The effect of vortices structures on the flow-induced vibration of three flexible tandem cylinders

Flow-induced vibration (FIV) is a phenomenon oftentimes occurring in practical situations where flexible cylinders are immersed in an oncoming flow. Understanding the FIV characteristics in different vortex structures plays an important role in vibration control and utilization of engineering structures, such as offshore stacks and bladeless wind turbines. A detailed investigation on FIV of flexible tandem cylinders in the atmospheric boundary layer (ABL) under various vortex structures through setting strakes were conducted experimentally and numerically. The typical FIV responses and dynamic vortex evolutions at two specific spans, 1.4 and 3.2, were obtained in U-r is an element of[2, 35] to demonstrate the effect of different vortices. At the small span, FIV of tandem cylinders depends on the shear layers interaction, which behave as a uncircular bluff body with a pivot point near the downstream cylinder. Under the irregular small-scale vortices generated by the disturbed shear layer, the second instability of FIV of tandem cylinders at U-r >= 20 can be excited. At the moderate span, the fully developed wake interference predominates. Among the effect of three different vortex structures, FIV of tandem cylinders cannot be promoted by steady vortices; clear FIV responses at U-r <= 10 with larger amplitudes can be observed in periodic oscillating vortices; FIV response of downstream cylinders at small reduced velocities decreases but another instability with divergent FIV will re-occur at U-r >= 20 in irregular oncoming small-scale vortices. Finally, the vortices structures in different scales are assessed through the turbulence kinetic viscosity, and the instabilities of aerodynamic forces affected by the small-scale vortices are discussed.