Momentum transport in the modular solvers is generalised and run-time
selectable, supporting laminar, generalised laminar or non-Newtonian as well LES
or RAS turbulence modelling so it is clearer to name the momentum transport
model instance 'momentumTransport' rather than 'turbulence'.
Given that the number of solid solver modules is currently 1 and unlikely to
exceed 3 it is not very useful to maintain solid and fluid sub-directories and
easier to see the correspondence between the solver modules and tutorial cases
without.
