Experimental investigation of wake and load modifications of a submerged horizontal cylinder by a wrap-around flexible appendage under wave–current conditions

Abstract

The hydrodynamic effects of a wrap-around flexible appendage attached to a submerged horizontal cylinder were experimentally investigated under current, wave, and wave-current conditions using synchronized force measurements, particle image velocimetry, and image-based motion tracking. The test covered Reynolds numbers Re = 1.2 × 10 4 – 6.0 × 10 4 and Keulegan–Carpenter numbers KC = 0.32 – 1.72. Under current action, the appendage effect strongly depends on its length: the mean Cd first decreased by approximately 22.7 – 39.3% and subsequently recovered by 4.9 – 14.7%, while C l-rms increased by 303 – 986%, with the largest increase at L / D = 2.5. The normalized recirculation length L r / D increases from 1.53 for the bare cylinder to 1.80 for L / D = 2.16, but decreases to 1.48 and 1.52 for L / D = 2.33 and 2.50, respectively. Under wave action, the L / D = 2.5 configuration has limited influence on the primary force level but redistributes the load spectrum by enhancing higher harmonics. Under wave-current conditions, it increases the horizontal loading while reducing the vertical response. Flow-field analyses show the appendage acts as an inflow-dependent hydrodynamic modifier by altering wake organization, recirculation, and local flow structures, rather than functioning as a universal load-reduction device.