Wake-induced vibration of tandem spheres

Flow-induced vibration (FIV) of spheres, especially wake-induced vibration (WIV), has received limited attention as compared with cylinder-based FIV systems. To date, no studies have examined how the unsteady, turbulent wake from an upstream sphere influences the FIV response of a downstream sphere. This study investigates the transverse FIV response of an elastically mounted sphere placed in the wake of a stationary upstream sphere with varying streamwise spacing ratios. The experiments were conducted in a low-turbulence free-surface water channel utilizing a low mass-damping ratio (m*+C_A)𝜁 = 0.064 (where m* is the ratio of the oscillating mass of the system to the displaced fluid mass , C_A is the added-mass coefficient, and 𝜁 is the damping ratio) setup in a subcritical Reynolds number range of Re = 11,000-42,200. A reduced velocity range of U* = U_∞/fD = 2.5-22.5 (where U_∞ is the free-stream velocity, f is the structural natural frequency, and D is the diameter of the elastic sphere) is explored. An upstream static sphere induced WIV in the downstream sphere, with oscillation amplitude increasing monotonically for U* > 10, where Mode III behavior is typically observed in isolated spheres. As the streamwise spacing ratio L* = L/D (where L is the center-to-center distance between the spheres) decreased, vibration amplitude increased significantly by up to 156% at L* = 1.5 compared to the isolated case at U* = 22.5. The upstream sphere also increased the periodicity of vibrations beyond U* > 10 as compared to the isolated sphere. The effect of spacing ratio, WIV dynamics, and flow-field measurements will be discussed in the presentation.