Air-entrained Cavity Formation Mechanism During Asymmetric Nanostructured Spheres Plunging Into Water With A Low Speed And Underwater Drag Reduction Performance

The underwater vehicles experience hydrodynamic drag in the sailing processes,which leads to the speed reduction and energy consumption.How to reduce the hydrodynamic drag in the sailing process is particularly important.It was found that cavities could greatly reduce the hydrodynamic drag of the vehicles with blunt shapes by up to 90%.Some creatures in nature use their surfaces with special superwettability(a combination of rough structure and specific chemical compound)to form stable cavities underwater.Inspired by the above,we introduced the nanostructure coatings on the hydrophobic and hydrophilic spheres.We found that the positions(impacting direction)and coverage ratios(r)of the nanostructure and the release heights influenced the cavity formation process and the drag reduction performance of underwater cavities.The main achievements were listed below:1.The cavity formation process after the hydrophobic spheres plunging into water with a low speed and the drag reduction performance of underwater cavities.By introducing the nanostructure coatings on the hydrophobic spheres,we found that the positions(impacting direction)and coverage ratios(r)of the nano-porous structure accurately manipulated the behavior of water film on the hydrophobic sphere surfaces.If the nanostructure faced downward,only a tiny region of the nanostructure(r ≤ 1/8)could help a thin liquid film directly develop into a splash crown and an air-entrained cavity.However,when the nanostructure faced upward,the detached thin film developed to a splash crown and an air-entrained cavity only if r increased to 1/3 or even above.At a depth of 60-80 cm from the water surface,the cavities were stably trapped on the hydrophobic sphere surfaces.The drag reduction efficiency of the spheres with complete cavities was more than 90% comparing with the spheres without cavities.2.The cavity formation process after the hydrophilic spheres plunging into water with a low speed and the drag reduction performance of underwater cavities.In this paper,the positions and coverage ratios of the nanostructure were designed to control the sputtering position of the water film on the sphere surfaces,and further influenced the formation of a sputter crown and an airentrained cavity.If the nanostructure faced downward,the coverage ratios had little effect on the formation of an air-entrained cavity until the release height reached 70 cm.If the nanostructure faced upward,the coverage ratios had little effect on the formation of an air-entrained cavity until the release height reached170 cm.It was also found that the nanostructure facing downward was more likely to form an air-entrained cavity than the nanostructure facing upward.At the same release height(coverage ratio),the spheres with the nanostructure facing upward required a higher coverage ratio(release height)to form an air-entrained cavity.