Fabrication Of Binary Structured Surface For Drag Reduction

The researches on skin friction drag reduction have attracted significant attentions due to the practical values in engineering applications such as increasing vehicle speed, decreasing energy consumption and so on. So far, enormous theoretical and experimental investigations on turbulent drag reduction have been carried out and many drag reduction technologies have been developed, for example compliant surface, polymer, hydrophobic surface, and micro-structued surface like riblets. However, the technologies based on turbulent drag reduction theory can only reduce skin friction drag of turbulent flow, and can not reduce that of laminar flow dominating in many flow conditions, which has hampered the implementation of turbulent drag reduction technologies in the industry.In the paper, a novel drag reduction technology was proposed by designing a lotus and fish scale like surface with submilli-microscale binary structure. On the designed surface, gas phase is supposed to be generated due to vortex cavitation and developed in low pressure condition provided by surface topography. With the generated gas phase at solid-liquid interface, the shear force at the solid-liquid interface will be replaced by gas-liquid interfacial shear, not only reducing skin friction of laminar flow, but also restraining the growth of turbulence flow. As a result, the skin friction will be reduced effectively.The designed surface was fabricated with a simple spray-painting technique. By using a self-developed spray-painting apparatus, a mixture including resins, solvents, and micro particles was coated on a substrate. The polymer coating with designed binary structure surface formed spontaneously after the solidification of wet coating film.Experiments were performed and computational fluid dynamic methods were conducted to investigate the formation mechanism of binary structure surface. And according to the results, Benard convection and interfacial deformation driven by the gradients of temperature and surface tension due to solvents evaporation played a determinant role in the formation process of surface topography of polymer coating. By controlling the gradients of temperature and surface tension in the wet coating film, the interfacial deformation and surface topography of polymer coating were controlled.Experimental reseaches on the optimization of surface characteristic parameters were carried out so as to maximize the drag reduction property of polymer coating. Coating samples with optimized surface were tested in a minisized high-speed water tunnel, and a relative drag reduction rate was calculated. The experimental results showed that, the fabricated coating with designed binary structured surface had a steady drag reduction rate of 13%~18% at the flow speed of 14~20m/s, and the drag reduction efficiency became more remarkable with the incrment of flow speed.To further investigate the drag reduction property of designed surface, comparison experiments between coated rowing shell and smooth rowing shell were performed in a deep towing tank, and the test results re-proved the validity of fabricated polymer coating in reducing skin friction drag.Finally, by means of computational fluid dynamic methods, the processes of vortex cavitation caused by surface topography were numerically simulated. Based on the simulation, the effect mechanism of binary structured surface on drag reduction was analyzed.