Velocity oscillations at the head of the gravity current were investigated in experiments and numerical simulations of a locked-exchange flow.A comparison of the experimental and numerical simulations showed that the ...Velocity oscillations at the head of the gravity current were investigated in experiments and numerical simulations of a locked-exchange flow.A comparison of the experimental and numerical simulations showed that the depth and volume of the released fluid affected the oscillations in the velocity of the gravity current.At the initial stage,the head moved forward at a constant velocity,and velocity oscillations occurred.The head maximum thickness increased at the same time as the head,which did not have a round,and accumulated buoyant fluid due to the buoyancy effect intrusion force.The period of accumulation and release of the buoyant fluid was almost the same as that observed for the head movement velocity;the head movement velocity was faster when the buoyant fluid accumulated and slower when it was released.At the viscous stage,the forward velocity decreased proportionally to the power of 1/2 of time,since the head was not disturbed from behind.As the mass concentration at the head decreased,the gravity current was slowed by the viscous stage in its effect.At the viscous stage,the mass concentration at the head was no longer present,and the velocity oscillations also decreased.展开更多
Granular packings under gravity in frictional and frictionless silos were simulated and the influence of the wall friction on the normal force distribution was investigated. Although there is an obvious Janssen effect...Granular packings under gravity in frictional and frictionless silos were simulated and the influence of the wall friction on the normal force distribution was investigated. Although there is an obvious Janssen effect in frictional silos, only a slight influence on the geometry of packing was found. The law of normal force distribution is different for frictional and frictionless walls, which is related to the pressure profile. A modified formula with consideration of the pressure profile was well fitted to the simulation results.展开更多
A plane strain finite element model was established to investigate the effect of friction between diaphragm wall and soil on braced excavation. The behavior of interface between diaphragm wall and soil was simulated w...A plane strain finite element model was established to investigate the effect of friction between diaphragm wall and soil on braced excavation. The behavior of interface between diaphragm wall and soil was simulated with the interface model of ABAQUS. Parametric studies were conducted with different diaphragm wall external friction angles δ. The results show that deflection of diaphragm wall and ground surface settlement decrease with the decrease of δ. However, the reduction effect on diaphragm wall deflection is the most significant at the depth where the maximum wall deflection occurs and can be neglected at the wall base. The ratio between wall deep inward component and wall cantilever component reaches its peak value 2.7 when δ=5°. The ratio of the maximum ground surface settlement to the maximum wall lateral deflection decreases at a reduced rate with the increase of δ. For excavation with braced diaphragm wall, the effect of friction between diaphragm and soil on the deflection of diaphragm wall and ground settlement, especially the distribution of ground surface settlement behind diaphragm, should be taken into account.展开更多
The effect of frictions between dies and tube on the cross section quality of thin-walled tube numerical controlled(NC) bending was studied by numerical simulation method, combined with theoretical analysis and experi...The effect of frictions between dies and tube on the cross section quality of thin-walled tube numerical controlled(NC) bending was studied by numerical simulation method, combined with theoretical analysis and experiment. The results show that the frictions between mandrel, wiper, pressure die, bending die and tube have a significant and complicate effect on the section quality of thin-walled tube NC bending. To improve the section quality, frictions between mandrel, wiper and tube should be decreased, but the frictions between the pressure die, bending die and tube increase. The effect on the section distortion is more significant from mandrel, wiper, pressure die to bending die and the effect on the wall thinning more significant from mandrel, pressure die, wiper, to bending die. The effects of frictions between all dies and tube on wall thinning are smaller than their effects on section distortion. Mandrel and wiper should be lubricated well and drawing oil is used to lubricate them in actual production. The frictions between pressure die, bending die and tube should be increased and the dry friction is used between pressure die, bending die and tube in actual production.展开更多
In this letter we present a novel wall shear stress measurement technique for a turbulent boundary layer using sandwiched hot-film sensors. Under certain conditions, satisfactory results can be obtained using only the...In this letter we present a novel wall shear stress measurement technique for a turbulent boundary layer using sandwiched hot-film sensors. Under certain conditions, satisfactory results can be obtained using only the heat generated by one of the hot-film and a calibration of the sensors is not required. Two thin Nickel films with the same size were used in this study, separated by an electrical insulating layer. The upper film served as a sensor and the bottom one served as a guard heater. The two Nickel films were operated at a same temperature, so that the Joule heat flux generated by the sensor film transferred to the air with a minimum loss or gain depending on the uncertainties in the film temperature measurements. Analytical solution of the shear stress based on the aforementioned heat flux was obtained. The preliminary results were promising and the estimated wall shear stresses agreed reasonablywell with the directly measured values （with errors less than 20%） in a fully developed turbulent pipe flow. The proposed technique can be improved to further increase precisions.展开更多
Achieving densely packed particles is desirable within the industries of ceramics,pharmaceuticals,defence and additive manufacturing.In this work,we use the discrete element method(DEM)to determine the effect of wall ...Achieving densely packed particles is desirable within the industries of ceramics,pharmaceuticals,defence and additive manufacturing.In this work,we use the discrete element method(DEM)to determine the effect of wall parameters on the final packing density of mono-disperse spheres subject to 4 varying three-dimensional vibration and fill conditions.We focus specifically on the impact of the container wall parameters on the particles'final packing density.Following on from the validation of the DEM simulation the particle-wall coefficient of restitution,the particle-wall coefficient of rolling friction and the particle-wall coefficient of sliding friction were varied individually and the effect on the final packing density analysed.For relatively low particle-particle friction glass beads,the effect of these wall properties had no discernible effect on the final packing density achieved.Following on from these findings the particle-wall properties were varied at the extreme values of particle-particle coefficient of rolling friction and particle-particle coefficient of sliding friction.For a particle-particle coefficient of sliding friction=1,increases in particle-wall coefficient of restitution resulted in a minor increase in the final packing density of particles though this was not statistically significant.For a particle-particle coefficient of sliding friction=1,increases in particle-wall coefficient of rolling friction resulted in a minor decrease in the final packing density of the particles though again not to a degree where the trend can,with complete certainty,be distinguished from the random error across the repeats.Finally,when the particle-particle coefficient of sliding friction=1,increases in particle-wall coefficient of sliding friction resulted in a significant decrease in the final packing density of particles.This decrease was attributed to the propagation of force chains throughout the packing.The significant decrease in final packing density with particle-wall coefficient of sliding friction highlights the need to choose appropriate vessel materials to optimise packing of particles with a high particle-particle coefficient sliding friction.Conversely,for particles with minimal particle-particle friction,the particle-wall friction coefficient has no effect on the final packing density of particles-a potentially valuable finding for certain industrial applications.All simulations were run using the open-source DEM package LIGGGHTS on the University of Birmingham's high-performance computer:BlueBEAR.All the code files used within this paper can be found on Github:https://github.com/Jack-Grogan/DEM-Vibropacking-Wall-Effects.展开更多
文摘Velocity oscillations at the head of the gravity current were investigated in experiments and numerical simulations of a locked-exchange flow.A comparison of the experimental and numerical simulations showed that the depth and volume of the released fluid affected the oscillations in the velocity of the gravity current.At the initial stage,the head moved forward at a constant velocity,and velocity oscillations occurred.The head maximum thickness increased at the same time as the head,which did not have a round,and accumulated buoyant fluid due to the buoyancy effect intrusion force.The period of accumulation and release of the buoyant fluid was almost the same as that observed for the head movement velocity;the head movement velocity was faster when the buoyant fluid accumulated and slower when it was released.At the viscous stage,the forward velocity decreased proportionally to the power of 1/2 of time,since the head was not disturbed from behind.As the mass concentration at the head decreased,the gravity current was slowed by the viscous stage in its effect.At the viscous stage,the mass concentration at the head was no longer present,and the velocity oscillations also decreased.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11705256,11772095,and 11572091)the National Key Research and Development Program of China(Grant No.2016YFB0700103)
文摘Granular packings under gravity in frictional and frictionless silos were simulated and the influence of the wall friction on the normal force distribution was investigated. Although there is an obvious Janssen effect in frictional silos, only a slight influence on the geometry of packing was found. The law of normal force distribution is different for frictional and frictionless walls, which is related to the pressure profile. A modified formula with consideration of the pressure profile was well fitted to the simulation results.
基金Project (07FDZDSF01200) supported by Tianjin Science and Technology Innovation Special Funds
文摘A plane strain finite element model was established to investigate the effect of friction between diaphragm wall and soil on braced excavation. The behavior of interface between diaphragm wall and soil was simulated with the interface model of ABAQUS. Parametric studies were conducted with different diaphragm wall external friction angles δ. The results show that deflection of diaphragm wall and ground surface settlement decrease with the decrease of δ. However, the reduction effect on diaphragm wall deflection is the most significant at the depth where the maximum wall deflection occurs and can be neglected at the wall base. The ratio between wall deep inward component and wall cantilever component reaches its peak value 2.7 when δ=5°. The ratio of the maximum ground surface settlement to the maximum wall lateral deflection decreases at a reduced rate with the increase of δ. For excavation with braced diaphragm wall, the effect of friction between diaphragm and soil on the deflection of diaphragm wall and ground settlement, especially the distribution of ground surface settlement behind diaphragm, should be taken into account.
基金Project(50225518) supported by the National Science Foundation of China for Distinguished Young Scholars Projects(50175092+4 种基金 59975076) supported by the National Natural Science Foundation of China Project supported by the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, China Project(04H53057) supported by the Aeronautical Science Foundation of China Project(20020699002) supported by the Specialized Research Fund for the Doctoral Program of Higher Education, China Project(Z200518) supported by the Graduate Starting Seed Fund of Northwestern Polytechnical University, China
文摘The effect of frictions between dies and tube on the cross section quality of thin-walled tube numerical controlled(NC) bending was studied by numerical simulation method, combined with theoretical analysis and experiment. The results show that the frictions between mandrel, wiper, pressure die, bending die and tube have a significant and complicate effect on the section quality of thin-walled tube NC bending. To improve the section quality, frictions between mandrel, wiper and tube should be decreased, but the frictions between the pressure die, bending die and tube increase. The effect on the section distortion is more significant from mandrel, wiper, pressure die to bending die and the effect on the wall thinning more significant from mandrel, pressure die, wiper, to bending die. The effects of frictions between all dies and tube on wall thinning are smaller than their effects on section distortion. Mandrel and wiper should be lubricated well and drawing oil is used to lubricate them in actual production. The frictions between pressure die, bending die and tube should be increased and the dry friction is used between pressure die, bending die and tube in actual production.
基金funded by the National Natural Science Foundation of China (11572078 and 91752101)973 Plan (2014CB744100)
文摘In this letter we present a novel wall shear stress measurement technique for a turbulent boundary layer using sandwiched hot-film sensors. Under certain conditions, satisfactory results can be obtained using only the heat generated by one of the hot-film and a calibration of the sensors is not required. Two thin Nickel films with the same size were used in this study, separated by an electrical insulating layer. The upper film served as a sensor and the bottom one served as a guard heater. The two Nickel films were operated at a same temperature, so that the Joule heat flux generated by the sensor film transferred to the air with a minimum loss or gain depending on the uncertainties in the film temperature measurements. Analytical solution of the shear stress based on the aforementioned heat flux was obtained. The preliminary results were promising and the estimated wall shear stresses agreed reasonablywell with the directly measured values （with errors less than 20%） in a fully developed turbulent pipe flow. The proposed technique can be improved to further increase precisions.
文摘Achieving densely packed particles is desirable within the industries of ceramics,pharmaceuticals,defence and additive manufacturing.In this work,we use the discrete element method(DEM)to determine the effect of wall parameters on the final packing density of mono-disperse spheres subject to 4 varying three-dimensional vibration and fill conditions.We focus specifically on the impact of the container wall parameters on the particles'final packing density.Following on from the validation of the DEM simulation the particle-wall coefficient of restitution,the particle-wall coefficient of rolling friction and the particle-wall coefficient of sliding friction were varied individually and the effect on the final packing density analysed.For relatively low particle-particle friction glass beads,the effect of these wall properties had no discernible effect on the final packing density achieved.Following on from these findings the particle-wall properties were varied at the extreme values of particle-particle coefficient of rolling friction and particle-particle coefficient of sliding friction.For a particle-particle coefficient of sliding friction=1,increases in particle-wall coefficient of restitution resulted in a minor increase in the final packing density of particles though this was not statistically significant.For a particle-particle coefficient of sliding friction=1,increases in particle-wall coefficient of rolling friction resulted in a minor decrease in the final packing density of the particles though again not to a degree where the trend can,with complete certainty,be distinguished from the random error across the repeats.Finally,when the particle-particle coefficient of sliding friction=1,increases in particle-wall coefficient of sliding friction resulted in a significant decrease in the final packing density of particles.This decrease was attributed to the propagation of force chains throughout the packing.The significant decrease in final packing density with particle-wall coefficient of sliding friction highlights the need to choose appropriate vessel materials to optimise packing of particles with a high particle-particle coefficient sliding friction.Conversely,for particles with minimal particle-particle friction,the particle-wall friction coefficient has no effect on the final packing density of particles-a potentially valuable finding for certain industrial applications.All simulations were run using the open-source DEM package LIGGGHTS on the University of Birmingham's high-performance computer:BlueBEAR.All the code files used within this paper can be found on Github:https://github.com/Jack-Grogan/DEM-Vibropacking-Wall-Effects.
