EXPERIMENTAL STUDY ON DRAG REDUCTION CHARACTERISTICS OF SUPERHYDROPHOBIC GROOVE SURFACES WITH VENTILATION

Drag reduction is one of the main technical approaches to solve the enhancing speed and extending voyage of the vehicle under water, which is extremely crucial to alleviate the increasingly severe energy crisis all over the world. In the gravity pipeline experimental system, drag reduction characteristics with ventilation and gas film spreading state on superhydrophobic groove surfaces are tested and raised in the turbulent state. The variation laws of drag reduction rate with Reynolds number and dimensionless spacing of grooves at different ventilation rates are obtained. In addition, it is the diffierence of ventilation drag reduction that is compared and analyzed between merely superhydrophobic surfaces and superhydrophobic groove surfaces. The material of the experimental plate is colorless acrylic. The groove structure is processed via mechanical method and is sprayed by superhydrophobic coating. Results reveal that continuing ventilation can settle the issue of easy loss of gas film on superhydrophobic groove surface, and the gas film can achieve perennial stabilization. As ventilation rate adds, the gas film spreads more uniformly and drag reduction rate rises under the constant Reynolds number, which result in the notable drag reduction effect. As ventilation rate affects the capability of scaling out of gas film, drag reduction presents two modes with Reynolds number under the constant ventilation rate. When the ventilation rate and the Reynolds number are unchanging, the drag reduction rate firstly increases and then decreases with the expansion of the groove size, and the maximum reduction rate is obtained when S^+\approx 76. The inherent mechanism on drag reduction characteristics of superhydrophobic groove surfaces with ventilation is that not only the spreadability and stability of gas film layer is enhanced significantly but also the wetted area is increased obviously due to groove structures, meanwhile, the maximum value of drag reduction is larger than both the groove surface and the superhydrophobic surface.