Annular-flow-induced vibrations of a simply-supported tube in a finite-length narrow-gap support

•Tube subjected to narrow annular flow losing stability by flutter; dynamic instability.•Instability caused by finite-length gap having a diffuser at the downstream side.•Critical flow velocity possible at very low flow with flow expansion at down stream.•Expansion channel at downstream side very harmful for SG tube and support design. The stability behavior of a simply-supported tube subjected to narrow annular flow in a finite-length gap support at the mid-length of the tube is experimentally investigated. For the experiments, a simply-supported tube and several finite-length gap supports were made considering different gap sizes. Several diffuser angles at the downstream of the support were also tested, since it is believed that the pressure recovery at the exit of the support is much more important than pressure loss at the entrance of the support. In this study, we observed that the tube lost its stability by flutter, and that the critical flow velocity at which the tube lost stability was dependent on the annular gap size and the diffuser angle. Interestingly, the tube with the diffuser angle of 10° lost stability at a lower flow velocity than with the diffuser angle of 20°, for relatively narrow gaps that are in the range of 0.018 to 0.042 gap-to-tube diameter. Differently, for the largest gap of 0.138 gap-to-tube diameter, the diffuser angle of 20° resulted in a lower instability flow velocity. We concluded that there may be different instability mechanisms depending on diffuser angle and related to the narrowness of the gap.