Multi-Modal VIV Excitations: Measurements and Predictions Available to Purchase

An elongated structure in a current will experience vibrations due to vortex shedding (VIV). In the offshore industry, the main concern is to quantify the fatigue damage due to VIV. For in-situ applications, the current will vary as a function of depth, the structure can be made of sections with different diameters, all these parameters will modify the shedding frequency, leading to multi-modal excitations. In order to better understand and quantify the response of such a structure, experiments were carried out in the BGO-First facilities in la Seyne sur Mer. This tank is equipped with a current generation system that permits to produce uniform currents all throughout the tank section, 16 m wide and 3 m deep. The model consisted in an inclined cable under tension, about 24 m long, in a vertical plane perpendicular to the current, with about half of its length in the water. Its submerged part was fitted with flexible sheaths of different diameters, inducing vortex shedding at varying frequencies along the cable and a multi-modal response. Seven different geometries were tested, at varying top tension and current velocities. The cable motion was measured in twenty locations of its aerial part, through an optical tracking system. Displacements were of the order of 10mm with a measurement accuracy of 0.1mm. In the present paper, one configuration with two sheath diameters is studied in details. Crossflow and in-line results on RMS displacements and frequencies are reported. Comparisons with two numerical methods are also performed. The first method is a simple modal approach restricted to crossflow vibrations. The modal lift forces are evaluated and the response of the structure is obtained in the frequency domain. The second technique is based on the resolution of the Navier-Stokes equations in two-dimensional slices perpendicular to the structure. The Navier-Stokes solver is coupled with a finite-element model used to discretize the structure. Good agreement between numerical and experimental results is obtained for the displacements and dominant frequencies.