Effect of hydrophobicity on turbulent boundary layer under water

Turbulent flow field on hydrophobic surface is measured in the gravitational low-speed water tunnel using hot-film anemometer. The mean velocity profile shows that hydrophobic surface has an apparent influence on the distribution of the turbulent boundary layer in the near-wall region (y+ < 80). The turbulence intensity and friction on hydrophobic surface are also decreased. In this study, drag reduction rate up to 14.2%, and slip length up to 18.290 μm are obtained. In view of the importance of the bursting events to the production of turbulent kinetic energy and Reynolds stress, the energy distributions of flow data at y+ = 20 (right in the buffer layer) are analyzed based on continuous wavelet transform (CWT), and the relation between the maximum value of energy and the bursting events is established. By proposing a new detecting method, phase averaged waveform of ejections and sweeps of the bursting events are extracted and analyzed. The results demonstrate that the intensity of the bursting events is decreased and the time scale is extended. Therefore, we concluded that hydrophobic surface decreases the turbulent fluctuations, by weakening the bursting events, and so decreases the Reynolds stress and wall friction in boundary layer, and finally results in macroscopic drag reduction in turbulence. At last, the prospect of hydrophobic drag reducing technology is discussed.