Section FLUX vs Node RFL

Hello, I’m using Calculix for thermal analysis of different geometries. To calculate the thermal Resistance, I need the heat flow through the simulated component. Hence I print the resulting Flux with the SECTION PRINT Keyword for my input and output surface. Additionally I print out the external concentrated heat sources for all nodes on the surfaces (NODE PRINT → RFL). In some cases the FLUX and the RFL values differ and i struggle to understand why. Also the FLUX values for the in- and output surface differ, which is not plausible to me, since I make static studies and all other boundary conditions are adiabatic and I have no heat sources in my geometry. Where is the difference between both variants and which is suited best to my purpose?

RFL (external concentrated heat sources, basically total flux) obtained with *NODE PRINT is a nodal variable while FLUX (heat flux) obtained with *SECTION PRINT is an integration point variable. More specifically, it’s printed in the integration points of the faces. For this reason I would expect some differences between them.

Thank you for the reply.
Which one do you think is more appropriate to determine the resulting heat flow?

For validation case 4 of ISO 10211 the resulting heat flux by RFL is similar to the FLUX result. But for other calculation, they differ significantly. Since the inflow and outflow values of the RFL variant are nearly the same, I would say this is more convenient. Can it be that the integration point values are inaccurate for less fine meshes?

I use PrePoMax for the preparation of my simulation studies. Since the *SECTION PRINT works on the integration points I assume, that not all elements needed for the Section Flux calculation are captured by the surface selection of PrePoMax. To verify this assumption, I repeated my last convergence study and added the heat flux values obtained from the Nodes. A comparison of RFL and FLUX shows, that RFL is very consistent across all meshes. Also, the difference between in- and outflow is for RFL in the range on 10^-7 to 10^-9, as I would expect. Hence, I conclude you have to pay close attention to the element selection for the use of *SECTION PRINT.

This is somewhat confusing but it seems that RFL is a reaction flux only (like reaction forces in solid mechanics but here it’s a reacton to prescribed temperatures). Even though it’s called total flux in one place in the user guide (it used to be called reaction flux there in manuals for older versions of CalculiX though).

FLUX variable, on the other hand, can be used to evaluate the total heat flow through the surface (made up of element faces).

I checked my Results again and I don’t understand the deviations appearing for the SURFACE FLUX results. Is it possible, that there is an error in the integration algorithm used to calculate the resulting heat flux? (@dhondt)