Fluidelastic stability of a rotatedsquare array with multiple flexible cylinders, subject to cross-flow

Previous experiments have established that a single flexible cylinder in a rotated squarearray of otherwise rigid cylinders, with P/d = 2·12, is fluidelastically stable when subject to cross-flow. However, new experimental results presented here show that instability is possible when the array has three or more flexible cylinders, at least when the flexible cylinders are positioned in the first few rows of the array. This is consistent with previously developed theoretical analyses, which show that for this array the instability is a fluid stiffness-controlled mechanism, and requires fluid coupling between at least two adjacent flexible cylinders, rather than a fluid damping-controlled mechanism where the instability requires only one degree-of-freedom of a single flexible cylinder. The experimental results show that increasing mechanical damping has a very smalleffect on the critical flow velocity at which instability occurs. Increasing cylinder mass, on the other hand, can result in a large increase of the critical flow velocity, although the quantitative effect depends on the actual value of the nondimensional mass; the effect of increasing cylinder mass being greater at higher values of nondimensional mass. Frequency detuning of adjacent cylinders is also shown to have an effect on the critical flow velocity.