  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 diffcult to observe the
hydrodynamic effects and measure the viscosity of the electronic fluid by means
of electric current measurements. Nevertheless one can observe nonuniform
current densities by confining the charge flow to a narrow channel, where the
current can exhibit 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.
