Vortex-induced vibration of two rigidly connected cylinders in side-by-side arrangement

In the offshore oil and gas engineering, multiple pipelines or riser pipes are often bundled together due to economical and operational considerations. Due to its engineering relevance, flow past multiple circular cylinders close to each other has been investigated extensively in the past decades. Extensive review of two circular cylinders in fluid flow can be found in Zdravkovich [1-3]. When two cylinders are in a side-by-side arrangement in a fluid flow, the biased flow regime was found when the gap between two side-by-side cylinders is between 0.1D to 1.2D, with D being the diameter of the cylinder, forming a narrow wake behind one cylinder and a wide wake behind another [I, 4, 5]. If two cylinders are in a tandem arrangement in a flow, the vortex shedding from the upstream cylinder was found to be suppressed if the gap between them is less than 2D to 2.5D depending on the Reynolds number [6-8]. Recently, vortex-induced vibration (VIV) of two circular cylinders has also been studied. Many studies have been conducted to study the VIV of an elastically mounted cylinder in the wake of a stationary cylinder. References [9, I 0] classified the response of two riser pipes in a tandem arrangement in two categories: Wake Induced Oscillation (WIO) and Vortex Induced Vibration (VIV). Numerical studies on VIV of two cylinders in a tandem arrangement were mainly conducted at relatively low Reynolds numbers. Cui et al. [II] simulated VIV of two elastically connected side-by-side cylinders in a fluid flow and found that the VIV locks onto the first- or the second mode frequency depends on the reduced velocity. VIV of two rigidly connected cylinders of different diameters has also been investigated both experimentally [12] and numerically [13] due to their relevance to the piggyback pipeline in offshore engineering. In this study, VIV of two identical rigidly connected cylinders in a side-by-side arrangement is investigated numerically by solving the two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations. Simulations are conducted for one-degree-of VIV of two rigidly connected cylinders for gap ratios (gap to diameter ratio) ranging from 0.5 to 3 with an increment of 0.5 and reduced velocities ranging from 2 to 30 with an increment of 0.5