Physical modeling and swirling strength analysis of vortex shedding from near-bed piggyback pipelines

Abstract The vortex shedding from near-bed piggyback pipelines in a steady flow has been investigated experimentally in a large water flume. A specially arranged PIV system with upward-illumination of pulsed laser arrays from the flume bottom was employed for the flow visualization and quantitative measurement of the lee-wake flow in a sub-critical regime around the piggyback pipelines in the proximity of a plane boundary. Based on dimensional analyses, a dimensionless maximum swirling strength ( W m ) is used for analyzing the vortex shedding intensity and its frequency. Time-averaged swirling strength analyses indicate that the lee-wake patterns for the near-bed piggyback pipelines are dependent on the configuration factors, including the gap-to-diameter ratio ( e / D ), the spacing-to-diameter ratio ( G / D ), and the diameter ratio of two pipes ( d / D ), etc. The swirling strength in the lee-wake is obviously asymmetric for piggyback pipelines with bed proximity. For the fixed values of G / D and d / D , the maximum swirling strength decreases with the decrease of e / D . Moreover, for the examined G / D range (0 ≤ G / D ≤ 0.5), minimum values of W m and corresponding VIV amplitude for the piggyback pipelines are evidently within the same range of spacing-to-diameter ratio G / D ≈ 0.05–0.20.