Vortex-induced vibration experiments with a long semi-immersed flexible cylinder under tension modulation: Fourier transform and Hilbert–Huang spectral analyses

Abstract

An experimental investigation of the vortex-induced vibration phenomenon on a long semi-immersed flexible cylinder in constant current profile were carried out at a recirculating water channel facility. The experiments are part of a comprehensive research project which aims a better understanding of non-linear riser dynamics. Vertical and prescribed monochromatic harmonic motions were imposed to the top with amplitude \(A_t/L\approx 1\,\%\) causing eigenfrequencies modulations. The mass ratio parameter is \(m^*\approx 10\), and the ratio between the total length \(L\) and the immersed length \(L_\text {im}\) is \(L/L_\text {im}\approx 3.5\). The Reynolds number \(Re\) range lies within the interval \(10^3<Re<10^4\). Three values of motion frequency \(f_t\) were imposed, being them \(f_t:f_{N,1}=\)1:3, 1:2 and 1:1 where \(f_{N,1}\) is the first eigenfrequency. The application of both the Fourier Transform and the Hilbert–Huang Transform allowed identifying subharmonic components and also frequency modulation aspects in the data.