EXPERIMENTAL STUDY ON DRAG REDUCTION BY AIR INJECTION ON HYDROPHILIC AND ALTERNATED SUPERHYDROPHOBIC SURFACES

The superhydrophobic surface is conducive to the formation of gas film on the wall, which is a potential new bionic drag reduction technology with potential anti-fouling function. However, the gas film is easy to be lost and damaged under the shear action of high-speed incoming flow. By constructing the hydrophilic and alternated superhydrophobic surfaces to enhance the stability of the superhydrophobic surface gas film, thus expect to achieve a better drag reduction effect. Using a gravity type water circulation pipeline testing system, The influence of superhydrophobic strip width and Reynolds number on drag reduction performance under turbulent conditions were tested. In addition, the corresponding gas film spreading state and its impact on drag reduction characteristics were analyzed. The results show that the continuous air injection of the hydrophilic and alternated superhydrophobic surfaces can solve the problem of the loss of the air film layer on the surface and realize the stable maintenance of the air film layer for a long time; the surface drag reduction rate shows a decreasing tendency with the increase of the water flow rate (Reynolds number), and the stability of the surface air film layer decreases gradually; the surface drag reduction rate shows a tendency of increasing and then decreasing with the increase of the width of the superhydrophobic strips and reaches a maximal reduction rate of 40.2% at the width of the superhydrophobic strips of 5.0 mm. The reason for this is that when the superhydrophobic strip is narrower, the liquid-solid interface with high shear stress accounts for a higher percentage, which brings a higher resistance; and when the strip is wider, the stability of the air film layer on the surface is not good. Therefore, the most suitable width of superhydrophobic strip exists under a certain flow condition, which makes the best drag reduction effect.