Dynamics and excitation in a low mass-damping cylinder in cross-flow with side-by-side interference

Abstract

Experiments have been conducted with a low mass-damping circular cylinder, elastically supported in a cross-flow, in the vicinity of a second stationary cylinder. The dynamic response, including amplitudes and frequencies of oscillation, together with the fluid excitation, were measured covering a large parametric space, consisting of variations in the gap distance between the cylinders as well as in the reduced velocity and Reynolds number. The flow dynamics in the near wake was also measured using planar particle image velocimetry. The results show how there is a strong wake interaction between the cylinders that greatly modifies the vortex-induced vibrations (VIV) of the elastically mounted cylinder when the centre-to-centre distance between the models is initially set to values smaller than $3.5D$, where $D$ is the external diameter. The wake interference leads to responding amplitudes that are reduced if compared to those of isolated cylinders undergoing VIV, while responding frequencies are increased. The transverse force coefficients observed in the lock-in region increase and the upper branch shifts to smaller reduced velocities. The phase between motion and excitation is also shifted and values measured in the lower branch of the response tend to be smaller than those typical of isolated cylinders. At the smallest separation distances investigated, the wakes of the cylinders are synchronised in an out-of-phase mode of shedding, characterised by a biased flow towards the oscillating cylinder.