In this study, both experiments and numerical simulations have been performed to study sinusoidal oscillations of an identical pair of circular cylinders in a side-by-side configuration for various gaps in the still fluid. The key parameter of Keulegan-Carpenter (KC) number in the experiment is chosen between 0.5 and 20, Strokes number (β=Re/KC) values are selected from 350 to 2810 and gap ratio is selected from 0.5 to 3 in the experiments. Compared to the single cylinder cases, a large drag coefficient increase has been observed for gap ratios from 0.5 to 1.0. This phenomenon has later been confirmed by numerical simulations (in a smaller fixed Reynolds number of 120) using Lily-Pad, a solver built on boundary data immersed method (BDIM). In the numerical results, wake visualization shows that vortices shed from the cylinder pair will induce a jet between the gap, forming a vortex pair and accelerating the fluid particles away. This jet motion helps to expel energy from the structure into the fluid, and is confirmed by the energy flux calculation on the control volume around the cylinder pair, thus explains the enhancement of the drag coefficient.
Compared to the fluid structure interaction problem of cylinders open to the uniform flow that has been widely investigated, cylinders in the oscillatory flow has attracted less attention (Xu, 2013). However its significance cannot be undermined for its rich physics as well as its prevailing existence in all kinds of engineering scenario, especially in the ocean engineering field. As Fan (2016) pointed out that examples can be found in the offshore field such as the wave induced oscillatory flow around the risers, mooring lines, point wave energy generators, pump towers in the LNG ship experiencing sloshing load in the liquid tank induced by ship motion and blow-out preventers (BOP) forced to vibrate under the influence of upper riser motion, etc. In all these scenarios, the hydrodynamic model of the problems can be sufficiently simplified as fluid structure interaction in the oscillatory flow.