Reply To: Coupling force in NS and AD lattice
At first glance, your new DensityDrivenBuoyancyCoupling coupling operator looks ok.
Are you sure that the problem is in the coupling? Your error / warning could just as well be caused by a problem in the image output only. How does the VTK look? It could also be the case that everything code-wise works but the simulation diverges (i.e. a model error occurs).