Time domain simulation of vortex-induced vibrations in stationary and oscillating flows

Abstract

This paper focuses on the further development of a previously published semi-empirical method for time domain simulation of vortex-induced vibrations (VIV). A new hydrodynamic damping formulation is given, and the necessary coefficients are found from experimental data. It is shown that the new model predicts the observed hydrodynamic damping in still water and for cross-flow oscillations in stationary incoming flow with high accuracy. Next, the excitation force model is optimized by simulating the VIV response of an elastic cylinder in a series of experiments with stationary flow. The optimization is performed by repeating the simulations until the best possible agreement with the experiments is found. The optimized model is then applied to simulate the cross-flow VIV of an elastic cylinder in oscillating flow, without introducing any changes to the hydrodynamic force modeling. By comparison with experiment, it is shown that the model predicts the frequency content, mode and amplitude of vibration with a high level of realism, and the amplitude modulations occurring at high Keulegan-Carpenter numbers are well captured. The model is also utilized to investigate the effect of increasing the maximum reduced velocity and the mass ratio of the elastic cylinder in oscillating flow. Simulations show that complex response patterns with multiple modes and frequencies appear when the maximum reduced velocity is increased. If, however, the mass ratio is increased by a factor of 5, a single mode dominates. This illustrates that, in oscillating flows, the mass ratio is important in determining the mode participation at high maximum reduced velocities.

Time domain model for calculation of pure in-line vortex-induced vibrations

“…This may induce transient effects and stability problems in the time integration process. This is why Thorsen et al (2016) proposed a smoothed amplitude response curve, which converges towards the solution obtained by equation 25, when the vibrations reach steady-state. The same smoothing technique is utilized in the proposed in-line VIV model.…”

“…In Thorsen et al (2014a), the model is seen to produce realistic results for both forced and free oscillation tests of rigid cylinders, and free oscillations of flexible cylinders. Through several case studies, a high degree of realism is found for numerical simulations of flexible pipes with circular cross section in uniform and sheared current (Thorsen et al, 2014b(Thorsen et al, , 2015a, and in oscillating flow (Thorsen et al, 2016).…”

Experimental analysis of fluid-structure interaction phenomena on a vertical flexible cylinder: modal coeficients and parametric resonance.

“…Besides, numerical models could lead to a full characterization of hydrodynamics coefficients in a wide range of Reynolds number and Keulegan-Carpenter number. Fu et al (2014) and Thorsen et al (2016) present interesting results of hydrodynamic forces on a flexible cylinder. Further experiments in order to assess the inertia coefficient would assist towards a deeper understanding of added mass results obtained in the present work.…”

“…In fact, the recent work by Thorsen et al (2016), e.g., on flexible cylinders in oscillating flows, still uses C a = 1 in its proposed empirical hydrodynamic model. The experimental work by Fu et.…”

“…Nevertheless, Pesce and Fujarra (2000) have also found an added mass value for a flexible cantilever greater than the unity (C a,1 = 1.17). Thorsen et al (2016), analyzing experimental data of a flexible cylinder subjected to an oscillatory flow presented by Fu et al (2014), also found added mass coefficient value greater than the unity (C a = 1.53). It should be recalled that the Sarpkaya charts were obtained for rigid cylinders under oscillatory flow, not for flexible cylinders.…”