A new type of microstructure inspired by the cross section of barchan dunes was proposed to reduce windage,which was considered as a passive drag reduction technology in aerospace manufacturing field.Computational flu...A new type of microstructure inspired by the cross section of barchan dunes was proposed to reduce windage,which was considered as a passive drag reduction technology in aerospace manufacturing field.Computational fluid dynamics method was carried out to discuss the effect of the microstructure on the skin friction reduction under high velocity flow condition.Different microstructure heights were employed to survey the reduction of drag.The results illustrated that the appearance of microstructure led to a generation of pressure drag in non-smooth model(with microstructures inspired by cross section of barchan dune)in contrast to smooth model.However,the microstructure significantly increased the thickness of the low-speed fluid by 11.4%in the near-wall flow field,causing the low-speed fluid to rise and decreasing the velocity gradient near the wall,thereby reducing viscous resistance.In addition,high-speed fluid flowed above the microstructure units instead of along the inner side of the units due to the influence of micro-vortex,resulting in a reduction of friction near the surface.Further-more,micro-vortex was considered to be the significant internal factor to achieve turbulent drag reduction since it could not only reduce the viscous resistance by promoting the fluid flow above the microstructure but also pro-vide a reverse thrust force.The understanding of the mechanism of drag reduction provides theoretical guidance for further fabrication of drag reduction coatings using renewable materials.展开更多
Superhydrophobic coatings with high flexibility and mechanical durability can well address many practical application problems.To this end,we proposed and fabricated a kind of bio-based superhydrophobic(multi-walled c...Superhydrophobic coatings with high flexibility and mechanical durability can well address many practical application problems.To this end,we proposed and fabricated a kind of bio-based superhydrophobic(multi-walled carbon nanotubes)CNT@PU(polyurethane)coatings.It was demonstrated that the CNT@PU coatings with 64%soft segment content possessed the preferable bonding strength(5B)with metal substrates.The multi-walled carbon nanotubes,as additive materials,were used to construct the microscopic structures of the coating surfaces,which made polyurethane surface superhydrophobic(water contact angle being 156.9°,and water sliding angle being 4.3°).Furthermore,the high bonding strength between CNT and coating matrix led to robust mechanical durability of supertiydrophobic CNT@PU coatings,and the coatings remained superhydrophobicity after 10 cycles of abrasion under 100 g load pressure.Also,the superhydrophobic coatings could well resist 5 cycles of tape-peeling action,and presented outstanding flexibility.The supernydrophobic CNT@PU coatings with high flexibility and mechanical durability could be applied to various substrates suggesting their big potential in future real-world application.展开更多
基金This work was supported by the National Natural Science Foundation of China(Nos.52075246 and U1937206)the Project Funded by China Postdoctoral Science Foundation(No.2019M661826)+1 种基金Open Fund of Key Laboratory of Icing and Anti/De-icing(No.IADL20190202)the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions(KYCX19_0181).
文摘A new type of microstructure inspired by the cross section of barchan dunes was proposed to reduce windage,which was considered as a passive drag reduction technology in aerospace manufacturing field.Computational fluid dynamics method was carried out to discuss the effect of the microstructure on the skin friction reduction under high velocity flow condition.Different microstructure heights were employed to survey the reduction of drag.The results illustrated that the appearance of microstructure led to a generation of pressure drag in non-smooth model(with microstructures inspired by cross section of barchan dune)in contrast to smooth model.However,the microstructure significantly increased the thickness of the low-speed fluid by 11.4%in the near-wall flow field,causing the low-speed fluid to rise and decreasing the velocity gradient near the wall,thereby reducing viscous resistance.In addition,high-speed fluid flowed above the microstructure units instead of along the inner side of the units due to the influence of micro-vortex,resulting in a reduction of friction near the surface.Further-more,micro-vortex was considered to be the significant internal factor to achieve turbulent drag reduction since it could not only reduce the viscous resistance by promoting the fluid flow above the microstructure but also pro-vide a reverse thrust force.The understanding of the mechanism of drag reduction provides theoretical guidance for further fabrication of drag reduction coatings using renewable materials.
基金supported by thc National Natural Science Foundation of China(Nos.52075246,51671105,51705244,and U1937206)the Natural Science Foundlation of Jiangsu Province(No.BK20170790)+2 种基金the Project Funded by China Postdoc-toral Science Foundation(No.2019M661826)Opcn Fund of Key Laboratory of lcing and Anti/Deicing(No.IADL20190202)the Projcct Funded by the Priority Academic Program Development of Jiangsu Highcr Education Institutions,and the NUAA Innovation Program for Graduate Education(No.kfjj20190617).
文摘Superhydrophobic coatings with high flexibility and mechanical durability can well address many practical application problems.To this end,we proposed and fabricated a kind of bio-based superhydrophobic(multi-walled carbon nanotubes)CNT@PU(polyurethane)coatings.It was demonstrated that the CNT@PU coatings with 64%soft segment content possessed the preferable bonding strength(5B)with metal substrates.The multi-walled carbon nanotubes,as additive materials,were used to construct the microscopic structures of the coating surfaces,which made polyurethane surface superhydrophobic(water contact angle being 156.9°,and water sliding angle being 4.3°).Furthermore,the high bonding strength between CNT and coating matrix led to robust mechanical durability of supertiydrophobic CNT@PU coatings,and the coatings remained superhydrophobicity after 10 cycles of abrasion under 100 g load pressure.Also,the superhydrophobic coatings could well resist 5 cycles of tape-peeling action,and presented outstanding flexibility.The supernydrophobic CNT@PU coatings with high flexibility and mechanical durability could be applied to various substrates suggesting their big potential in future real-world application.