[thomasmoura]

Hello all,

I'm doing some comparisons between CFX and Fluent in terms of speed and accuracy.

I've done CFX runs where I set the target residuals at 1e-6 (as company methodology). When doing the Fluent runs I've came across that the recommended values were 1e-3...

Anyone could explain me why this 3 order magnitude difference, as this has direct impact in the run duration?

[SPH_CFD]

Originally Posted by thomasmoura

Hello all,

I'm doing some comparisons between CFX and Fluent in terms of speed and accuracy.

I've done CFX runs where I set the target residuals at 1e-6 (as company methodology). When doing the Fluent runs I've came across that the recommended values were 1e-3...

Anyone could explain me why this 3 order magnitude difference, as this has direct impact in the run duration?

1e-3 is just default in numerical theory (lower this value incraeses the accuracy of solution). But we always set to 1e-6 for convergence criteria.

Sent from my SM-A520F using CFD Online Forum mobile app

[ghorrocks]

I suspect CFX and Fluent calculate the residuals by different methods, so the numbers need to be compared with care.

To compare CFX vs Fluent objectively you need to compare a well resolved model in both cases. This means mesh independent, adequate convergence and any other tuneable parameters correctly set. It also means that the same settings are not necessarily the best for both - for instance the mesh which gives mesh independence could well be different in both software. Do not assume the same mesh is suitable for both codes.

[thomasmoura]

Originally Posted by SPH_CFD

1e-3 is just default in numerical theory (lower this value incraeses the accuracy of solution). But we always set to 1e-6 for convergence criteria.

Sent from my SM-A520F using CFD Online Forum mobile app

Yes I understand that, but the "default" of CFX is 1e-4... Do you currently set 1e-6 in Fluent?

[thomasmoura]

Originally Posted by ghorrocks

I suspect CFX and Fluent calculate the residuals by different methods, so the numbers need to be compared with care.

Yes I suspect the same, so I am asking if anyone can confirm this and have a guideline like "fluent residuals are 10x greater" or something like that.

[SPH_CFD]

Originally Posted by thomasmoura

Yes I understand that, but the "default" of CFX is 1e-4... Do you currently set 1e-6 in Fluent?

Yes, always 1e-6 (at least) for both FLUENT và CFX or the orther commercial softwares.

[elia_l]

Originally Posted by ghorrocks

I suspect CFX and Fluent calculate the residuals by different methods, so the numbers need to be compared with care.

To compare CFX vs Fluent objectively you need to compare a well resolved model in both cases. This means mesh independent, adequate convergence and any other tuneable parameters correctly set. It also means that the same settings are not necessarily the best for both - for instance the mesh which gives mesh independence could well be different in both software. Do not assume the same mesh is suitable for both codes.

I think the same.. Residuals for fluent "deal more" with imbalances (conservation equations); for CFX should be more or less the difference between values that the same variable (p, u, v, w) assumes for two consecutive iterations.


Please correct me if I'm wrong.

[ghorrocks]

I don't about know the residuals in Fluent, but for CFX the residuals are calculated by a propriety algorithm which has only a general outline described in the documentation. So it is not possible to compare the exact approach as it is not public.