Abstract
Force measurements made on a square-section cylinder forced to oscillate in a direction normal to a stream are presented. These measurements are used to estimate the values that the combined mass-damping parameter of an equivalent spring-mounted cylinder should take if it is to perform oscillation at prescribed values of amplitude ratio and reduced velocity. The mass-damping parameter appears to be predicted well by the quasisteady theory of galloping when the reduced velocity is well above lock-in. Below resonance, the parameter is negative, contrary to the predictions of quasisteady theory, and this indicates why galloping is not usually observed. It is found from the forced-vibration measurements that at a given value of reduced velocity the combined mass-damping parameter may have the same value for more than one amplitude of oscillation, and this observation explains why a spring-mounted cylinder can vibrate at more than one amplitude. An attempt is made to fit the measurements of force and phase angle around vortex resonance with predictions of the nonlinear lift oscillator model proposed by Hartlen and Currie [5]. It is found that the phase angle between the lift and the cylinder displacement is not predicted well by the theory.
Published Version
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