  Electrons behave like a classical fluid with a momentum distribution function
that varies slowly in space and time when the quantum mechanical
carrier-carrier scattering dominates over all other scattering processes.
Recent experiments in monolayer and bilayer graphene have reported signatures
of such hydrodynamic electron behavior in ultra-clean devices. In this
theoretical work, starting from a microscopic treatment of electron-electron,
electron-phonon and electron-impurity interactions within the Random Phase
Approximation, we demonstrate that monolayer and bilayer graphene both host two
different hydrodynamic regimes. We predict that the hydrodynamic window in
bilayer graphene is stronger than in monolayer graphene, and has a
characteristic `v-shape' as opposed to a `lung-shape'. Finally, we collapse
experimental data onto a universal disorder-limited theory, thereby proving
that the observed violation of Wiedemann-Franz law in monolayers occurs in a
regime dominated by impurity-induced electron-hole puddles.
