Reply To: wetting with free surface or large density ratio
The FORCE field describes the local force field to be applied per-cell and is read by the local (forced) dynamics. I.e. for simple cases one can set the FORCE field directly while more complex cases as Shan Chen require a separation between a user-defined EXTERNAL_FORCE and the computed FORCE.
As all fields, including (EXTERNAL_)FORCE are local to each cell a code modification should only required if multiple models apply to the same cell (in which case there likely are further model problems that have to be considered). => You are correct in your assumptions but feel free to ask further questions.
As a general note I would recommend to implement any new post processors and coupling operators in the new style introduced by OpenLB 1.5. This will require much less boilerplate than the legacy generator / processSubDomain approach while also transparently working on GPUs and improving performance also on CPU targets. The recent user guide for 1.6 contains FAQ entries explaining on how to write new operators in this style.