MECHANISM OF VIBRATION SUPPRESSION EFFECT FOR CONNECTED CANTILEVERED STRUCTURE OSCILLATING IN-LINE DIRECTION IN CROSS FLOW

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This study is related to self-excited in-line vibration of a thermo-well in liquid sodium cross flow experienced in Fast Breeder Reactor Monju. Although quite a large number of research on the in-line oscillation of a cantilevered cylinder have been conducted in Japan after the failure of the thermo-well, clear cut explanation of the cause of the vibration has not yet been done. Especially, the explanation on the reason why only single particular thermo-well was broken is not yet given and is still under investigation. From the viewpoint of academic interest, this is an exciting topic.Therefore, we decided, to conduct precise measurement of fluid dynamic forces acting on the vibrating cylinder by using water tunnel at CRIERI Abiko Research Center under small forcing amplitude external excitations. In a series of experiments, we changed forcing amplitudes, oncoming flow velocities and excitation frequencies to check the effect of the dependency on the amplitude of the cylinder and reduced velocity.As a result, we found the fact that fluid force is composed of the component proportional to acceleration, velocity and displacement of the vibrating circular cylinder. We also found that excitation parameter which determines the instability is strongly dependent on the cylinder amplitude. In addition, added mass coefficient varies depending on reduced velocity and the stability criteria of reduced damping is slightly affected by the mass ratio. Finally, we collected unsteady fluid dynamic data as a database for further studies.