Numerical study on hydrodynamic interaction characteristics of vortex-induced vibration of two side-by-side cylinders near the wall

This study investigates the two-dimensional (2D) vortex-induced vibration (VIV) of two side-by-side cylinders near a wall at a low Reynolds number of 200, using the direct numerical simulation. The intention is to reveal the VIV characteristics of the cylinders under the combined effects of the wall-cylinder interaction and cylindercylinder interaction. We examined four spacing ratios between the centers of two cylinders (T/D = 2, 3, 4, 5) and three gap ratios between the lower surface of the cylinder and the wall (G/D = 0.4, 0.9, 1.6) in the reduced velocity range of Ur = 3-8. The motion responses, fluid loads, vortex shedding frequencies, and wake modes are systematically analyzed in comparison with the cases of an isolated single cylinder and a near-wall single cylinder. The results show that the lock-in regime of the reduced velocity depends on the spacing ratio. The wall effects suppress the transverse vibration and enhances the streamwise vibration of the cylinders, leading to a larger transverse amplitude on the upper cylinder and a larger streamwise amplitude on the lower cylinder. Moreover, both the streamwise and transverse vibration frequencies on two cylinders are locked around the natural frequency for three gap ratios, significant different from those for isolated cylinders. Particularly, the soft lock-in phenomenon is observed for T/D = 2 or G/D = 0.4, where the vibration frequency is locked just below the natural frequency. Moreover, a modulation instability state is observed where the main peak switches frequently between the two peaks at Ur = 6 for T/D = 3 and G/D = 0.9. In addition, the wake patterns are sensitive to the gap ratio and spacing ratio, leading to abundant vortex shedding modes such as the irregular, 2S, P + S, dual-C (2S), dual-2SI, and dual-2SII modes.