Experimental investigation on the vibration response of an inclined near-wall flexible pipe span conveying different internal flows

This paper reports the experimental results of the vortex-induced vibration (VIV) and pipe-wall impact of an inclined near-wall flexible pipe transporting three representative internal flows with oblique angle (θ) ranging from 0° to 45°. The flexible pipe with an aspect ratio of 57 was fixed at both ends. The tests were performed in a recirculation water flume in the reduced velocity range of Ur = 7.92–23.92 using the non-intrusive optical measurement. The experimental results indicate that the existence of internal flow causes the tilt of the peaks of root-mean-squared (RMS) amplitude toward the downstream of internal flow. Nevertheless, with increasing θ, the peaks of RMS amplitude tilt to the upstream of internal flow. The mode competition of the pipe conveying gas flow is the weakest, but it is the most sensitive to θ. In contrast, the most intense mode competitions are observed in the pipe conveying gas–liquid flow and the pipe conveying liquid flow for the in-line and cross-flow directions, respectively. Five pipe-wall impact patterns are identified based on the impact position and frequency, and the partition of wall-impact pattern is provided in the s/L–Urn diagram. The impact position undergoes spatial transfer with the change of θ, especially for the pipe conveying gas flow. By comprehensive consideration of the spatial-temporal evolution of VIV response and impacting segments, the independence principle is invalid in predicting the VIV of near-wall flexible pipe span conveying internal flow.