  Using atomistic simulations, we investigate the dynamical behavior of a
single skyrmion interacting with an asymmetric linear protrusion array under
external ac driving. When the ac drive is applied along the $x$ direction, the
skyrmion moves along the hard direction of the substrate asymmetry in three
phases: a pinned phase with localized skyrmion orbits, a constant velocity
phase where the orbits become delocalized, and a reentrant pinned phase with
larger localized orbits. We measure the dependence of the skyrmion velocity on
the frequency and amplitude of the ac drive. All three phases appear for all
frequency values, and in the constant velocity phase the skyrmion velocity
depends only on the frequency and not on the amplitude of the ac drive. When ac
driving is applied in the $y$ direction, the skyrmion moves along the easy
direction of the substrate asymmetry and exhibits the same three phases as for
$x$ direction driving along with a fourth phase which, at high driving
frequencies, consists of a series of constant velocity phases, each with
different average skyrmion velocities. For low frequencies, the constant
velocity phase is lost and the skyrmion speed increases linearly with
increasing ac drive amplitude due to a Magnus boost effect. Our findings
suggest new ways to create reliable data transport for spintronic devices using
skyrmions as information carriers, where the skyrmion direction and speed can
be controlled by varying only the ac drive amplitude and frequency.
