An insight into the vortex-induced vibration of a near-wall flexible pipe in the presence of wall impact

Abstract

The vortex-induced vibration (VIV) and wall impact of a near-wall flexible pipe arranged perpendicular to the incoming flow are experimentally investigated in a water flume with an initial gap-to-diameter ratio G/D ranging from 0.2 to 1.5. The neutrally buoyant submerged flexible pipe with fixed-end supports possesses a length-to-diameter ratio of 75. The non-intrusive measurement with high-speed cameras was employed to simultaneously capture the space–time varying vibration displacements as well as the wall impact process in the reduced velocity range of 4.76–17.55 with the maximum Reynolds number of about 2900. The experimental results highlight the effect of gap ratio on the VIV and wall impact. The highest excited mode decreases with the reduction of G/D, accompanied by the prolongation of the lower branch of the fundamental modal vibration. Meanwhile, the flexible pipe possessing the same in-line and cross-flow dominant frequency is elongated, indicating the enhanced coupling between the in-line and cross-flow responses. Four pipe-wall impact patterns are identified, including the first-mode dominant response with single segment impact, transition I, transition II and the second-mode dominant response with two segments alternating impact, depending on the dominant response mode and the spatial-temporal evolution of contacting pipe segment. The time-varying contacting length and the spatial transfer of contacting segment are two main features of wall impact. As the dominant mode transfers from the fundamental to the second, the impact frequency increases from the same as the dominant frequency to double of the latter, which is associated with the modal weights.