file.

Have you looked at the documentation monkey suggested? For instance it also says the name for Mach Number is "Mach". But there other restrictions as well - some variables are restricted by which variables they can be a function of. I think this is actually the problem you are seeing - you cannot make the total temp at an opening a function of the variables it lists. If you kludge the RULES file to allow you to do this you will just make the solver unstable and the run will crash.... that is why the restriction is there in the first place.
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Old   April 1, 2016, 07:30
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Thank you, Glenn.
Yes, I had looked at the documentation. To make the trial simple, I just abbreviated total pressure and total temperature. ptot and toto still appeared in the message describing error.
Yes, I agree with you that my modification makes the restriction too slack. In case of any bad results from the case after modifying RULES file, I am also conducting a case with a constant and educated-guess opening temperature on another computer meanwhile.
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