  Hydrodynamic flow of charge carriers in graphene is an energy flow unlike the
usual mass flow in conventional fluids. In neutral graphene, the energy flow is
decoupled from the electric current, making it difficult to observe the
hydrodynamic effects and measure the viscosity of the electronic fluid by means
of electric current measurements. In particular, we show that the hallmark
Poiseuille flow in a narrow channel cannot be driven by the electric field
irrespective of boundary conditions at the channel edges. Nevertheless one can
observe nonuniform current densities similarly to the case of the well-known
ballistic-diffusive crossover. The standard diffusive behavior with the uniform
current density across the channel is achieved under the assumptions of
specular scattering on the channel boundaries. This flow can also be made
nonuniform by applying weak magnetic fields. In this case, the curvature of the
current density profile is determined by the quasiparticle recombination
processes dominated by the disorder-assisted electron-phonon scattering -- the
so-called supercollisions.
