Effect of the Body Number and Fluid Resistance on Stability of the Multi-Connected Moving Body

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Unstable vibration of multi-connected bodies supported by damper and spring systems moving in a narrow flow passage are reported. These vibration phenomena have been often observed in high-speed trains running through tunnels, cleaning robots going through pipings, medical machines in human blood vessels and core internals in nuclear reactor vessels. The equations of motion of multi-connected bodies are derived by Lagrangian method. The fluid forces acting on the multi-connected rigid bodies are obtained analytically on the basis of the Navier-Stokes equations applied to a narrow flow passage. The equations of coupled motion of the multi-connected bodies and fluid are derived. Using coupled equation, a stability analysis is performed. The critical velocities at the onset of the unstable behavior are estimated by plotting root locus. The flutter type instability and the divergence type instability are observed when the flow velocity increases. The variation of the coupled mode shape corresponding to the increment of the flow velocity is shown, and the relation between the coupled mode shape and unstable phenomena is discussed. Furthermore, the effect of the number of bodies and the pressure loss at the connecting points on the coupled mode and pressure distribution is investigated. The mechanism of occurrence of unstable phenomena is studied.
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