Numerical investigation on the effect of diameter ratio on wake induced vibrations of tandem cylinders

Abstract

Numerical simulations of WIV (wake induced vibration) for tandem cylinders with different diameter ratio in the impingement regime are performed for a low Reynolds number of Re = 150 and low mass ratio of m* = 2.0. Five different diameter ratios of d/D = 1/2, 3/4, 1, 4/3, and 3/2 are examined, in order to identify the effects of diameter ratio on 2-DOF (two-degree-of-freedom) vibration responses of the cylinder in the range of 2≤Ur ≤ 14. The results show that the diameter ratio and vortex mode play important roles in the vibration amplitudes and frequencies of the downstream cylinder. The WIV can be further classified into WIEV (wake interference and enhancement vibration) and WCV (wake controlled vibration) according to two distinct mechanisms of the amplification of cross-flow vibration amplitude. As d/D ≤ 1, the vibration frequencies of the downstream cylinder are significantly pulled down and interfered by the small vortices shedding from the small upstream cylinder. Meanwhile, a wider range of lock-in and enhanced vibration amplitudes are the features for WIEV. The inline direction of the downstream cylinder have large response amplitudes in a wide range of Ur (reduced velocity) and reach to the maximum value at d/D = 1. As d/D > 1, the hydrodynamic characteristics of the downstream cylinder are mainly controlled by the larger vortices shedding from the upstream cylinder. The downstream cylinder vibrates as the vortex shedding frequency of the larger upstream cylinder and the lock-in phenomenon is absent. WCV significantly increases the risk of fatigue failure of the cylindrical structure.