  Flows over two side-by-side circular cylinders exhibit fascinating flow
physics due to complex interactions between the coupled wakes. However, their
mutual interference effects have not been elucidated in a quantitative manner
thus far. In this paper, we create a mismatch between the two wakes by
introducing a size difference in the cylinder pair, such that the effects of
one wake on the other can be distinguished. Depending on the size and gap
ratios between the two cylinders, the coupled wake exhibits distinct dynamical
features including the quasi-periodic flow, synchronized flow, and chaotic
flow. Through advanced spectral analysis of lift coefficients and dynamic mode
decomposition of the flow fields, we reveal that the quasi-periodic flows are
mainly composed of two primary frequencies associated with vortex shedding in
the near wakes of the two cylinders. Both wakes impose their own frequencies on
the other, resulting in the beating phenomenon in the lift coefficients. The
triad interactions between the two wakes generate the sideband frequencies,
which are associated with modal structures that are mostly active in the far
wake. The transition from quasi-periodicity to synchronization is dominated by
the vortex shedding behind the larger cylinder, to which the wake of the
smaller cylinder locks in. These results reveal new insights on the coupled
wakes of two circular cylinders, and are pivotal for understanding more general
wake interaction problems.
