Solution diverging on using double precision

qrie · January 22, 2013, 04:11

Hi,

I have an incompressible Navier Stokes code that was working fine as long as I was using single precision. When I converted to double precision, the residues are very high. Why is this happening?

Thanks!
Qrie

Martin Hegedus · January 22, 2013, 12:29

A bug in your code?

Sometimes issues like the following may occur

if (xx == 0.0) {
then do something
}

Under single precision this might work but double does not if xx is very small but not equal to 0.0, for example xx=1.0e-12. Single precision would round value to zero. Error tolerances should be used.

if (abs(xx) < 1.0e-8) {
then do something
}

qrie · January 23, 2013, 00:11

That helps! Thank you!

January 22, 2013, 04:11	Solution diverging on using double precision	#1
qrie New Member Join Date: Jun 2010 Posts: 20 Rep Power: 16	Hi, I have an incompressible Navier Stokes code that was working fine as long as I was using single precision. When I converted to double precision, the residues are very high. Why is this happening? Thanks! Qrie

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January 22, 2013, 12:29		#2
Martin Hegedus Senior Member Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19	A bug in your code? Sometimes issues like the following may occur if (xx == 0.0) { then do something } Under single precision this might work but double does not if xx is very small but not equal to 0.0, for example xx=1.0e-12. Single precision would round value to zero. Error tolerances should be used. if (abs(xx) < 1.0e-8) { then do something }

January 23, 2013, 00:11		#3
qrie New Member Join Date: Jun 2010 Posts: 20 Rep Power: 16	That helps! Thank you!