  Wind turbine's wake, being convectively unstable, may behave as an amplifier
of upstream perturbations and make the downstream turbine experience strong
inflow fluctuations. In this work, we investigate the effects of the
side-to-side motion of a floating offshore wind turbine (FOWT) on wake dynamics
using large-eddy simulation and linear stability analysis (LSA) on the NREL 5MW
baseline offshore wind turbine. Simulation results reveal that the turbine
motion can lead to wake meandering for motion frequencies with the Strouhal
number $St = fD/U_\infty \in (0.2,0.6)$ (where $f$ is the motion frequency, $D$
is the rotor diameter, and $U_\infty$ is the incoming wind speed), which lie in
the range of the natural roll frequencies of common FOWT designs. This
complements the existing wake meandering mechanism, that the side-to-side
motion of a FOWT can be a novel origin for the onset of wake meandering. The
amplitude of the induced wake meandering can be one order of magnitude higher
than the initial perturbation for the most unstable frequencies. The
probability density function of the spanwise location of the instantaneous wake
centers is observed having two peaks on the time-averaged wake boundaries and a
trough near the time-averaged wake centerline, respectively. It is also found
that the LSA can predict the least stable frequencies and the amplification
factor with acceptable accuracy for motion amplitude $0.01D$. Effects of
non-linearity are observed when motion amplitude increases to $0.04D$, for
which the most unstable turbine oscillations shift slightly to lower
frequencies and the amplification factor decreases.

Fluid Dynamics