  Theoretical and experimental studies have revealed that electrons in
condensed matter can behave hydrodynamically, exhibiting fluid phenomena such
as Stokes flow and vortices. Unlike classical fluids, preferred directions
inside crystals lift isotropic restrictions, necessitating a generalized
treatment of electron hydrodynamics. We explore electron fluid behaviors
arising from the most general viscosity tensors in two and three dimensions,
constrained only by thermodynamics and crystal symmetries. Hexagonal 2D
materials such as graphene support flows indistinguishable from those of an
isotropic fluid. By contrast 3D materials including Weyl semimetals, exhibit
significant deviations from isotropy. Breaking time-reversal symmetry, for
example in magnetic topological materials, introduces a non-dissipative Hall
component to the viscosity tensor. While this vanishes by isotropy in 3D,
anisotropic materials can exhibit nonzero Hall viscosity components. We show
that in 3D anisotropic materials the electronic fluid stress can couple to the
vorticity without breaking time-reversal symmetry. Our work demonstrates the
anomalous landscape for electron hydrodynamics in systems beyond graphene, and
presents experimental geometries to quantify the effects of electronic
viscosity.
