  The focus of this work is to dissect the physical mechanisms that drive and
sustain flow-induced, transverse vibrations of cylinders. The influence of
different mechanisms is quantified by using a method to partition the fluid
dynamic force on the cylinder into distinct, physically relevant components. In
conjunction with this force partitioning, calculations of the energy extracted
by the oscillating body from the flow are used to make a direct connection
between the phenomena responsible for force generation and their effect on
driving flow-induced oscillations. These tools are demonstrated in a study of
the effect of cylinder shape on flow-induced vibrations. Relatively small
increases in cylinder aspect-ratio are found to have a significant influence on
the amplitude of oscillation, resulting in a large drop in oscillation
amplitude at reduced velocities that correspond to the upper range of the
synchronization regime. By mapping out the energy transfer between the fluid
and structure as a function of aspect-ratio, we identify the existence of a
low-amplitude stationary state as the cause of the drop in amplitude.
Partitioning the fluid dynamic forces on cylinders of varying aspect-ratio then
allows us to uncover the physical mechanisms behind the appearance of the
underlying bifurcation. The analysis also suggests that while vortex shedding
in the wake is necessary to initiate oscillations, it is the vorticity
associated with the boundary layer over the cylinder that is responsible for
the sustenance of flow-induced vibrations.
