Reply To: component velocity in Shanchen multicomponent model
The _external_ velocity field is used to realize the coupling og the lattices. As you observed this was originally stored / referred to using offsets into an array and has since been replaced by a significantly more flexible and maintainable descriptor field concept.
ForcedBGKdynamics use the forcing scheme by Guo (10.1103/PhysRevE.65.046308).
The fully model of the system results of these dynamics and the coupling instantiated by ShanChenForcedGenerator2D. There the value of the VELOCITY field is set to the local momentum (using computeJ i.e. computed from the populations).