Drag reduction performance of discrete superhydrophobic surfaces in von Kármán swirling flow

Enhancing the energy efficiency and mitigating environmental pollution in maritime transportation relies significantly on minimizing frictional drag of ships. In this study, we employ wettability dividing lines and convex stripes to discretize the continuous air layer on superhydrophobic surfaces, offering a mechanism to enhance their drag reduction efficacy. Initially, we theoretically analyze the stabilization of the air layer on superhydrophobic surfaces through wettability dividing lines and convex stripes. Subsequently, torque measurements are conducted to characterize drag reduction performance in von–Kármán swirling flow. The drag reduction parameters that performed optimally were determined experimentally by parametric analysis, with a 20–mm spacing identified for the hydrophilic ring on the wettability difference surface, and a 20–mm spacing with a 2–mm height for the convex stripe on the superhydrophobic surface. Under these specified parameter values, the maximum Reynolds number for effective drag reduction (Rec) increases from 1.62 × 105 to 3.24 × 105, and the drag reduction rate at Rec = 3.24 × 105 reaches 32.29% and 26.61%, respectively. Moreover, coupling wettability dividing lines with superhydrophobic convex stripes further enhances Rec to 4.05 × 105, and the drag reduction rate stabilizes at around 30% within the maximum Reynolds number.