CFX Post Expressions number decimals

njiang · November 29, 2013, 15:48

Hi, there,

I am creating an expression in CFX post using the trendline created in the Excel as following:

(-798936626.593750*z^6/1[m^6] + 4687121164.076870*z^5/1[m^5] - 11450097996.080200*z^4/1[m^4] + 14908481102.838800*z^3/1[m^3] - 10912008484.852500*z^2/1[m^2] + 4256979247.793130*z/1[m] - 691537634.412929)*1[K]

when I evaluate the expression using 0.91 for z, the CFX gives me wrong number 1536K, while Excel is 762K.

How do I change the decimal setup/precision in CFX?

Thanks.

Nan

ghorrocks · November 30, 2013, 05:40

When you start the solver select double precision. But I vaguely recall that CEL is evaluated in single precision even with the rest of the numerics being double precision. I am not certain of this, but keep it is mind.

njiang · November 30, 2013, 09:29

Hi, Glenn,

Thanks. I do have double precision turned on for solver.

mvoss · December 13, 2013, 05:39

maybe you can use the "function"-function from cfx pre to overcome the rounding errors if the expression is supposed to be used in the simulation.
Be very careful about the resolution of the points being used in the function, because they will be interpolated piece-wise between two points.
You can also import data (like *.csv) for comparison in post - directly within the graph-tool.

November 29, 2013, 15:48	CFX Post Expressions number decimals	#1
njiang New Member Join Date: Jun 2011 Posts: 5 Rep Power: 15	Hi, there, I am creating an expression in CFX post using the trendline created in the Excel as following: (-798936626.593750z^6/1[m^6] + 4687121164.076870z^5/1[m^5] - 11450097996.080200z^4/1[m^4] + 14908481102.838800z^3/1[m^3] - 10912008484.852500z^2/1[m^2] + 4256979247.793130z/1[m] - 691537634.412929)*1[K] when I evaluate the expression using 0.91 for z, the CFX gives me wrong number 1536K, while Excel is 762K. How do I change the decimal setup/precision in CFX? Thanks. Nan

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November 30, 2013, 05:40		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	When you start the solver select double precision. But I vaguely recall that CEL is evaluated in single precision even with the rest of the numerics being double precision. I am not certain of this, but keep it is mind.

November 30, 2013, 09:29		#3
njiang New Member Join Date: Jun 2011 Posts: 5 Rep Power: 15	Hi, Glenn, Thanks. I do have double precision turned on for solver.

December 13, 2013, 05:39		#4
mvoss Senior Member Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 449 Rep Power: 20	maybe you can use the "function"-function from cfx pre to overcome the rounding errors if the expression is supposed to be used in the simulation. Be very careful about the resolution of the points being used in the function, because they will be interpolated piece-wise between two points. You can also import data (like *.csv) for comparison in post - directly within the graph-tool.