Fluid forces acting on a cylinder undergoing streamwise vortex-induced vibrations

•Amplitude response of a streamwise oscillating cylinder is examined.•Phase and amplitude of unsteady drag force estimated using equations of motion.•No change in phase of drag force observed in low amplitude, lock-in region.•Low amplitude region coincides with sudden drop in amplitude of unsteady drag force.•Influence of choice of damping ratio is examined.

This brief communication examines the fluid forces acting on a cylinder free to move in the streamwise direction throughout its response regime. The amplitude and phase of the unsteady drag coefficient are estimated from the displacement signals and a simple harmonic oscillator model. We examine the counter-intuitive reduction in vibration amplitude observed in streamwise vortex-induced vibrations (VIV) at resonance, which has remained one of the most poorly understood aspects of VIV. Our results show that it is not caused by a change in the phase of the fluid forcing with respect to the cylinder displacement, as suggested by previous researchers; instead, we show that there is a sudden decrease in the amplitude of the unsteady drag coefficient in this region. The possible cause of this result, relating to three-dimensionality in the wake, is briefly discussed.