This paper investigates the flow-induced vibration (FIV) and possibility of fluidelastic instability occurrence in a rotated square geometry tube array through a series of experimental tests. All experiments presented here were conducted in water cross-flow. The array pitch spacing ratio of approximately P/D=1.64 is somewhat larger than that commonly found in typical steam generators. The stability of a single flexible tube as well as multiple flexible tubes were investigated. The tubes were free to vibrate purely in the streamwise direction or the transverse direction relative to the upstream flow. A single flexible tube, in the otherwise rigid tube array, was found to undergo large amplitude vibrations (up to 40 % D) in the transverse direction. Tube vibration frequency analysis indicated the presence of two frequency components related to vorticity shedding in the array. This potential vorticity-induced-vibrations (VIV) and potential coupling between VIV and FEI are discussed in the paper. Test results for streamwise flow-induced vibrations are also presented. Results in water flow show a possible effect related to flow periodicity at low velocity. At significantly high flow velocities, the tubes are found to fully restabilize. This restabilization after VIV locking has not been previously reported as an unlocking result. The present results suggest that the flow-induced vibration of tubes in a rotated square array configuration is significantly more complex than in other geometries, particularly for the streamwise vibration case.

The present paper presents experimental tests investigating flow-induced vibration and possible fluidelastic instability (FEI) in a rotated square geometry tube array. The pitch spacing ratio of approximately P/D = 1.64 is somewhat larger than that found in typical array geometries. Experiments were conducted in water flow. The stability of a single flexible tube as well as multiple flexible tubes was investigated. The tubes were free to vibrate purely in the streamwise direction or the transverse direction relative to the upstream flow. A single flexible tube, in the otherwise rigid tube array, was found to undergo large amplitude vibrations (up to 40% D) in the transverse direction. Tube vibration response PSDs indicated the presence of two frequency components related to vorticity shedding in the array. This potential vorticity-induced-vibrations (VIV) and potential coupling between VIV and FEI is discussed in the paper. Test results for streamwise flow-induced vibrations are also presented. Results in water flow show a possible effect related to VIV at low velocity. At significantly high flow velocity, the tubes are found to fully restabilize. This restabilization after VIV locking has not been previously reported as an unlocking result. The present results suggest that flow-induced vibration of tubes in a rotated square array configuration is significantly more complex than in other geometries, particularly for the streamwise vibration case.