On the horseshoe vortex characteristics of bed-mounted inclined cylinders

This experimental study investigates the influence of inclination of bed-mounted cylinders on the horseshoe vortex (HSV) generated around them. To this end, the flow past vertical (VC), backward-inclined (BIC), forward-inclined (FIC), and side-inclined (SIC) cylinders were studied using the same fully developed approach flow. Velocity fields upstream of the cylinder-bed junction were measured using particle image velocimetry at a Reynolds number based on cylinder diameter of 26 000. Mean flow analysis revealed well-defined HSV structures in the VC, BIC, and SIC configurations, whereas no distinct HSV was observed for the FIC case. Instantaneous flow field analysis showed that the weakest downflow along the cylinder surface occurred in BIC due to the formation of a vortex above the primary HSV, which trapped and suppressed the downward flow. In contrast, the FIC configuration exhibited the strongest downflow, as its forward inclination provided a more streamlined path to the descending flow. Swirling strength analysis indicated that the strength of the HSV was maximum in BIC, followed by SIC, VC, and FIC. Probability density functions (PDFs) and joint PDFs of velocity fluctuations identified three dominant flow conditions: zero-flow, backflow, and intermediate flow in VC, BIC, and SIC, while FIC exhibited only zero-flow and backflow conditions. Joint PDFs, two-point correlation, and proper orthogonal decomposition analyses further supported the presence of localized, oscillatory HSV structures.