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Galloping is a destructive type of flow-induced vibration (FIV) affecting engineering structures subjected to wind. In this paper, we introduce a simple, low-cost device able to passively absorb the energy of vibrating structures and reduce the galloping amplitudes. This purely nonlinear energy sink (NES) consists of multiple balls rotating freely in a circular track without direct mechanical coupling with the primary system. We assess the benefit of the proposed NES in mitigating the galloping of a square prism through wind tunnel tests and explain the main factors affecting its behaviour. The NES balls rotate due to dynamic interaction with the prism, exhibiting a way of nonlinear energy absorption, which is enhanced by ball collisions. We test different NES configurations varying the number of NES balls to highlight the number of balls' effect on shifting the mass centre and changing the dynamics of the NES. Whereas a single ball NES with a mass 8% that of the prism can increase the critical reduced velocity by 68%, a 2 or 3-ball NES of equivalent mass increases this speed by 87%. • A multi-ball nonlinear energy sink is effective for galloping mitigation in wind tunnel tests. • Balls in a circular racetrack damp galloping through rotation and collisions. • Using multiple balls eliminates the strongly modulated response of the prism generated by a single ball NES in the rotational regime. • Changing the mass distribution due to balls' motion is a unique advantage of the multi-ball NES. • An optimum number of balls achieves the best suppression for galloping without the over-crowdedness of the NES track. [ABSTRACT FROM AUTHOR]

Galloping is a critical type of flow-induced vibration (FIV) arising on power transmission lines, high rise buildings, pipe and cables bundles in the oil and gas industry. In this paper, we present a purely nonlinear energy sink (NES) that mitigates the galloping of a square prism. The NES is composed of a ball rotating freely in a circular track attached to the prism. The ball's dynamics is coupled to that of the prism in a purely nonlinear way by inertia. We experimentally assess how this simple NES reduces the prism vibration by comparing the prism amplitude responses with and without the NES. A supplementary video presents these experiments, during which the NES ball exhibits different dynamics in three regimes; oscillatory, intermittent, and rotational. We characterize the ball behaviour and its effect on the prism response in each regime. The oscillatory regime appears at low flow speeds at which both the prism and the ball oscillate with small amplitude. The intermittent regime represents a transition mode within a small range of flow speeds and corresponds to a small jump in the vibration amplitude of the prism. The rotational regime appears at higher flow speeds, where the ball oscillates with relatively high angular speeds resulting in a strong modulated response of the prism. The design of the NES allows to easily vary its track dimensions to use a ball of different sizes and masses. Accordingly, we demonstrate the influence of the main NES parameters, which are the ball mass, NES track radius, ball friction, and radial clearance between NES track walls and the rotating ball, on both the prism response and the ball behaviour. The NES we present is directly amenable to mitigate other types of FIV. • A free ball in a circular track acts as a NES and delays the occurrence of galloping. • The proposed NES design is robust and works over a large parameter range. • The NES experiences three successive response modes with increasing flow speed. • The main NES parameters have a significant effect on its behaviour. [ABSTRACT FROM AUTHOR]