| Owing to the unique microstructure and composition,the body surfaces of some organisms have shown the special superhydrophobic property during the evolution,allowing these organisms to be more easily adapted to the surrounding environment.For the bladed fluid machines,fluid medium,as the carrier,can be driven by impeller to transfer and convert the energy,inevitably generating a large amount of friction loss on the blade surface,and the greater the speed,the greater the energy loss.If the drag reduction property of superhydrophobic surface can be successfully applied to the blade surface,the energy loss on turbulent boundary layer can be greatly reduced,thereby significantly improving the efficiency of the devices.Based on this inspiration,this paper selects a simple plane sample instead of the complex blade to carry out the theoretical analysis of the preparation of the superhydrophobic surface and drag reduction.Under the guidance of such ideas,this paper provides three plants with less attention as the research objects,and further explores the crucial role of microstructure played in the superhydrophobic surface from four aspects including the test of plant samples,the bionic surface preparation,numerical simulation of droplet impacting and slipping drag reduction.The research can be carried out from four aspects,and the specific contents are as below:(1)In this section,the Setaria Viridis,Trifolium Tepens and Canna Indica are selected as the research objects to separately test the surface wettability and surface microstructure.Then,the corresponding idealized mathematical model of surface microstructure is established according to the observation results.The results show that the dynamic contact angle on the Setaria leaf is 150.2°±0.5°,and the sliding angle is 7°±1°.The parallel submillimeter perthitic textures are evenly distributed in the leaf surface of Setaria Viridis,resulting in that the droplet can be directed to rolling.The inverted triangular prism structures are randomly distributed on the leaf surface of Trifolium Tepens,and the dynamic contact angle is 151.1°±0.9°,the sliding angle is 8°±1°.For the Canna Indica,the dynamic contact angle is 154.2°± 1.9°,and the sliding angle is 7°±1°.The typical quadrilateral concavity is distributed evenly on the Canna leaf.Special microstructures can make the air effectively exist on the surface of plant leaves,thereby reducing the contact area between the droplet and the leaf surface,resulting in the easily rolling of droplet on the leaf surfaces.Based on the ideal models of wettability,the microstructural geometry and physical dimension of superhydrophobic surface have the obvious effect on the wetting behavior of droplet on solid surface.Heuristically,when the superhydrophobic surface is fabricated,the wettability of the solid surface can be improved by adjusting the microstructural shape and size.(2)Surface microstructures of Setaria Viridis is relatively simple and easy to copy,so it is really possible to machine the ideal biomimetic perthitic textures on the aluminum alloy substrate using precision machining with Electrical Discharge Machining.Then the superhydrophobic surface with ideal effect can be successfully prepared after using the chemical modification to reduce the surface free energy of biomimetic substrate.After testing,the micron perthitic textures are evenly distributed on the surface of biomimetic substrate,which of the dynamic contact angle is 151.2°± 1.4° and the sliding angle is 9°±1°.By comparing the trajectories of droplets on bionic and ordinary smooth substrates,it can be further demonstrated that the mechanical and chemically modified biomimetic substrate has excellent superhydrophobic property.(3)The dynamic process and internal velocity,pressure distribution of droplet impacting solid surface are numerically simulated by CFD method,and then the effects of droplet velocity,surface wettability and surface tension on droplet dynamics are also further analyzed.The results show that the process of droplet impacting is divided into four stages: falling,spreading,shrinking and bouncing.During spreading and shrinking stages,droplet needs to overcome the adhesive force of solid surface,and the velocity and pressure inside the droplet are distributed symmetrically along the center axis.The contact angle and surface tension have a significant influence on the flow pattern of droplet.With the increase of the contact angle and surface tension,the smaller the spread of the droplet on the solid surface,the less time it takes to reach the maximum spreading stage.To continue to increase the impacting velocity,breakage occurs when the droplet contacts the solid surface.The breaking process of the droplet includes two stages.The first stage occurs at the droplet edge,mainly caused by the insufficient surface tension;the second stage occurs inside the droplet,mainly due to the uneven internal force.(4)The effect of the perthitic textures on movement pattern of droplet on the superhydrophobic surface are analyzed by the numerical simulation method.The results show that there are three main characteristics when the droplet impacts on the superhydrophobic surface with perthitic textures.The first is that the droplet can spread easily in a direction parallel to the perthitic texture.Second is that two obvious convex spheres occur at the droplet edge along the perthitic texture.Third is that the retracting of the droplet presents a long and narrow crosswiss shape.The effect of the perthitic textures on the flow drag reduction are also analyzed in the microchannel instead of blade surface.The results show that when the fluid flows through the superhydrophobic surface,a parabolic gas-liquid interface is formed,and the sliding velocity is formed to reduce the friction resistance at the interface.For the purpose of drag reduction and efficiency increasing on the blade surface,the submillimeter perthitic textures of Setaria Viridis are selected as the bionic prototype to carry out the theoretical analysis involving to the preparation of superhydrophobic surface and the fluids' flow pattern on the surface.In this process,the wetting mathematical model and the analysis method for superhydrophobic surface based on CFD method can provide a theoretical reference for the bionic drag reduction optimization of blade surface,simplify the design process and shorten the development cycle.Meanwhile,the mechanical treatment for biomimetic microstructure and nanostructure on the surface can improve the stability of superhydrophobic surface to some extend,and avoid the environment pollution brought by chemical method,which is a green processing method.The conclusions of this paper can provide a reference for the application of biomimetic superhydrophobic surface to reduce the resistance in fluid machinery,and eatablish a foundation for its engineering application. |
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