The importance of the in-flow oscillation of a single cylinder in cross-flow has been highlighted since an accident in a FBR-type reactor. In-flow oscillations have also been observed in tube arrays. This report is an experimental study on this phenomenon using a maximum of nine cylinders in a water tunnel. Six patterns of cylinder array, one single cylinder, two & three cylinders in parallel and in tandem, and a nine cylinder bundle, are examined. The cylinders are constrained to move only in the in-flow direction. The cylinder motion is measured by strain gages and by a high-speed digital video camera. The results of these motions are compared with the visualized vortex motion. As main results, two excitation mechanisms, symmetric vortex shedding and alternate vortex shedding, are observed. A transition range is found between these two mechanisms where large amplitude vibrations are observed. An additional test has been done to determine the root cause of the large vibrations in the transition range.
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