Stream-wise fluidelastic instability has recently been in the spotlight due to a practical event in steam generators at a nuclear power plant [1]. The instability has been reported to occur easily in rotated triangular arrays, but at all hardly in normal triangular arrays [2][3]. In square arrays, no instability has been reported in the authors’ test facility [4].

This paper presents the test results on rotated square arrays of the pitch-diameter ratios from 1.2 to 1.5. The tests have been conducted with a wind tunnel, where cylinders have been constrained to move in only one specific direction, either stream-wise or transverse to the flow. The results indicate that fluidelastic vibrations occur only in the stream-wise direction. The critical factor corresponds to the previous data reported in some guidelines [5], but here the factor is related mainly to stream-wise instability.

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