Research On The Mechanism And Maintenance Of Drag Reduction Effect On Hydrophobic Surface

The drag reducing mechanism, properties and maintenance of air-water interface on hydrophobic surface are investigated through experiments, simulations and theoretical analyses. A method of maintaining the drag reducing effect is achieved. The main research contents and results are as follows:(1) The turbulent boundary layer on hydrophobic surface is measured using the hot-wire anemometer. It is observed that the velocity profile is lifed up, and the turbulence intensity is reduced in the near-wall region. The existence of the slip is verified. Based on the wavelet analysis, a method of extracting the coherent structures accurately from turbulence signals is proposed. It is discovered that the surface hydrophobicity can inhibit the fluctuations in turbulence.(2) Two existing modes of the air layer on hydrophobic surface are discovered, film mode and bubble mode. And the corresponding drag reducing model is established. The vanishing of air in both modes is concluded to explain the unsteadiness of drag reducing effect on hydrophobic surface.(3) The Lattice Boltzmann Method(LBM) is improved to characterize the wettability of solid surface. The LBM models of the film mode and bubble mode are established. And the effect of the flow, the solid-liquid interaction and the micro-structures on the air layer and drag reduction is concluded.(4) Based on the cavitation theory, a method of maintaining the air layer on hydrophobic surface is proposed. Rules of designing the micro-sturcture are deduced. The cavitation on micro-structure is realized using numerical simulations. And the effect of the flow, the micro-structure and the wettability on cavitation is concluded.(5) Based on the artificial ventilation, the boundary layer is investigated on the air-water interface. The effect of the air layer and bubble on the velocity profile, slip, vortical structures and drag reduction are discovered. It is confirmed that this method can maintenance the air layer and the drag reducing effect on hydrophobic surface.