A few analytical models for a simply-supported cylinder subjected to a concentric annular flow have been developed considering small perturbations in the flow components. The flow perturbation is believed to play a key role in causing instabilities or reducing the effective tube damping. However, it is generally accepted that there is no dynamic instability of a simply-supported cylinder subjected to concentric annular flows; the dynamic instability may materialize only with a very flexible rod where traveling wave solutions are expected. The dynamic behavior of a concentric annular flow in a finite-length-narrow gap at the mid-span of a cylinder is studied to investigate the possibility of a dynamic instability in the case of a simply-supported cylinder. In this study, typically used stainless steel tubing and geometry in engineering application are applied with some fluid boundary conditions at the finite-length-narrow gap such as a contraction entrance and a diffuser exit. The research results from this study are as follows: (1) determination of an analytical solution for tube dynamics in terms of fluid boundary conditions such as contraction loss at the entrance and pressure recovery at the exit of the finite-length-annular gap, (2) confirmation of the effect of leakage-flow to system dynamics, and (3) confirmation of flutter instability (negative damping type) at the fluid boundary conditions.

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