Force-displacement measurements on closely spaced tandem cylinders

Abstract

The static lift force (resulting from static displacement) and the dynamic lift force (resulting from forced vibration) acting on the downstream member of a pair of slightly staggered tandem cylinders in a free-surface water channel flow were measured. The downstream cylinder could undergo transverse forced vibrations while the upstream cylinder remained fixed. The motivation of this study was to obtain reasonable estimates for the phase lag, or delay time, between cylinder motion and the dynamic lift force acting on the oscillating downstream cylinder when it undergoes interference galloping. A preliminary empirical correlation of delay time with a dimensionless parameter, which the product of the reduced velocity and the square root of the nondimensional amplitude of cylinder motion, is presented. A comparison of the dynamic force-displacement diagrams with the force displacement diagrams derived by a time-delayed quasi-steady model reveals that even the time-delayed quasi-steady analysis cannot accurately reproduce the dynamic loading of the vibrating cylinder. However, the force-displacement diagrams from the quasi-steady analysis appear to qualitatively resemble the dynamically recorded force-displacement diagrams, especially at higher reduced velocities, perhaps explaining the success of the time-delayed quasi-steady model in predicting a first estimate of the onset velocity for interference galloping.