  A hydrodynamic flow of electrons driven by an oscillating electric field in
Dirac and Weyl semimetals is investigated. It is found that a double-peak
profile of the electric current appears and is manifested in a stray magnetic
field with peaks in one of the field components. The nontrivial current profile
originates from the interplay of viscous and inertial properties of the
electron fluid as well as the boundary conditions. Analytical results are
supported by numerical calculations in samples of different geometries such as
straight channels, nozzles, and cavities. The double-peak profile of the
current is found to be qualitatively insensitive to a specific form of the time
dependence of the oscillating electric field and is stable with respect to the
sample geometry. A phase diagram and criteria for observing the double-peak
structure are determined. In addition, it is shown that nozzles and cavities
provide an efficient means to locally enhance or reduce the fluid velocity.
