Temporal-spatial analysis on multi-mode interaction of a flexible pipe subject to external current and internal flow

Abstract

The vortex-induced vibration (VIV) in relation to a flexible pipe is numerically studied in order to examine the coupled effects occurring between the external shear current and the varying density internal flow. The analysis is performed using numerical methods and interpreted with position-frequency-energy (PFE) and time-frequency-energy (TFE) illustrations. The results indicate that the external shear current and varying density internal flow play different roles in affecting the VIV response. Due to temporal-spatial multi-mode competition, the phenomenon of mode switching is captured, showing alternating amplitude trough and peak in the TFE spectrum. The vibration response at the amplitude trough is commonly characterized by a wide range of frequencies, but the vibration response at the amplitude peak is usually dominated by a single frequency or a narrow range of frequencies. Based on the analysis related to spanwise waveforms, it is found that the standing wave commonly corresponds to a relatively stable equilibrium state of energy transfer between fluid-structure and mode-mode at local segments, and that the traveling waveform corresponds to an unsteady status of energy transfer between fluid-structure along the pipe span. Finally, through the contrast analysis of lift force and vibration response, one can find that the lift force along the pipe span is highly periodic and is in phase with the vibration displacement.