CFD Online Discussion Forums - The precision of "Foam::pow"?

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The precision of "Foam::pow"?

Hello guys,

I use OpenFOAM to do a simple calculation:

Code:

             scalar z = Foam::exp(Foam::sqr(0.544830258187397)/2.0);

             Info<<"z = "<<z<<endl;

             Info<<"z 16= "<<Foam::pow(z, 16.0)<<endl;

This is the output:

Code:

z = 1.16

z 16= 10.748

However, especially this z, it should be 1.16219666037989. Then my z16 will be 11.0783.

This is what I got from matlab:

Code:

z = 1.1621966603798944501

z6 = 11.0783

How can I handle this? if I use z with 1.16, z16 is not precise.

Thanks guys.

does it change if you change the writePrecision in the controlDict?

update, sorry, sorry, I have a typo in my code....Its my fault not the code

------------------------------------------------------------------

Quote:

Originally Posted by Henning86 (Post 565751)

does it change if you change the writePrecision in the controlDict?

Hi,

yeah, I tried in my complete solver to debug, it does not have any effect.

And this code above (just a test) does not need controlDict file..

Code:

#include "fvCFD.H"



int main()

{

             double sigma = 0.544830258187397;

        Info<<"sigma = "<<sigma<<endl;

        Info<<"Foam::sqr(sigma)/2.0 ="<<Foam::sqr(0.544830258187397)/2.0<<endl;

        Info<<"Foam::exp(Foam::sqr(sigma)/2.0) = "<<Foam::exp(Foam::sqr(sigma)/2.0)<<endl;

        double z = Foam::exp(Foam::sqr(sigma)/2.0);

        Info<<"Foam::pow(ans, 16) = "<<Foam::pow(z, 16.0)<<endl;

}

Code:

sigma = 0.54483

Foam::sqr(sigma)/2.0 =0.14842

Foam::exp(Foam::sqr(sigma)/2.0) = 1.16 //????????????????

Foam::pow(ans, 16) = 10.748

The key of this problem is: this function:

Foam::exp(Foam::sqr(0.544830258187397)/2.0)

is 1.16.

But it should be 1.16219666037989

The problem is probably the Info command

try Info.precision(16) i dont know what the exact command is.

google "precision cout". the message stream class should be similar