Flow-induced vibrations and instabilities in a rotated-square cylinder array in cross-flow

Abstract

This paper reports on experiments on a rotated-square array of cylinders with pitch-to-diameter ratio of 1·5, in air- and water-crossflow, where one of the cylinders is flexibly mounted and the others rigid. In air-flow, fluidelastic instability is observed, predominantly in the flow direction, in the first two or three rows of the array, but not deeper in. Furthermore, special tests revealed the existence of divergence—a static fluidelastic instability—in the second row of the array. In water-flow, oscillatory fluidelastic instability never developed, but the existence of divergence, mostly in the flow direction, was very clear: the oscillation frequencies of the system vanished, and the cylinder migrated statically away from the original point of equilibrium. Multiple Strouhal numbers were observed in both air- and water-flow: S = 0·8 and 1·4 in the first couple of rows of the array, and S = 0·6–0·7 and 1·4 farther in. In the case of water-flow these periodicities gave rise to resonances and lock-in of cylinder vibration; beyond the lock-in range and prior to the onset of divergence, the cylinder was fluidelastically overdamped and curiously inert. Some work was also done on the buffeting response of the cylinder, and amplitude-flow correlations have been obtained.