Experiments were carried out with a transversely oscillating cylinder in steady current, to investigate the sensitivity to small cylinder pitch rotations and in-line motions on the vortexinduced lift forces. The in-line motions of a spring blade mounted cylinder are convex when towing forward (pushing) and concave when towing backwards (pulling). At the same time the cylinder performs a small pitch rotation (about its central axis), which could induce Magnus-like lift forces. An unexpectedly large sensitivity was found to the in-line motions in certain tests with oscillation amplitudes of over half a diameter. The observed sensitivity to the cylinder pitch rotations was rather small. The tests were carried out in the lock-in regime for VIV and at sub-critical Reynolds numbers, which are close to the critical Reynolds regime. The results presented in this paper will initiate further investigation and will potentially have practical implications as well.
One of the great challenges in the offshore industry is still the assessment of the motions of a circular cylinder in waves and current for application to risers or riser bundles in up to 3,000 m (approximately 10,000 ft) water depth. User defined current profiles in two directions can be specified. Large scale VIV tests were carried out at MARIN for filling the VIV database at prototype Reynolds number up to 0.7 million. A flexible mounted model riser of 3.84 m length and 206 mm diameter was towed at high speed through the towing tank. The overhead carriage allowed for the tests with large drag and oscillating lift forces and an electric oscillator was deployed for forced oscillation tests. Earlier experiments in the critical Reynolds regime (drag bucket), have revealed some very important scale effects for the smooth cylinder, similar to what has been reported by Allen and Henning (OTC, 2001).