  We investigate the features arising from hydrodynamic effects in graphene and
phosphorene devices with finite heat sources, using ab initio calculations to
go beyond Callaway's model and inform a full linearized scattering operator,
and solving the phonon Boltzmann transport equation through energy-based
deviational Monte Carlo methods. We explain the mechanisms that create those
hydrodynamic features, showing that boundary scattering and the relation of
sample dimensions to the nonlocal length are the determinant factors,
regardless of the relative importance of normal versus resistive scattering.
From this point of view, the nonlocal length reflects the ability of scattering
to randomize the heat flux, and we show that approximations made on the
scattering operator may have, through the value of nonlocal length, qualitative
consequences on the signatures of hydrodynamic behavior.
