Characterizing Flow Perturbations in Tube Arrays Under Cross-Flow

Abstract Fluidelastic instability (FEI) remains a critical concern for steam generator tube arrays, as it can lead to rapid tube failure if not adequately addressed during the design phase. Numerous investigations were conducted to experimentally predict the onset of instability and to uncover the underlying physics of the mechanism. A theoretical framework, the cell flow model, was developed by Hassan and Weaver to model fluidelastic instability (FEI) in both transverse and streamwise directions for flexible tube array. The model incorporates a time lag expression for flow channel area perturbations, derived from computational fluid dynamics (CFD) simulations. This study presents a comprehensive experimental investigation aimed at characterizing flow perturbations induced by tube motion in a parallel triangular tube bundle subjected to cross-flow. A sinusoidally oscillating tube within an otherwise rigid array was tested over a range of reduced flow velocities (Ur=10,15,20, and 30) to analyze the temporal and spatial dynamics of flow perturbations. Particle Image Velocimetry (PIV) was employed to capture detailed velocity fields and visualize the evolution of flow structures. The experimental results revealed that disturbances caused by tube motion decay as they propagate upstream and downstream, contradicting classical assumptions of constant disturbance amplitude. Furthermore, the time lag exhibited a bilinear variation along the flow path, emphasizing localized effects near the flow attachment and separation regions. While time lag increased with reduced velocity, the area perturbation amplitude and profile remained largely unaffected. These findings validate and could extend existing theoretical models, such as those by Lever and Weaver, by incorporating new observations of disturbance propagation and decay mechanisms.