Hydrodynamic force model for flexible pipe based on energy competition and applications into flow induced vibration prediction in uniform flow

A modified hydrodynamic force model based on the energy competition is developed. The model divides the hydrodynamic loads on flexible cylinder pipes undergoing bidirectional VIV into three parts: vortex-induced force, drag force and added mass force. The vortex-induced force is directly defined to be in phase with the VIV velocity response, which makes the key hydrodynamic coefficients of the synchronization model can be identified. The energy roles of the hydrodynamic force components are analyzed. The vortex-induced force acts as an energy input, while the energy is dissipated by the drag force. The energy competition results in flow-induced vibration. To determine the hydrodynamic coefficients, experiments on the flexible pipe are conducted. The identification methods of hydrodynamic force and coefficients are then introduced. The identified results show that the drag and vortex-induced force and coefficients are positively correlated with the VIV response amplitudes. It confirms that the hydrodynamic coefficients are not independent of the VIV response amplitudes. The empirical models of the drag and vortex-induced force coefficients are established by fitting the measured values. By instantaneous phase adjustment between the vortex-induced force and VIV velocity response, the empirical hydrodynamic coefficient models are effectively applied to a time-domain prediction method. Compared with experimental results, the good consistencies of the dominant mode number, dominant frequency, mean deformation and VIV response amplitudes can be found in the numerically simulated results. These consistencies prove the feasibility and advantages of the modified force model and the developed empirical coefficient model. Some discrepancies caused by the preliminary coefficient model should be improved by further optimization through intensive systematical experiments.