Experimental investigation of split flexible flags on vortex-induced vibration control for a rectangular cylinder

This study investigates the influence of split flexible flags on the vortex-induced vibration (VIV) characteristics of a rectangular cylinder with a 5:1 breadth-to-depth ratio. Wind tunnel experiments were conducted, incorporating pressure and displacement measurements alongside flow visualization techniques. A systematic comparison was made between the uncontrolled VIV behavior and the cases where split flags were applied at the leading and trailing edges. The results demonstrate that the development of primary and secondary resonance regions is governed by the number of motion-induced leading-edge vortices (MILEVs) generated along the cylinder's sides. In the secondary resonance region, MILEVs are shorter, slower, and require two vibration cycles to shed from the trailing edge, while in the primary resonance region, they complete shedding within one cycle. Furthermore, the split flags prove effective in mitigating VIV by altering the pressure distribution around the cylinder, promoting a more uniform energy dissipation, and reducing fluctuating lift coefficients by up to 67.4%. The flapping-induced vortices generated by the flags interfere with the formation of original vortex structures, thereby suppressing the motion-induced vortices that drive VIV. This study presents a novel approach to VIV mitigation in structures with large breadth-to-depth ratios, offering substantial potential for practical engineering applications.