Entrainment phenomenon by forced displacement excitation in vortex induced vibration

Abstract

This paper deals with the entrainment phenomenon by forced displacement excitation in vortex induced vibration of structures. This phenomenon can occur when a long-period earthquake occurs in a super high-rise structure that may have vortex induced vibration and can occur also when an offshore floating wind turbine is affected by wind and swell simultaneously. Moreover, it is important to examine this phenomenon from the viewpoint of the multi-degree-of-freedom forced self-excited system. By the experiment using wind tunnel and the numerical analysis using shooting method and ordinally numerical integration, following was made clear: (1) Entrainment phenomenon was confirmed in the excitation frequency region including the natural frequency . (2) There is the case that the entrainment region is wider at the higher wind speed than at the wind speed with the largest amplitude by vortex excitation. (3) Regarding the resonance amplitude in entrainment region, there is the case that its amplitude is slightly larger at the higher wind speed than that at the wind speed when the amplitude by the vortex excitation is the largest. (4) The largest vibration amplitude at entrainment is a value almost close to the sum of the largest amplitude at the time of only the vortex excitation and that at the time of only the forced displacement excitation. (5) The experimental results and the numerical analysis results mentioned above were almost qualitatively consistent.

Keywords

• nonlinear vibration
• vortex induced vibration
• selfexcited vibration
• forced vibration
• entrainment