Transverse natural frequencies and flow-induced vibrations of double bellows expansion joints

This paper considers the transverse vibrations of fluid-filled double-bello ws expansion joints. The bellows are modelled as a Timoshenko beam, and the fluid added mass includes rotary inertia and bellows convolution distortio n effects. The natural frequencies are given in terms of a Rayleigh quotien t, and both lateral and rocking modes of the pipe connecting the bellows un its are considered. The theoretical predictions for the first six modes are compared with experiments in still air and water and the agreement is foun d to be very good. The flow-induced vibrations of the double bellows are th en studied with the bellows downstream of a straight section of pipe and a 90 degrees elbow. Strouhal numbers are computed for each of the flow-excite d mode resonances. The bellows natural frequencies are not affected by the flowing fluid but the presence of an immediate upstream elbow substantially reduces the flow velocity required to excite resonance. (C) 1999 Academic Press.