We describe recent results showing the dynamic response, excited by vortex shedding, of a long flexible cylinder subject to a stepped current immersed in the wake of another cylinder, placed upstream in tandem configuration. Experiments were conducted at the E.T.S.I. Navales towing tank of the Technical University of Madrid during March 2012. The tank is 80 m long with a cross-section of 4 × 2.5 m. A supporting structure was designed in order to provide support for a 3 m long cylinder with an external diameter of 16 mm. The cylinder was instrumented with strain gauges providing curvature measurements in the in-line and the cross-flow directions at 11 locations along its length. Tension and drag forces were also measured at both ends of the model. For these experiments, the upstream rigid cylinder was made stationary by fixing it at both ends, and it was located at different centre to centre distances.
More than 200 runs were conducted, with its lower 65% length under the water free surface, connected to the structure by means of universal joints. The supporting structure allowed to configure different top end conditions and to apply different top tensions. Tests were conducted with speeds up to 1.4 m/s. The cylinder had a low flexural stiffness of 6.04 Nm2 and low mass ratio of 2.7. Fundamental natural frequencies were in the range from about 2.3 to 6.2 Hz, and the cylinder responded in modes up to the third cross-flow.
