  We present a numerical study on the transverse flow-induced vibration (FIV)
of an elastically mounted sphere in the vicinity of a free surface at
subcritical Reynolds numbers. To begin, We verify and analyze the mode
transitions and the motion trajectories of a fully submerged sphere vibrating
freely in all directions for the Reynolds number up to $30\,000$. Next, the
response dynamics of a transversely vibrating sphere is studied for three
values of normalized immersion ratio ($h^*=h/D$, where $h$ is the distance from
the top of the sphere to undisturbed free-surface level and $D$ is the sphere
diameter), at $h^*=1$ (fully submerged sphere with no free-surface effect),
$h^*=0$ (where the top of the sphere touches the free surface) and $h^*=-0.25$
(where the sphere pierces the free surface). At the lock-in range, we observe
that the amplitude response at $h^*=0$ is decreased significantly compared to
the case at $h^*=1$. It is found that the vorticity flux is diffused due to the
free-surface boundary and the free surface acts as a sink of energy that leads
to a reduction in the transverse force and amplitude response. When the sphere
pierces the free surface at $h^*=-0.25$, the amplitude response at the lock-in
state is found to be greater than all the submerged cases studied with the
maximum peak-to-peak amplitude of $\sim2D$. We find that the interaction of the
piercing sphere with the air-water interface causes a relatively large surface
deformation and has a significant impact on the synchronization of the vortex
shedding and the vibration frequency. Increased streamwise vorticity gives rise
to a relatively larger transverse force to the piercing sphere at $h^*=-0.25$,
resulting in greater positive energy transfer per cycle to sustain the
large-amplitude vibration. Lasty, we study the sensitivity of FIV response on
the mass ratio, $m^*$, and Froude number, $Fr$, at the lock-in state.
