  Models of cylinders in the oscillatory flow can be found virtually everywhere
in the marine industry, such as pump towers experiencing sloshing load in a LNG
ship liquid tank. However, compared to the problem of a cylinder in the uniform
flow, a cylinder in the oscillatory flow is less studied, let alone multiple
cylinders. Therefore, we experimentally and numerically studied two identical
circular cylinders oscillating in the still water with either a side-by-side or
a tandem configuration for a wide range of Keulegan-Carpenter number and Stokes
number $\beta$. The experiment result shows that the hydrodynamic performance
of an oscillating cylinder pair in the still water is greatly altered due to
the interference between the multiple structures with different configurations.
In specific, compared to the single-cylinder case, the drag coefficient is
greatly enhanced when two cylinders are placed side-by-side at a small gap
ratio, while dual cylinders in a tandem configuration obtain a smaller drag
coefficient and oscillating lift coefficient. In order to reveal the detailed
flow physics that result in significant fluid forces alternations, the detailed
flow visualization is provided by the numerical simulation: the small gap
between two cylinders in a side-by-side configuration will result in a strong
gap jet that enhances the energy dissipation and increase the drag, while due
to the flow blocking effect for two cylinders in a tandem configuration, the
drag coefficient decreases.
