Effect of diameter ratio on the flow-induced vibrations of two rigidly coupled circular cylinders in tandem

Abstract

We present an experimental study on the effects on the vortex-induced vibrations (VIV), of the relative size of two rigidly connected circular cylinders in a tandem arrangement, with a fixed centre-to-centre separation of 1.3D, where D represents the diameter of the upstream cylinder. This separation distance was selected after the results obtained by Jiménez-González and Huera-Huarte (2017), where a high VIV sensitivity region was identified at such location in the wake of a cylinder oscillating in cross-flow. The flow-induced response of the system, which has one degree of freedom, has been analysed for several values of the diameter ratio d∕D≤1, where d is the diameter of the downstream cylinder. As the value of d∕D increases, the VIV response is attenuated, it being associated with important reductions in the transverse and in-line force coefficients. In addition, Digital Particle Image Velocimetry (DPIV) has allowed us to study the changes in the near wake and forcing introduced by the presence of different downstream cylinders. Thus, the flow dynamics is clearly driven by the relative size of the cylinders, and therefore, the size of the gap region. In particular, for small values of d∕D, a moderate disruption in the vortex formation and shear layer interaction of the wake of the upstream cylinder takes place. For intermediate values of d∕D, the downstream cylinder generates a strong local wake that results in the shedding of two co-rotating vortices at each side of the system, while markedly undermining the shedding from the upstream cylinder. In the limit of d∕D=1, a stagnant region develops in the gap between the cylinders, and the tandem behaves as a single bluff body, featuring a very attenuated flow-induced response.