A cylinder in a steady current beneath a free surface is subjected to oscillations in the
streamwise direction. Techniques of high-image-density particle image velocimetry and
instantaneous force measurement provide the relationship between the instantaneous,
global flow patterns and the unsteady loading on the cylinder.
The existence of locked-on states for the fully submerged cylinder is addressed
in the companion study of Cetiner & Rockwell (2001). The present investigation
shows that it is possible to generate distinctly different locked-on states of vortex
formation, provided the cylinder is located immediately beneath the free surface. As
a consequence, the time-dependent transverse force is phase-locked to the cylinder
motion. In the event that a finite gap exists between the cylinder and the free surface,
however, instantaneous, jet-like flow through the gap acts to destabilize such locked-on
states. Lissajous representations of the forces demonstrate the degree of phase-locking
or, in some cases, a loss of lock-on and associated phase drift. Moreover, the degree of
submergence of the cylinder beneath the free surface has remarkable consequences for
the magnitudes of positive and negative spikes of the time-dependent force signatures,
as well as the averaged spectra of the transverse force. In turn, these alterations of the
unsteady transverse force are accompanied by substantial changes of the averaged
in-line and transverse forces.
Vortex systems can exist at locations both upstream and downstream of the cylinder.
They are due to vorticity from the cylinder surface and/or the free surface. The space–time
development of the entire system of vorticity concentrations is interpreted in
terms of the time histories of the relative velocity of the cylinder and the instantaneous
forces on the cylinder. In turn, these features of the vorticity field are related to critical
points near the free surface, deduced from topologies of the corresponding velocity
and streamline patterns. Despite the fact that changes in the patterns of vorticity
and the corresponding topologies occur in conjunction with large fluctuations of the
transverse force coefficient, the dimensionless strength of the vortices is below the
threshold for which distinguishable, localized deformations of the free surface occur.