Experimental study of annular-flow-induced instabilities of cylindrical shells

An experimental study is presented of the dynamical behaviour of a cylindrical shell within a coaxial rigid cylindrical pipe, with flow in the annulus. The shell could be clamped at both ends or clamped at one end and free at the other. The frequencies of some of the principal modes of the shell were monitored with fibre-optic sensors as the flow velocity was incremented, and the signals were analyzed on a FFT Signal Analyzer, up to the point of loss of stability. It was found in these experiments that a clamped-clamped shell loses stability by divergence (buckling) in its second or third circumferential mode, depending on the length/radius ratio and the internal pressure in the shell. The experimental observations are extensively compared with a previously developed theoretical model. The qualitative dynamical behaviour in general and the mode associated with instability are in good agreement with theory; the critical flow velocity for divergence, however, is substantially smaller than theoretically predicted, presumably as a result of imperfections. Clamped-free shells, on the other hand, lose stability by flutter, in these experiments in the third or fourth circumferential mode of the shell, depending on the annular-gap/radius ratio and the length/radius ratio.

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