An experimental investigation on the narrow annular-flow-induced vibration instability of a tube

The stability of a simply-supported tube subjected to annular flow in a finitelength narrow gap support is experimentally investigated. The exit boundary condition of the annular-leakage passage is known to be a possible cause of instability of the inner tube. In this study, therefore, several finite-length gap supports considering two diffuser angles were investigated on the effects of the annular gap and diffuser angle to the instability of the inner tube. Numerous experiments with the different supports have been performed in a transparent plexiglass tube so that a basic annular-flow can be provided for the inner tube and the movement of the tube can be measured by several laser vibrometers. It is observed that (1) the tube loses stability by flutter at very low flow velocity; the highest critical flow velocity is 3 m/s, (2) the critical flow velocity is strongly dependent on the annular gap size, the diffuser angle and the combination of both, and (3) generally the smaller the gap the lower the critical flow velocity.