We demonstrate a simple and controllable way to synthesize large-area, few-layer graphene on iron substrates by an optimized chemical vapor deposition （CVD） method using a mixture of methane and hydrogen. Based on a...We demonstrate a simple and controllable way to synthesize large-area, few-layer graphene on iron substrates by an optimized chemical vapor deposition （CVD） method using a mixture of methane and hydrogen. Based on an analysis of the Fe-C phase diagram, a suitable procedure for the successful synthesis of graphene on Fe surfaces was designed. An appropriate temperature and cooling process were found to be very important in the synthesis of highly crystalline few-layer graphene. Graphene-based field-effect transistor （FET） devices were fabricated using the resulting few-layer graphene, and showed good quality with extracted mobilities of 300-1150 cm2/（V.s）.展开更多
Widespread usage of bump-type foil journal bearing(BFJB) in oil-free microturbomachinery requires accurate predictions of dynamic performance characteristics, although it remains a challenging issue because BFJB refle...Widespread usage of bump-type foil journal bearing(BFJB) in oil-free microturbomachinery requires accurate predictions of dynamic performance characteristics, although it remains a challenging issue because BFJB reflects nonlinear both structurally and aerodynamically.This paper presented a simple experimental method to semi-directly obtain the minimum film thickness and dynamic stiffness of BFJB using the journal orbit. Numerical calculations and simulations are conducted to validate the experimental method. The micro-deformation and interaction of various foils are taken into consideration to improve the model precision. The results from the numerical model regarding the BFJB dynamic characteristics are compared with the experimental results coming from a dedicated test rig, which shows that the experimental results fluctuate obviously and agree not well with the numerical results at the start stage due to the presence of dry friction at that time, nevertheless, they show fantastic agreement as soon as a gas film is gradually generated to separate the shaft from the top foil. Therefore, the proposed experimental method is effective to predict film thickness and dynamic characteristics during the period from the lift-off time to the land-off time. The dynamic characteristics, along with the journal orbits also can be used to rapidly predict the dynamics behavior of rotor-bearing systems.展开更多
This paper presents analytical and numerical results of vapor bubble dynamics and acoustics in a variable pressure field.First,a classical model problem of bubble collapse due to sudden pressure increase is introduced...This paper presents analytical and numerical results of vapor bubble dynamics and acoustics in a variable pressure field.First,a classical model problem of bubble collapse due to sudden pressure increase is introduced.In this problem,the Rayleigh–Plesset equation is treated considering gas content,surface tension,and viscosity,displaying possible multiple expansion–compression cycles.Second,a similar investigation is conducted for the case when the bubble originates near the rounded leading edge of a thin and slightly curved foil at a small angle of attack.Mathematically the flow field around the foil is constructed using the method of matched asymptotic expansions.The outer flow past the hydrofoil is described by linear(small perturbations)theory,which furnishes closed-form solutions for any analytical foil.By stretching local coordinates inversely proportionally to the radius of curvature of the rounded leading edge,the inner flow problem is derived as that past a semi-infinite osculating parabola for any analytical foil with a rounded leading edge.Assuming that the pressure outside the bubble at any moment of time is equal to that at the corresponding point of the streamline,the dynamics problem of a vapor bubble is reduced to solving the Rayleigh-Plesset equation for the spherical bubble evolution in a time-dependent pressure field.For the case of bubble collapse in an adverse pressure field,the spectral parameters of the induced acoustic pressure impulses are determined similarly to equivalent triangular ones.The present analysis can be extended to 3D flows around wings and screw propellers.In this case,the outer expansion of the solution corresponds to a linear lifting surface theory,and the local inner flow remains quasi-2D in the planes normal to the planform contour of the leading edge of the wing(or screw propeller blade).Note that a typical bubble contraction time,ending up with its collapse,is very small compared to typical time of any variation in the flow.Therefore,the approach can also be appli展开更多
A doublet integral equation is formulated for the two-dimensional dissipative potential flow around a hydrofoil submerged below a free-water surface. The free-water surface is assumed to involve energy dissipation, an...A doublet integral equation is formulated for the two-dimensional dissipative potential flow around a hydrofoil submerged below a free-water surface. The free-water surface is assumed to involve energy dissipation, and thus it is the source of damping. A doublet panel method is developed from incorporation of the dissipative Green function approach and the doublet distributions on the hydrofoil surface. Numerical computations are implemented, and the derived numerical results are in good agreement with analytic solutions and experimental measurements.展开更多
Two-dimensional numerical simulations are performed to study the propulsive performance of fish-like swimming foils using the immersed-boundary method. A single fish as well as two fishes in tandem arrangement are stu...Two-dimensional numerical simulations are performed to study the propulsive performance of fish-like swimming foils using the immersed-boundary method. A single fish as well as two fishes in tandem arrangement are studied. First, the effect of the phase speed on the propulsive performance of a single fish is analyzed. The wake structures and pressure distribution near the wavy fish are also examined. The results show good correlation with those by previous researchers. Second, two tandem fishes with the same phase speed and amplitude are studied. The results show that the fish situated directly behind another one endure a higher thrust than that of a single one.展开更多
文摘We demonstrate a simple and controllable way to synthesize large-area, few-layer graphene on iron substrates by an optimized chemical vapor deposition （CVD） method using a mixture of methane and hydrogen. Based on an analysis of the Fe-C phase diagram, a suitable procedure for the successful synthesis of graphene on Fe surfaces was designed. An appropriate temperature and cooling process were found to be very important in the synthesis of highly crystalline few-layer graphene. Graphene-based field-effect transistor （FET） devices were fabricated using the resulting few-layer graphene, and showed good quality with extracted mobilities of 300-1150 cm2/（V.s）.
基金funded by the Natural Science Foundation of China (No. 51775025, 51205015)China Key Research and Development Plan (No. 2017YFB0102102, 2018YFB0104100)。
文摘Widespread usage of bump-type foil journal bearing(BFJB) in oil-free microturbomachinery requires accurate predictions of dynamic performance characteristics, although it remains a challenging issue because BFJB reflects nonlinear both structurally and aerodynamically.This paper presented a simple experimental method to semi-directly obtain the minimum film thickness and dynamic stiffness of BFJB using the journal orbit. Numerical calculations and simulations are conducted to validate the experimental method. The micro-deformation and interaction of various foils are taken into consideration to improve the model precision. The results from the numerical model regarding the BFJB dynamic characteristics are compared with the experimental results coming from a dedicated test rig, which shows that the experimental results fluctuate obviously and agree not well with the numerical results at the start stage due to the presence of dry friction at that time, nevertheless, they show fantastic agreement as soon as a gas film is gradually generated to separate the shaft from the top foil. Therefore, the proposed experimental method is effective to predict film thickness and dynamic characteristics during the period from the lift-off time to the land-off time. The dynamic characteristics, along with the journal orbits also can be used to rapidly predict the dynamics behavior of rotor-bearing systems.
基金Supported by the Ministry of Science and Higher Education of the Russian Federation as part of the World-class Research Center Program:Advanced Digital Technologies(contract No.075-15-2020-903 dated 16.11.2020).
文摘This paper presents analytical and numerical results of vapor bubble dynamics and acoustics in a variable pressure field.First,a classical model problem of bubble collapse due to sudden pressure increase is introduced.In this problem,the Rayleigh–Plesset equation is treated considering gas content,surface tension,and viscosity,displaying possible multiple expansion–compression cycles.Second,a similar investigation is conducted for the case when the bubble originates near the rounded leading edge of a thin and slightly curved foil at a small angle of attack.Mathematically the flow field around the foil is constructed using the method of matched asymptotic expansions.The outer flow past the hydrofoil is described by linear(small perturbations)theory,which furnishes closed-form solutions for any analytical foil.By stretching local coordinates inversely proportionally to the radius of curvature of the rounded leading edge,the inner flow problem is derived as that past a semi-infinite osculating parabola for any analytical foil with a rounded leading edge.Assuming that the pressure outside the bubble at any moment of time is equal to that at the corresponding point of the streamline,the dynamics problem of a vapor bubble is reduced to solving the Rayleigh-Plesset equation for the spherical bubble evolution in a time-dependent pressure field.For the case of bubble collapse in an adverse pressure field,the spectral parameters of the induced acoustic pressure impulses are determined similarly to equivalent triangular ones.The present analysis can be extended to 3D flows around wings and screw propellers.In this case,the outer expansion of the solution corresponds to a linear lifting surface theory,and the local inner flow remains quasi-2D in the planes normal to the planform contour of the leading edge of the wing(or screw propeller blade).Note that a typical bubble contraction time,ending up with its collapse,is very small compared to typical time of any variation in the flow.Therefore,the approach can also be appli
文摘A doublet integral equation is formulated for the two-dimensional dissipative potential flow around a hydrofoil submerged below a free-water surface. The free-water surface is assumed to involve energy dissipation, and thus it is the source of damping. A doublet panel method is developed from incorporation of the dissipative Green function approach and the doublet distributions on the hydrofoil surface. Numerical computations are implemented, and the derived numerical results are in good agreement with analytic solutions and experimental measurements.
基金Project supported by the National Natural Science Foundation of China (Grant No: 10472104) and National Laboratory of Hydrodynamics of China.
文摘Two-dimensional numerical simulations are performed to study the propulsive performance of fish-like swimming foils using the immersed-boundary method. A single fish as well as two fishes in tandem arrangement are studied. First, the effect of the phase speed on the propulsive performance of a single fish is analyzed. The wake structures and pressure distribution near the wavy fish are also examined. The results show good correlation with those by previous researchers. Second, two tandem fishes with the same phase speed and amplitude are studied. The results show that the fish situated directly behind another one endure a higher thrust than that of a single one.
