Reply To: Printing velocity data
Functors such as BlockReduction2D2D are function objects, i.e. they implement C++’s function call operator operator().
You can look up documentation for BlockReduction2D2D and any other of OpenLB’s classes using DoxyGen (Documentation -> Developer Guide in the navigation, I seem to be unable to post links here). There one can see that for this particular class the function call operator is inherited from the BlockDataF2D functor.
Your guess at how your goal can be accomplished in 3D is correct. You can use BlockReduction3D2D to reduce a given super functor on arbitrary hyperplanes.