The mechanisms underlying flow-induced instabilities of cylinder arrays in crossflow

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Abstract
This paper aims to shed some light on the physical mechanisms involved in flowinduced instabilities of arrays of cylinders in crossflow. In the framework of quasi-steady fluid-dynamic theory, two distinct mechanisms are discussed. The first is similar but not identical to that associated with classical galloping; i.e. it is a negative fluid-dynamic damping mechanism and may obtain even if a single cylinder in the array is free to oscillate with only one degree of freedom. Unlike classical galloping, it is intimately related to the time delay experienced in the wake structure, and hence the fluid forces, adjusting to displacements of the cylinder. The second mechanism is similar to wake flutter; i.e. it is controlled by non-conservative fluid-dynamic stiffness effects and generally requires relative motion between adjacent cylinders in the array, although there is no reason why it should not occur for a single flexible cylinder with two degrees of freedom. The two mechanisms generally coexist, but each is predominant over different ranges of system parameters.