Method (1) and method (2) both produce similar results, however method (2)
is preferred for use in the rest of the simulations. This is because the
body force appears in a natural manner: as an extra term in the
governing equation. In method (1) the body force is produced by
considering the equilibrium distribution, not of the
actual fluid but of a fluid with a non-physical
`equilibrium velocity'. The model tests described in
sections 5.2.3, 5.2.4 and 5.3 and the
interfacial wave simulations in chapters 7 and 8 will all
have gravity implemented using method (2).