https://www.openfoam.com/documentation/guides/latest/doc/guide-fos-field-wallHeatFlux.html
https://www.cfd-online.com/Forums/openfoam-post-processing/91755-calculating-heat-flux.html
https://www.cfd-online.com/Forums/openfoam-post-processing/237465-how-get-wall-heat-flux-using-bouyantbossinesqsimplefoam-openfoam-v2006.html