Study On Turbulent Drag Reduction Mechanism Of Microstructured Surfaces And Its Influence On Anti-clogging Performance Of Emitters

Microstructured surfaces such as superhydrophobic and micro-riblet surfaces can reduce wall friction by constructing micro and nano structures,which have been successfully used in the fields of wall drag reduction,antifouling and self-cleaning.The flow channel of drip irrigation emitters is slightly curved,and the sediment and impurities in the irrigation water were easy to deposit in the low-speed zone of the flow channel or adhere to the inner wall of the flow channel,resulting in emitter clogging.The application of biomimetic microstructured surfaces is an effective way to reduce emitter clogging.It is of great significance and practical value to study the drag reduction characteristics of superhydrophobic and micro-riblet surfaces,as well as the mechanism of turbulent drag reduction in the near wall region.The microstructured surfaces were used to improve the anti-fouling and anti-clogging performance of pipelines and emitters.To solve the scientific issue of how the wall flow field characteristics would change when the microstructured surfaces was applied in pipelines and emitters,the turbulent and drag reduction characteristics of the superhydrophobic and micro-riblet surfaces in pipelines and emitters were investigated by using the hydraulics test,numerical simulation and theoretical analysis methods.Firstly,this study compared the turbulent flow field characteristics of superhydrophobic and micro-riblet surfaces with the smooth surface.Secondly,the superhydrophobic and micro-riblet surfaces were applied in the pipeline to analyze the pressure difference changes,wall friction resistance and flow fields near the surfaces,and to explore the drag reduction effect of the superhydrophobic and micro-riblet surfaces in pipelines.Then,the distribution of vortices near the superhydrophobic and micro-riblet surfaces was studied based on vortex dynamics theory.The flow vortex structures on different microstructured surfaces were compared and analyzed to further study the drag reduction mechanism of the microstructured surfaces.Finally,the superhydrophobic and micro-riblet surfaces were applied in the emitter’micro-channel,and a drag reduction model combining superhydrophobic and micro-riblet was established.The influence of the combined drag reduction model on the anti-clogging performance of emitters was studied.The main conclusions are as follows:(1)The turbulence drag reduction characteristics of superhydrophobic and micro-riblet surfaces were analyzed,and the drag reduction effect of microstructured surfaces was determined.The results showed that the average velocity of water flow on the superhydrophobic and micro-riblet surfaces increased in the turbulent boundary layer,and the Reynolds shear stress and turbulence intensity decreased in different degrees.The two different types of microstructured surfaces had certain drag reduction effect.In the Reynolds number range of the experiments,the maximum drag reduction of the superhydrophobic surface was 9.48%.Among the three micro-riblet surfaces with different angles,the drag reduction effect of the micro-riblet surface with 60°angle was the best,and the maximum drag reduction rate was 7.60%.The drag reduction effect of the superhydrophobic surface was better than that of the micro-riblet surface.The concepts of wall roughness?and effective contact area S were introduced into the velocity formulas of the superhydrophobic and the micro-riblet surface.The velocity distribution formulas of the superhydrophobic and the micro-riblet surface were derived to theoretically demonstrate the drag reduction characteristics of the two surfaces.(2)The influence of superhydrophobic and micro-riblet surfaces on the flow field in pipelines was expounded and the drag reduction rate of microstructured surfaces in pipelines was analyzed.The results showed that the pressure difference of the pipeline with superhydrophobic and micro-riblet surfaces decreased,indicating that the two surfaces reduce the flow velocity loss in pipelines.Microstructured surfaces played a significant role in reducing drag.In the Reynolds number range of the test(Re_d was 5375-32250),the maximum drag reduction rates of the superhydrophobic and micro-riblet surfaces were 8.37%and9.67%,respectively.The average velocity of the two types of turbulent boundary layer increased and the Reynolds shear stress decreased.The turbulence intensity near the surface was weakened,showing that the microstructured surfaces had a certain inhibition effect on water flows.(3)The spatial correlation of pulsation velocity on superhydrophobic and micro-riblet surfaces was determined.The spatial correlation analysis of the transient flow field on the microstructured surfaces showed that the characteristic length Lx of the superhydrophobic and micro-riblet surfaces was smaller than that of the smooth surface,and the maximum reduction rate was 9.75%and 7.18%,respectively.The structural angle of the two kinds of microstructural surfaces was decreased compared with that of smooth surface.The maximum structural angles of vortex structures on the superhydrophobic,micro-riblet and smooth surfaces were 7.7°,6.9°and 9.7°,respectively.Microstructured surfaces inhibited the rise of vortex structures.The drag reduction was realized by changing the distribution of vortex structures near the surfaces.The spatial correlation analysis method was used to study the transient flow field in the pipeline,and the changes of the water flow structure on the superhydrophobic,micro-riblet,and smooth surfaces were analyzed,which laid a foundation for the preliminary study of the vortex structure near surfaces.(4)The drag reduction mechanism of superhydrophobic and micro-riblet surfaces was revealed,and the distribution of water vortex structures near microstructured surfaces was clarified.The average velocity near the the superhydrophobic surface increased,while the vorticity decreased.The main reason for the velocity increase of the superhydrophobic surface was the decrease of the frictional shear stress.The riblets on the micro-riblet surfaces restricted the development of water flow.The flow velocity in micro-riblets was low.The vorticity at the tip of riblets was large,while the vorticity at the bottom of the riblets was small.Similarly,the wall frictional shear stress on the riblet tip was larger,and the wall frictional shear stress on the bottom of riblets was smaller.However,the whole micro micro-riblet surface still presented a drag reduction effect.Three different vortex identification methods were used to identify vortices in the near-wall region of the microstructured surfaces,and it was found that the flow vortices on the superhydrophobic surface increased.The streamwise vortices on the micro-riblet surface also increased,and the spanwise vortices were confined to the interior and tip of the riblets and were all small broken vortices.The main reason for the drag reduction of the micro-riblet surfaces was to restrict the development of the spanwise vortex structures near the surface.An improvedΩ_M vortex identification method was proposed,and the concept of vortex density in aerodynamics was introduced to analyze the vortex structures with different strengths near the surface from the perspective of statistics.(5)Three kinds of microstructured surface models of emitters were established and the influence of microstructured surface on the anti-clogging performance of emitters was investigated.The results showed that the emitter flow channel model of the superhydrophobic surface changed the flow velocity distribution on the cross section of the flow channel,but does not significantly affect the pressure along the streamwise direction.Compared with the emitter flow channel model of the smooth surface,both the micro-riblet and combined surface models had greater influence on the flow rate and pressure of the whole flow channel.The widening of the main flow zone and the increase of the flow rate in the low speed zone of the two surfaces were beneficial to improve the anti-clogging performance of emitters.From the analysis of frictional shear stress of surfaces,the frictional shear stress of the combined surface was small,and the anti-clogging index was the largest.Compared with the three surface models,the combined surface model had the best anti-clogging performance.