Effects of upstream wake on vortex-induced vibrations and wake patterns of side-by-side circular cylinders

Abstract

In this paper, vortex-induced vibrations and wake patterns of two side-by-side circular cylinders in the wake of a stationary upstream cylinder are numerically investigated for a uniform free stream flow at the Reynolds number of 100. The three cylinders are in equilateral triangle arrangements with a center-to-center spacing ratio of 2.0–5.0. The considered mass ratio is 2.0 and the reduced velocity range is 0.5–15.0. The side-by-side cylinders are allowed to vibrate only in the transverse direction. Variations of response amplitudes, frequencies, mean position shifts and wake patterns with the reduced velocity and spacing ratio are investigated using direct numerical simulations. In comparison with the case without an upstream cylinder, amplitudes, frequencies, mean position shifts and hydrodynamic forces of two side-by-side cylinders are significantly impacted by the upstream cylinder wake and the associated wake-vortex-structure interactions. Key observations include (a) vibrations initiated at higher reduced velocities for a small to medium spacing ratio, (b) a wider range of large-amplitude vibrations, (c) a lower-frequency component, (d) appearances of periodic, quasi-periodic, chaotic and asymmetric vibrations, and (e) a hysteresis phenomenon. The asymmetric vibration is characterized by dissimilar vibration amplitudes of the two cylinders whereas the hysteresis is related to a transition of wake patterns when increasing versus decreasing the reduced velocities. Behind two side-by-side circular cylinders, the flip-flopping, anti-phase synchronized, in-phase synchronized, and chaotic patterns are identified.