In-flow instability of tube arrays is one of recent major issues to be examined in the heat exchanger design since the event at a nuclear power plant in California [1]. In our previous tests [2], the effect of the pitch-diameter ratio in triangular arrays is reported. This is one of the present major issues in the nuclear industry.

On the other hand, there are some trials [3] to produce the electric power by the phenomenon of the flow-induced vibration. They are mainly using the vortex shedding, but in this report the power generation by the unstable vibration is investigated. It is spotlighted the combination between the unstable vibration and the power generation whether the limit cycle exists or not. This report introduces the result on the existence of the limit cycle.

Many engineers have attempted to produce electric power using semi-conductor devices, but they could not obtain enough power. Then, the device using the electro-magnetic system is used here. The in-flow instability has a small effect on the fluid flow compared with the usual fluidelastic vibration in the transverse direction, because the in-flow movement does not disturb the flow path compared to the transverse fluidelastic vibration.

The coil-magnet system is set on the top of the flexible cylinder cantilevered from the bottom. A three by three array of cylinders in a triangular pattern is set in a small wind tunnel. When the flow velocity increases, the cylinder array becomes unstable in the in-flow direction as similarly to the transverse direction, and it produces the electricity in the coil. However, as the power generation means energy dissipation, resulting in increase of the system damping, the effect on the instability is examined.

At the end of this report, an analytical solution is introduced to explain the measured results. It is successfully completed to show the existence of the limit cycle.

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