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

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 (Wm) 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 Wm 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.

▸ Physical modeling of vortex-shedding from near-bed piggyback pipelines in currents. ▸ PIV measurement system with upward-illumination in a large water flume. ▸ Lee-wake patterns for the near-bed piggyback pipelines in currents. ▸ Swirling strength analysis of the vortex shedding for piggyback configurations.