  The relationships between characteristic large-scale wake structures
appearing behind a freestream-aligned circular cylinder are investigated and
discussed from the velocity field obtained by wind tunnel tests. The tests were
conducted under a supportless condition using a magnetic suspension and balance
system and stereo PIV measurements at a Reynolds number of $3.46\times10^4$.
The velocity fields were analysed with a modal decomposition combining
azimuthal Fourier decomposition and proper orthogonal decomposition. The wake
behind the freestream-aligned circular cylinder with three different fineness
ratios of 1.0, 1.5 and 2.0 was investigated, and the wake structures in a
nonreattaching flow formed by the cylinder at the fineness ratio of 1.0 are
mainly discussed in the present study. Four characteristic large-scale wake
structures of recirculation bubble pumping, azimuthal shear mode, large-scale
vortex shedding and streaks are identified and mainly focused on in the present
study. The state of the vortex shedding is classified into three;
anticlockwise/clockwise circular and flapping patterns. Each state has a
relationship with the azimuthal shear mode and it tends to appear when the
state is circular. Furthermore, from the analysis of the relationship between
modes, the recirculation bubble pumping is found to be related to the vortex
shedding position in the radial direction and the strength of the streaks.
Particularly, analysis of causality shows that the recirculation bubble pumping
is affected by them in the low-frequency range.
