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July 6, 2015, 09:53 
Confusing units in CFDPost

#1 
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Dear All,
I wonder if someone can clear this up for me: I find the units in CFDPost confusing, particularly wrt flow angles etc: For instance: 1) You seem to be able to 'set' the units for the various flow angles (imported Fluent quantities as well as native / calculated CFDPost quantities) in the Variables tab. This would lead me to assume that whatever unit you choose in the Variables tab (i.e. radians or degrees), is what is used throughout the remainder of the software. 2) However, if i define a custom variable, say alpha, using Velocity in Stn Frame Flow Angle (set in degrees) in the Variables tab, as Code:
alpha = 90  Velocity in Stn Frame Flow Angle Further, if i take say: Code:
Velocity Angle Code:
aveangle = massFlowAve(Velocity Angle)@X3 3) There is also a Set units pane in the Options / Preferences menu, but i'm not sure how this affects everything else. 4) Finally, do the trig functions in CFDPost want the angles defined as radians or degrees ... THen despite everything, it seems that contours etc are always display as DEGREES, so i actually have no idea how CFDPost imports / works with variables, especially flow angles ... so despite whatever units your variables are set / defined as, it will display them as Degrees. This is all very confusing, and since i am working with secondary flows in turbines, getting the flow angles in the various reference frames correct is very important. Any help from CFX gurus would be much appreciated  many thanks Cheers Jonathan PS I am importing my solutions from Fluent data files, if that makes any difference. Jon 

July 6, 2015, 11:24 

#2 
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ok, so after some reading i have some additional clarity, but not totally solved!
So for the benefit of anybody else reading: 1) If importing solutions from any nonCFX solver source, use: Code:
Edit > Options > Solution units ... to set the units of the data quantities which come from your solver. CFX includes the type of unit in its data files automatically, and these are automatically read in by CFDPost Code:
Edit > Options > Common > Units 3) If you check the following checkbox: Code:
Always convert units to Preferred Units i.e. Code:
3.1415 [rad]  Velocity Flow Angle I am still a little confused by the following: 1) If i set the correct solution file units, and then specified my preferred display units in the Options tabs, when i am defining a custom Expression, ie. alpha like so: Code:
alpha = 90 [deg]  Velocity in Stn Frame Flow Angle Confusing! Does anyone understand this? 2) Finally, can anyone tell me which angular units i should use for the trig functions (degrees or radians) or do you think this is consistent with the set Preferred Display Units as above? Many thanks and kind regards in advance, cheers jonathan 

July 6, 2015, 12:39 

#3 
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Not sure about the confusion. The documentation is very clear stating the units used for every builtin mathematical function in CEL (see CFX Reference Guide/Functions in ANSYS CFX/CEL Mathematical Functions  Table 15.1)
In any case, since you are doing trial and error, I would create an expression (not a variable), and input (for example) sin(30 [deg]) and evaluate to see the output. Try again by input sin( pi [rad] / 6) and compare. My initial test indicates the input is irrelevant since CFDPost will convert it to [rad], evaluate and display. Your mileage may vary though. Hope the above helps, 

July 6, 2015, 15:07 

#4  
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Hi, thanks for your reply, however, i am still confused by this ...
Quote:
Why does it evaluate to [radians] when preferred units are specified as [degrees]. This seems counter what even the CFDPost manual seems to suggest about how it works. This tends to make me unsure as to whether my interpretation as posted above is correct, hence the question on the expert CFX user forum for clarification ... best regards Jon 

August 31, 2016, 03:15 

#5 
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naveen kumar s
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i think similar doubt i have,,
how ansys will take if i give just sin((omega*t)90) 90 degree is for phase difference (and omega is in rad/sec) my question that 90 what it will consider as degree or radian, which i should give in degree or radian to get phase difference 90degree 

August 31, 2016, 12:27 

#6  
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The expression you are proposing is syntactically incorrect. Why ?
Quote:
omega = omega_value [rad s^1] Since t is time, omega x t = omega_value x time_value_in_s [rad] The next step in the expression is omega_value x time_value_in_s [rad]  90 which has units/dimensions mismatch, and the software will complain. However, if you write as sin((omega*t)90 [deg]) The evaluation step becomes omega_value x time_value_in_s [rad]  90 [deg] which can be evaluated w/o issues. Summary, if you use the units in your expressions consistently (regardless of units systems), the software will convert them as needed to whichever system it requires, and display the results in the "display" units requested. Hope the above helps, 

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