Unfortunately external forces can’t currently be used with the multicomponent model. It is something that I have been meaning to implement for a while. The reason that it doesn’t work is because the force is overwritten by the free energy model on each time step. (Line 361 of this method)
If you don’t mind editing the OpenLB source code directly then it would be quite simple to include a gravity force by editing the method linked above.
For example, to include gravity in the negative z-direction with a force density of 0.01 (in lattice units), the following line could be added before line 361:
`forceZ -= 0.01;
Note that this applies the same force to all of the different components. If you need different forces for the different components then it must depend upon the values of phi and psi.
Let me know if anything is unclear.