Numerical investigation of flow-induced vibrations of two cylinders in tandem arrangement with full wake interference

Abstract

This paper presents a numerical investigation on flow-induced vibration (FIV) of two elastically mounted cylinders in a tandem arrangement at subcritical Reynolds numbers. The tandem spacing between the cylinder centers is set at four cylinder diameters, placing the FIV problem within the full wake interference regime. A fluid-structure interaction numerical methodology based on a two-dimensional discrete vortex method is developed and applied to the FIV simulation of two cylinders. To investigate the effect of the upstream wake frequency and Reynolds number on the FIV response of the downstream cylinder separately, two experimental cases are designed. In one case, the FIV response is investigated with the Reynolds number varying and the upstream wake frequency fixed. In another case, the Reynolds number is fixed and the upstream wake frequency varies. In both cases, the FIV response of the upstream cylinder roughly resembles the lower branch of the typical vortex-induced vibration response of the single cylinder with the upstream reduced velocity varying from 6 to 9. For the downstream cylinder, the FIV response is characterized by two frequency branches: a dominant frequency branch associated with the wake interference mechanism and a secondary frequency branch associated with the upstream vortex shedding mechanism. It is found that the variation of the vortex shedding frequency in the upstream wake has little effect on the amplitude and dominant frequency of the downstream FIV response but directly causes the variation of the secondary frequency as the secondary frequency strictly follows the upstream vortex shedding frequency. The FIV response amplitude and dominant frequency of the downstream cylinder shows a strong dependence on the Reynolds number. The wake pattern of FIV shows that the FIV vortex shedding of each cylinder is directly related to the motion of itself but slightly modified by the wake of other cylinders for the full wake interference regime. The upstream wake vortices interfering with the downstream motion induce a flow in favor of the downstream FIV response.