Effects of superhydrophobicity on VIV control of a circular cylinder

We conducted an experimental study on the vortex-induced vibration (VIV) dynamics of cylinders featuring a specific superhydrophobic band. The superhydrophobic band refers to the pattern where the cylinder's surface is featured with equispaced bands of normal surface and superhydrophobic coating in an alternate manner. The experiments were conducted over a range of reduced velocities from U_r = 3 to 11, corresponding to Reynolds numbers between 1500 and 5900. To capture the near-field wake of the cylinders, a time-resolved particle image velocimetry (TR-PIV) system was employed, while the hydrodynamic forces were acquired using a six-component load cell. We found that the fully coated cylinder consistently displays the smallest amplitude of oscillation in the VIV initial branch, achieving a maximum reduction of approximately 38.9% at U_r = 5.0. Upon applying a superhydrophobic coated band, the patterned cylinder experiences a substantial enhancement in VIV amplitude (about 22.5% at U_r = 5.0) compared to the normal cylinder. On the other hand, in the VIV lower branch, the patterned cylinder effectively suppresses oscillation, whereas the fully coated cylinder exhibits slightly larger oscillation amplitudes than the normal cylinder. This finding agrees well with the trend of lift forces. The analysis of phase-averaged flow structures suggests that this difference can be attributed to the delayed vortex shedding from the superhydrophobic surface and the emergence of three-dimensional vortex structures created by the superhydrophobic band.