Significant digits of variables in CFX

Jerry190607 · July 8, 2020, 16:00

Hi everyone,

I am using the expression in CFX to express a polynomial function with the coefficients of 16 significant digits. It seems that CFX does not read all the significant digits of the coefficient because the calculation result of the expression is incorrect. I am wondering how many significant digits are defaulted in CFX. Is there any chance to modify the significant digits in CFX? If so, how to do it? Thank you!

Regards,
Jerry

Gert-Jan · July 8, 2020, 17:59

Do you use double precision?

Jerry190607 · July 9, 2020, 03:06

Quote:

Originally Posted by Gert-Jan

Do you use double precision?

Yes, I did.

Opaque · July 9, 2020, 09:46

Can you share the expression?

I would like to see an expression that requires that many digits to be accurate within engineering limits. What kind of precision is being asked on the left-hand side? Perhaps the expression is not being written in a "less sensitive form" to round off.

Polynomials are supposed to be written using Horner's method or something along those lines to reduce round off as the higher powers come into play.

evcelica · July 13, 2020, 13:44

You could calculate a table in different software, and use this table as a user function in CFX Pre instead of the polynomial fit.

July 8, 2020, 16:00	Significant digits of variables in CFX	#1
Jerry190607 New Member Jerry Join Date: Sep 2019 Location: Queensland, Australia Posts: 17 Rep Power: 7	Hi everyone, I am using the expression in CFX to express a polynomial function with the coefficients of 16 significant digits. It seems that CFX does not read all the significant digits of the coefficient because the calculation result of the expression is incorrect. I am wondering how many significant digits are defaulted in CFX. Is there any chance to modify the significant digits in CFX? If so, how to do it? Thank you! Regards, Jerry

July 9, 2020, 09:46		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,873 Rep Power: 33	Can you share the expression? I would like to see an expression that requires that many digits to be accurate within engineering limits. What kind of precision is being asked on the left-hand side? Perhaps the expression is not being written in a "less sensitive form" to round off. Polynomials are supposed to be written using Horner's method or something along those lines to reduce round off as the higher powers come into play. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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July 8, 2020, 17:59		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,913 Rep Power: 28	Do you use double precision?

July 13, 2020, 13:44		#5
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	You could calculate a table in different software, and use this table as a user function in CFX Pre instead of the polynomial fit.