Fluidelastic instability is a short term failure mode that occurs in tube bundles subjected to cross flow. It is believed that instability occurs due to two possible mechanisms; one is related to fluid coupling of neighboring tubes, the so called “stiffness mechanism”, and the other is related to a “negative fluid damping mechanism” i.e., fluidelastic forces in phase with tube velocity. The usage of a single flexible tube in a rigid array will eliminate the stiffness mechanism effect and leave only the damping mechanism, which makes the problem less complex. This paper presents a fundamental study of fluidelastic instability in a parallel triangular tube array subjected to air cross flow. It is found that a single flexible tube located in the third row of a rigid parallel triangular array does become fluidelastically unstable at essentially the same velocity as for a fully flexible array. However, when the single flexible tube is located in the first, second, fourth, or fifth rows, no instability behavior is detected. It is concluded from this work that, the tube location inside the array affects significantly its fluidelastic instability behavior when tested as a single flexible tube in a rigid array. It follows that a single flexible tube can be used for fundamental study of the phenomenon but not generally to generate stability maps for practical use.

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