Experimental Study Of The Wall-Bounded Turbulence Drag Reduction Effect On Stainless Steel Superhydrophobic Surface Fabricated By Laser Texturing

Turbulent viscous resistance is the main cause of energy consumption of various transport vehicles,and its friction resistance accounts for a very large proportion of the total resistance.For conventional transport plane and surface vessels,the turbulent frictional resistance accounts for about 50% of the total resistance,while for underwater vehicles such as torpedoes and submarines,it can reach up to 70%.Therefore,under certain conditions of power and energy,the reduction of the surface friction resistance of the vehicle will greatly increase its navigation speed and range.Turbulence drag reduction can reduce transport flow resistance and energy consumption which is an effective means to save energy and reduce consumption.So turbulence drag reduction has become a hot topic in the world in recent years.It has been found that superhydrophobic surface has the characteristics of anti-fouling,self-cleaning,water-proof and moisture-proof,anti-icing,surface protection and anticorrosion,and has significant wall turbulence drag reduction effect,which is of great significance to reduce liquid flow resistance,improve the speed of underwater vehicle,and save energy.In this paper,the micro-nano rough structure was constructed on the metal surface by laser etching,and then the surface energy of the material was reduced by chemical treatment.The adjustable superhydrophobic surface was prepared,and the experimental platform was designed and built for the experimental study of the wall-bounded turbulence drag reduction effect on stainless steel superhydrophobic surface fabricated by laser texturing.The main findings are as follows:1.The superhydrophobic surface on AISI 304 stainless steel with different spacing grid structure was prepared on stainless steel surface by ultraviolet nanosecond laser etching and coating with modifier.The static contact angle was161.5 degrees and the dynamic rolling angle was 2 degrees.Super-hydrophobic surface was prepared on AISI 304 stainless steel by infrared laser etching and stearic acid modification to reduce surface energy.The maximum contact angle can reach 169 degrees.After 5 days of natural aging,the superhydrophobic stainless steel surface prepared by this method still has the same contact angle as before,which indicates the good durability of the superhydrophobic property.2.From the point of view of fluid drag reduction effect,two different methods of wall turbulence drag reduction were designed,and test platforms were built.In the high speed video camera and vertical pipeline method,the drag reduction effect of superhydrophobic surface was tested by using the combined mechanism of pulley andguide rail.The results show that different drag reduction efficiencies of superhydrophobic surface(SHS)can be achieved by adjusting the line spacing of grid structure.When the laser etching groove line spacing is 80 microns,the maximum drag reduction(DR)efficiency reaches 29.7%,which shows great potential for application.Using the self-built horizontal guide laser ranging device,the displacement,speed,driving force and other experimental parameters are collected for data analysis.Combining with the experimental results of the vertical pipeline,it is further concluded that the superhydrophobic surface made on the stainless steel with80 microns laser etching groove line spacing can achieve the best drag reduction effect.3.The drag reduction mechanism of laser etching V-type superhydrophobic grooves is preliminarily analyzed.The special structure of V-groove can keep the fluid stable and low speed,make it retain the ability of low friction state,reduce the turbulence,limit the turbulence of the fluid,reduce the momentum exchange between the fluid micro-clusters,and reduce the wall-bounded friction resistance of the valley.Moreover,because of the low surface free energy property of the superhydrophobic surface,the shear force caused by the fluid flow will make the attraction between solid surface molecule and liquid molecule balance,which makes it easier to form velocity slip on solid surface and produce velocity gradient.The shear force between fluid and wall decreases,which reduces the viscosity of viscous fluid and further reduces friction resistance.