The dynamic behaviors of continuous and discrete flows in superhydrophobic microchannels are investigated with a lattice Boltzmann model.Typical characters of the superhydrophobic phenomenon are well observed from our...The dynamic behaviors of continuous and discrete flows in superhydrophobic microchannels are investigated with a lattice Boltzmann model.Typical characters of the superhydrophobic phenomenon are well observed from our simulations,including air trapped in the surface microstructures,high contact angles,low contact angle hysteresis,and reduced friction to fluid motions.Increasing the roughness of a hydrophobic surface can produce a large flow rate through the channel due to the trapped air,implying less friction or large apparent slip.The apparent slip length appears to be independent to the channel width and could be considered as a surface property.For a moving droplet,its behavior is affected by the surface roughness from two aspects:the contact angle difference between its two ends and the surface-liquid interfacial friction.As a consequence,the resulting droplet velocity changes with the surface roughness as firstly decreasing and then increasing.Simulation results are also compared with experimental observations and better agreement has been obtained than that from other numerical method.The information from this study could be valuable for microfluidic systems.展开更多
A new method is proposed for the effective roughness length (ERL) in heterogeneous terrain based on the principle of equalisation of momentum or heat fluxes calculated by the drag coefficient parameterization scheme u...A new method is proposed for the effective roughness length (ERL) in heterogeneous terrain based on the principle of equalisation of momentum or heat fluxes calculated by the drag coefficient parameterization scheme used in the ECMWF numerical model. Compared with the area-weighted logarithmically averaged ERL (drag coefficient), the newly calculated ERL (drag coefficient) is about 40% (16%) larger with a roughness step of 2.3. These differences reach their maximum values when the ratio of smooth to rough surface is 60% to 40%. Since the determination by this method is not sensitive to the atmospheric stratification, it is suitable for use in climate models.展开更多
The dependence of perpendicular magnetic anisotropy (PMA) on the barrier layer MgO thickness in MgO/CoFeB /Ta multilayers is investigated. The results show that the strongest PMA occurs in a small window of about 2 ...The dependence of perpendicular magnetic anisotropy (PMA) on the barrier layer MgO thickness in MgO/CoFeB /Ta multilayers is investigated. The results show that the strongest PMA occurs in a small window of about 2 4nm with the increase of MgO thickness from 1-1Onto. The crystalline degree of MgO and the change of interatomic distance along the out-of-plane direction may be the main reasons for the change of PMA in these multilayers. Moreover, the roughnesses of 2- and 4-nm-thick MgO samples are 3.163 and 1.8 nm, respectively, and both the samples show PMA. These results could be used to tune the magnetic characteristic of the ultra thin CoFeB film for future applications in perpendicular magnetic devices.展开更多
A quasistatic homogenized projection is made to characterize the effective cohesive zone behavior for rough-surface adhesion. In the context of the homogenized projection, the traction versus separation relation for t...A quasistatic homogenized projection is made to characterize the effective cohesive zone behavior for rough-surface adhesion. In the context of the homogenized projection, the traction versus separation relation for the homogenized cohesive zone (HCZ) of a rough interface can be highly oscillatory due to instabilities during microscopic adhesion and decohesion processes. The instabilities are found to occur not only individually but also collectively among the adhesive micro-asperity contacts, leading to extensive energy dissipation. Based on the behaviors of the HCZ relations, a framework for describing instability-induced energy dissipation in rough-surface adhesion is proposed to elucidate the effect of roughness on apparent interface adhesion. Two non- dimensional parameters, α related to roughness morphology and n related to flaw distribution, are identified to be most crucial for controlling the energy dissipation. For an interface with a shallow roughness and a strong intrinsic adhesive strength, the interface adhesion can be stronger if we make it rougher (reducing α) or lower its flaw density (increasing n). The HCZ projection method can be potentially extended and employed to bridge the apparent adhesion from intrinsic adhesion properties for engineering surfaces with multi-scale shallow roughness.展开更多
There are many examples that fluid flows on rough wall, such as channel flow in nature, pipe flow, etc. In order to know the flow structure of real fluids, it is important to study the effects of wall roughness on coh...There are many examples that fluid flows on rough wall, such as channel flow in nature, pipe flow, etc. In order to know the flow structure of real fluids, it is important to study the effects of wall roughness on coherent structure in turbulent shear flows. The experiments were carried out in a square glass channel, which is 600cm long, with the cross section of 30×25cm^2. The flow velocity was varied from 2 to 40 cm/s. Uniform sands whose diameters were 0.0012cm, 0.2gcm, 0.385cm, 0.594cm and 0.896cm respectively were glued to the floor of the channel. The rough Reynolds number Re_Δ= U_*Δ/ν=0.04~73, where U_*is the shear velocity, Δ is the diame- ter of uniform sand, v is the kinematic viscosity coefficient. Hydrogen bubble technique for flow visualization and HWL-II hot-film anemometer for velocity mea- surement were used in the experiments.展开更多
Surface roughness and thermal action are of remarkable importance in the lubrication performance of mechanical components,especially in extreme conditions.However,available studies mainly focus on the full-film lubric...Surface roughness and thermal action are of remarkable importance in the lubrication performance of mechanical components,especially in extreme conditions.However,available studies mainly focus on the full-film lubrication conditions without considering temperature rise and real 3D surface roughness due to the complexity of surface topography and temperature characteristics.Moreover,studies on the interfacial thermal behaviors of 3D rough surface lubricated contact in an extended range of working conditions remain limited.In this paper,a deterministic mixed thermal elastohydrodynamic lubrication model considering real 3D surface roughness and thermal effects is proposed.In this model,pressure and temperature are coupled with each other,the computation of elastic deformation is accelerated through the discrete convolution and fast Fourier transform method,the temperature field is calculated with the column sweeping technique,and the semi-system method is introduced to improve convergence and numerical stability under severe conditions.The model is validated by comparing its results with available published numerical and experimental results.The thermal behaviors of the contact interface are studied in a wide range of working conditions.The influences of surface roughness and thermal effect on lubrication performance are revealed.The results show that the proposed model can be used as a powerful analysis tool for lubrication performance and temperature prediction in various heavy-load,high-speed lubricated components over a wide range of lubrication conditions.展开更多
The elastohydrodynamic(EHD)friction properties of seven ISO VG 320 gear oils including three polyalphaolefins(PAOs),three polyglycols(PG)and a mineral oil have been investigated in rolling/sliding conditions at six di...The elastohydrodynamic(EHD)friction properties of seven ISO VG 320 gear oils including three polyalphaolefins(PAOs),three polyglycols(PG)and a mineral oil have been investigated in rolling/sliding conditions at six different temperatures and three roughnesses.Film thickness,Stribeck and traction curves have been generated using a ball‐on‐disc tribometer.Film thickness results are in agreement with previous work that it is primarily controlled by pressure‐viscosity coefficient and viscosity of lubricants.The results with smooth surface show that all oils experience significant shear heating leading to friction reduction at higher strain rates or lambda ratios but only PGs reach limiting friction whereas mineral oil and PAOs do not.Friction curves obtained at different temperatures and roughnesses enable simulating an extensive range of lubrication regimes and allow isothermal friction correction for shear heating.Stribeck curves with rough surfaces show an increase in friction in the lambda range of 0.5–3.5,where asperity separation varies from partial to full–indicating that roughness effects can be expected even under full film condition.This increase in friction is attributed to formation of a micro‐EHD region,and is seen only with mineral oil and PAOs whereas not with PGs.The results also highlight how EHD friction properties of different family of fluids could be influenced by roughness effects,and the possible mechanisms are discussed.展开更多
基金This work was supported by the Natural Sciences and Engineering Research Council of Canada(NSERC)and the Laurentian University Research Fund to JZ.
文摘The dynamic behaviors of continuous and discrete flows in superhydrophobic microchannels are investigated with a lattice Boltzmann model.Typical characters of the superhydrophobic phenomenon are well observed from our simulations,including air trapped in the surface microstructures,high contact angles,low contact angle hysteresis,and reduced friction to fluid motions.Increasing the roughness of a hydrophobic surface can produce a large flow rate through the channel due to the trapped air,implying less friction or large apparent slip.The apparent slip length appears to be independent to the channel width and could be considered as a surface property.For a moving droplet,its behavior is affected by the surface roughness from two aspects:the contact angle difference between its two ends and the surface-liquid interfacial friction.As a consequence,the resulting droplet velocity changes with the surface roughness as firstly decreasing and then increasing.Simulation results are also compared with experimental observations and better agreement has been obtained than that from other numerical method.The information from this study could be valuable for microfluidic systems.
基金This work was supportedjointly by the National Key Basic Research DevelopmentProgram(G1999043400)and the National Natural ScienceFoundation of China under Grant Nos.40275021,and49975016.
文摘A new method is proposed for the effective roughness length (ERL) in heterogeneous terrain based on the principle of equalisation of momentum or heat fluxes calculated by the drag coefficient parameterization scheme used in the ECMWF numerical model. Compared with the area-weighted logarithmically averaged ERL (drag coefficient), the newly calculated ERL (drag coefficient) is about 40% (16%) larger with a roughness step of 2.3. These differences reach their maximum values when the ratio of smooth to rough surface is 60% to 40%. Since the determination by this method is not sensitive to the atmospheric stratification, it is suitable for use in climate models.
基金Supported by the National Basic Research Program of China under Grant No 2011CB921804the Beijing Key Subject Foundation of Condensed Matter Physics under Grant No 0114023
文摘The dependence of perpendicular magnetic anisotropy (PMA) on the barrier layer MgO thickness in MgO/CoFeB /Ta multilayers is investigated. The results show that the strongest PMA occurs in a small window of about 2 4nm with the increase of MgO thickness from 1-1Onto. The crystalline degree of MgO and the change of interatomic distance along the out-of-plane direction may be the main reasons for the change of PMA in these multilayers. Moreover, the roughnesses of 2- and 4-nm-thick MgO samples are 3.163 and 1.8 nm, respectively, and both the samples show PMA. These results could be used to tune the magnetic characteristic of the ultra thin CoFeB film for future applications in perpendicular magnetic devices.
基金Project supported in part by the Nano and Bio Mechanics Program,under award CMS-0511961in part by the MRSEC Program,under award DMR-0520651,of the National Science Foundation
文摘A quasistatic homogenized projection is made to characterize the effective cohesive zone behavior for rough-surface adhesion. In the context of the homogenized projection, the traction versus separation relation for the homogenized cohesive zone (HCZ) of a rough interface can be highly oscillatory due to instabilities during microscopic adhesion and decohesion processes. The instabilities are found to occur not only individually but also collectively among the adhesive micro-asperity contacts, leading to extensive energy dissipation. Based on the behaviors of the HCZ relations, a framework for describing instability-induced energy dissipation in rough-surface adhesion is proposed to elucidate the effect of roughness on apparent interface adhesion. Two non- dimensional parameters, α related to roughness morphology and n related to flaw distribution, are identified to be most crucial for controlling the energy dissipation. For an interface with a shallow roughness and a strong intrinsic adhesive strength, the interface adhesion can be stronger if we make it rougher (reducing α) or lower its flaw density (increasing n). The HCZ projection method can be potentially extended and employed to bridge the apparent adhesion from intrinsic adhesion properties for engineering surfaces with multi-scale shallow roughness.
文摘There are many examples that fluid flows on rough wall, such as channel flow in nature, pipe flow, etc. In order to know the flow structure of real fluids, it is important to study the effects of wall roughness on coherent structure in turbulent shear flows. The experiments were carried out in a square glass channel, which is 600cm long, with the cross section of 30×25cm^2. The flow velocity was varied from 2 to 40 cm/s. Uniform sands whose diameters were 0.0012cm, 0.2gcm, 0.385cm, 0.594cm and 0.896cm respectively were glued to the floor of the channel. The rough Reynolds number Re_Δ= U_*Δ/ν=0.04~73, where U_*is the shear velocity, Δ is the diame- ter of uniform sand, v is the kinematic viscosity coefficient. Hydrogen bubble technique for flow visualization and HWL-II hot-film anemometer for velocity mea- surement were used in the experiments.
基金the National Key R&D Program of China (Grant No. 2018YFB0703804).
文摘Surface roughness and thermal action are of remarkable importance in the lubrication performance of mechanical components,especially in extreme conditions.However,available studies mainly focus on the full-film lubrication conditions without considering temperature rise and real 3D surface roughness due to the complexity of surface topography and temperature characteristics.Moreover,studies on the interfacial thermal behaviors of 3D rough surface lubricated contact in an extended range of working conditions remain limited.In this paper,a deterministic mixed thermal elastohydrodynamic lubrication model considering real 3D surface roughness and thermal effects is proposed.In this model,pressure and temperature are coupled with each other,the computation of elastic deformation is accelerated through the discrete convolution and fast Fourier transform method,the temperature field is calculated with the column sweeping technique,and the semi-system method is introduced to improve convergence and numerical stability under severe conditions.The model is validated by comparing its results with available published numerical and experimental results.The thermal behaviors of the contact interface are studied in a wide range of working conditions.The influences of surface roughness and thermal effect on lubrication performance are revealed.The results show that the proposed model can be used as a powerful analysis tool for lubrication performance and temperature prediction in various heavy-load,high-speed lubricated components over a wide range of lubrication conditions.
文摘The elastohydrodynamic(EHD)friction properties of seven ISO VG 320 gear oils including three polyalphaolefins(PAOs),three polyglycols(PG)and a mineral oil have been investigated in rolling/sliding conditions at six different temperatures and three roughnesses.Film thickness,Stribeck and traction curves have been generated using a ball‐on‐disc tribometer.Film thickness results are in agreement with previous work that it is primarily controlled by pressure‐viscosity coefficient and viscosity of lubricants.The results with smooth surface show that all oils experience significant shear heating leading to friction reduction at higher strain rates or lambda ratios but only PGs reach limiting friction whereas mineral oil and PAOs do not.Friction curves obtained at different temperatures and roughnesses enable simulating an extensive range of lubrication regimes and allow isothermal friction correction for shear heating.Stribeck curves with rough surfaces show an increase in friction in the lambda range of 0.5–3.5,where asperity separation varies from partial to full–indicating that roughness effects can be expected even under full film condition.This increase in friction is attributed to formation of a micro‐EHD region,and is seen only with mineral oil and PAOs whereas not with PGs.The results also highlight how EHD friction properties of different family of fluids could be influenced by roughness effects,and the possible mechanisms are discussed.