A great interest of using curved pipes was shown in the industrial applications due to many advantages such as developed agitation which can be obtained as an alternative to the conventional agitation at lower energy ...A great interest of using curved pipes was shown in the industrial applications due to many advantages such as developed agitation which can be obtained as an alternative to the conventional agitation at lower energy consumption and reduced cost.A computational investigations were conducted in this study to contemplate on the enhancement occur in the mixing flow upon using single and multi loops helical coil compared with a straight tube.The objective was to optimize the number of loops in the helical coil.The computations were achieved via“Fluent software”in which 3d,second order upwind were used for all cases studied.The study consisted of four parts in which the numerical error study was conducted in the first part to optimize the grid meshing and to make sure that the results are independent of it.The effect of using the helical coil on the pressure drop was conducted in the second part.The third and fourth parts of the study were devoted to envisage the enhancement of applying the curved loops in the coil on the heat and mass transfer,respectively.The main findings of the study were limited effect of single loop coil on the heat and mass transfer processes.Better effect was depicted upon increasing the number of loops in the helical coil into 2.However,applying 2 loops has resulted in building up higher pressure drops.Moreover,the pressure drop was shown slightly higher in the case of applying water when compared with air.展开更多
The definition of the thixotropy is a decrease in viscosity with time in shear and a subsequent recovery of viscosity after the shear deformation is removed.We used therefore start-up experiments with constant shear r...The definition of the thixotropy is a decrease in viscosity with time in shear and a subsequent recovery of viscosity after the shear deformation is removed.We used therefore start-up experiments with constant shear rates and can speak consequently of shear stress instead of viscosity.The shear stress increases sharply at the beginning of the stress growth experiment,passes an overshoot and begins to decrease up to the steady state shear stress.We can assume that a stress growth curve with an overshoot points out to thixotropic behavior.After the steady state shear stress is reached,the shear deformation is stopped,and the stress relaxation begins.It is to be expected that the shear stress will recover after the stress relaxation.Systems with shear thinning and plastic flow behavior were examined.The thixotropy is discussed in relation to the flow behavior.Stress growth experiments were performed with shear rates from the first Newtonian region and from the shear thinning region of the systems with shear thinning flow behavior.Stress start-up experiments were carrying out with shear rates from the first and second yield stress regions and from the transition section of a system with plastic flow behavior.展开更多
A circulating fluidized bed evaporator(including down-flow,horizontal,and up-flow beds) was constructed to study the effect of flow directions on multiphase flow boiling heat transfer.A range of experimental investiga...A circulating fluidized bed evaporator(including down-flow,horizontal,and up-flow beds) was constructed to study the effect of flow directions on multiphase flow boiling heat transfer.A range of experimental investigations were carried out by varying amount of added particles(0-2%),circulation flow rate(2.15-5.16 m^3/h) and heat flux(8-16 kW/m^2).The comparison of heat transfer performance in different vertical heights of the horizontal bed was also discussed.Results reveal that the glass bead particle can enhance heat transfer compared with vapor-liquid two-phase flow for all beds.At a low heat flux(q = 8 kW/m),the heat-transfer-enhancing factor of the horizontal bed is obviously greater than those of the up-flow and down-flow beds.With the increase in the amount of added particles,the heat-transfer-enhancing factors of the up-flow and down-flow beds increase,whereas that of the horizontal bed initially increases and then decreases.However,at a high heat flux(q=16 kW/m),the heat-transfer-enhancing factors of the three beds show an increasing tendency with the increase in the amount of added particles and become closer than those at a low heat flux.For all beds,the heat-transfer-enhancing factor generally increases with the circulation flow rate but decreases with the increase in heat flux.展开更多
A better understanding of airflow characteristics in the upper airway(UA) is crucial in investigating obstructive sleep apnea(OSA), particle sedimentation, drug delivery, and many biomedical problems. Direct visualiza...A better understanding of airflow characteristics in the upper airway(UA) is crucial in investigating obstructive sleep apnea(OSA), particle sedimentation, drug delivery, and many biomedical problems. Direct visualization of air flow patterns in in-vitro models with realistic anatomical structures is a big challenge. In this study, we constructed unique half-side transparent physical models of normal UA based on realistic anatomical structures. A smoke-wire method was developed to visualize the air flow in UA models directly. The results revealed that the airflow through the pharynx was laminar but not turbulent under normal inspiration, which suggested that compared with turbulent models, a laminar model should be more suitable in numerical simulations. The flow predicted numerically using the laminar model was consistent with the observations in the physical models. The comparison of the velocity fields predicted numerically using the half-side and complete models confirmed that it was reasonable to investigate the flow behaviors in UA using the half-side model. Using the laminar model, we simulated the flow and evaluated the effects of UA narrowing caused by rostral fluid shift on pharyngeal resistance. The results suggested that fluid shift could play an important role in the formation of hypopnea or OSA during sleep.展开更多
Impinging-jet injectors are widely used in liquid propulsion applications, since their simple configuration provides reliable and efficient atomization. The flowfield involves a series of complicated spatio-temporal e...Impinging-jet injectors are widely used in liquid propulsion applications, since their simple configuration provides reliable and efficient atomization. The flowfield involves a series of complicated spatio-temporal evolutions. Much effort has been directed toward understanding the underlying physics and developing quantitative predictions of impinging-jet atomization. This paper summarizes the recent advances in this direction, including state-of-the-art theoretical, experimental, and numerical studies, along with representative results. Finally, concluding remarks address remaining challenges and highlight modeling capabilities of high-fidelity simulations.展开更多
A series of three-dimensional numerical simulations is carried out to investigate the effect of inclined angle on flow behavior behind two side-by-side inclined cylinders at low Reynolds number Re=100 and small spacin...A series of three-dimensional numerical simulations is carried out to investigate the effect of inclined angle on flow behavior behind two side-by-side inclined cylinders at low Reynolds number Re=100 and small spacing ratio T/D=1.5 (T is the center-to-center distance between two side-by-side cylinders, D is the diameter of cylinder). The instantaneous and time-averaged flow fields, force coefficients and Strouhal numbers are analyzed. Special attention is focused on the axial flow characteristics with variation of the inclined angle. The results show that the inclined angle has a significant effect on the gap flow behaviors behind two inclined cylinders. The vortex shedding behind two cylinders is suppressed with the increase of the inclined angle as well as the flip-flop gap flow. Moreover, the mean drag coefficient, root-mean-square lift coefficient and Strouhal numbers decrease monotonously with the increase of the inclined angle, which follows the independent principle at small inclined angles.展开更多
The improved delayed detached-eddy simulation(IDDES) method is used to simulate the reverse flows past an NACA0012 airfoil at medium(10°) and large(30°) angles of attack. The numerical results of the baselin...The improved delayed detached-eddy simulation(IDDES) method is used to simulate the reverse flows past an NACA0012 airfoil at medium(10°) and large(30°) angles of attack. The numerical results of the baseline configuration are compared with the available measurements. The effects of the undulating leading edge with four different amplitudes are compared and analyzed at angle of attack of 10°. Based on these analyses, the amplitude of A/C=0.04 yields the best performance. Compared with the uncontrolled case, the performances of the undulating leading edge are greatly improved with reducing of the aerodynamic fluctuations. Furthermore, the mechanisms of performance are explored by comparing the local flow structures near the undulations.展开更多
The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical c...The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street's forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented.展开更多
The development of shale gas reservoir is mainly based on horizontal well production.Slug flow of gas-liquid two-phase is invariably encountered in inclined wells and horizontal wells of a producing environment.Due to...The development of shale gas reservoir is mainly based on horizontal well production.Slug flow of gas-liquid two-phase is invariably encountered in inclined wells and horizontal wells of a producing environment.Due to gravitational differentiation,oil-water two-phase flow pattern,the local velocity and local phase holdup along the radial direction of pipe in near horizontal wells will perform complicatedly.This paper presented the results of an experimental study and a theoretical analysis of two-phase gas/water flow in horizontal and highly inclined systems.Extensive experiments were conducted using a test loop made of 124 mm diameter acrylic pipe with inclination angles from the horizontal of 0°,5°,15°,45°,°2°,°5°and°10°,and with the total flow rate ranging from 50 to 800 m3/day.Based on the research on the law of slug flow dynamics model for gas-water two-phase flow in near horizontal pipeline,the theoretical analysis and experimental researches were done to propose the expressions of stable and exact production logging interpretation model for two-phase flow in near horizontal pipeline.The performance of the proposed method for estimating water holdup and water superficial velocity is in good agreement with our measurements.As a result,the slug flow dynamics model of gas-water two-phase flow in near horizontal wellbore was developed.The application effect of production logging in near horizontal wells had been improved.展开更多
Experimental studies on the flow pattern and the pressure gradient of gas-liquid co-current two-phase flow in a mini-channel were conducted.The test section was a transparent circular channel of 1.6 mm inner diameter....Experimental studies on the flow pattern and the pressure gradient of gas-liquid co-current two-phase flow in a mini-channel were conducted.The test section was a transparent circular channel of 1.6 mm inner diameter.The working fluids were air and water.The superficial velocities of gas and liquid were in the range of 0.025-66.300 m/s and 0.033-4.935 m/s,respectively.In the present work,the flow pattern and the pressure gradient data were obtained by analyzing the flow images captured by a high-speed camera,and by using the pressure transducer,respectively.As a result,it was found that (1) the obtained flow patterns were bubbly,plug,slug-annular,annular,and chum flows,(2) new experimental correlations on the bubble and plug lengths were proposed,whereas the lengths are the function of the homogeneous void fraction,(3) both gas and liquid superficial velocities affect proportionally to the pressure gradient,whereas it increases with the increase both of JG,JL.In addition,the obtained flow patterns are in a good agreement with that of the available flow pattern maps in the open literatures,such as,Triplett et al.(1999) and Chung and Kawaji (2004).展开更多
Most unsteady channel flows in nature and practical engineering appear as gradually varied ones,and in the free surface,the deformation conforms to the long wave hypothesis.One-dimensional total flow models were usual...Most unsteady channel flows in nature and practical engineering appear as gradually varied ones,and in the free surface,the deformation conforms to the long wave hypothesis.One-dimensional total flow models were usually used to for the numerical simulation of long-term and long-distance reaches to describe the water movements,however,the models lack a clear relationship between the three-dimensional flow field and the total flow field.Moreover,few studies of the variations of the roughness coefficient against the time in unsteady flows were conducted.The following results are obtained through the theoretical analysis and the numerical simulations in this paper.(1) One-dimensional total flow control equations of the unsteady gradually varied flow in open channels are obtained directly from the mathematical model of the viscous fluid motion,and can both reflect the influence of the turbulence and provide an explicit expression of the energy slope term.These equations establish a direct connection between the descriptions of the three-dimensional flow fields and the one-dimensional total flows.(2) Synchronous prototype observation data and planar two-dimensional numerical simulation results are used to extract the one-dimensional total flow information and discuss the total flow characteristics.(3)The orders of magnitude for terms in the total flow motion equation are compared,and the variation of the roughness coefficient against the time is analyzed.展开更多
Direct numerical simulations of temporally evolving supersonic turbulent channel flows of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800 for various values of the dimensional wall temp...Direct numerical simulations of temporally evolving supersonic turbulent channel flows of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800 for various values of the dimensional wall temperature to study the influence of the latter on the velocity-temperature correlations. The results show that in a fully developed turbulent channel flow, as the dimensional wall temperature increases, there is little change in the mean velocity, but the mean temperature decreases. The mean temperature is found to be a quadratic function of the mean velocity, the curvature of which increases with increasing dimensional wall temperature. The concept of 'recovery enthalpy' provides a connection between the mean velocity and the mean temperature, and is independent of dimensional wall temperature. The right tails of probability density function of the streamwise velocity fluctuation grows with increasing dimensional wall temperature. The dimensional wall temperature does not have a significant influence on the Reynolds analogy factor or strong Reynolds analogy(SRA). The modifications of SRA by Huang et al. and Zhang et al. provide reasonably good results, which are better than those of the modifications by Cebeci and Smith and by Rubesin.展开更多
In a crystalline material, structural defects such as dislocations or twins are well defined and largely determine the mechanical and other properties of the material. For metallic glass(MG) with unique properties in ...In a crystalline material, structural defects such as dislocations or twins are well defined and largely determine the mechanical and other properties of the material. For metallic glass(MG) with unique properties in the absence of a long-range lattice, intensive efforts have focused on the search for similar ’defects’. The primary objective has been the elucidation of the flow mechanism of MGs. However,their atomistic mechanism of mechanical deformation and atomic flow response to stress, temperature, and failure, have proven to be challenging. In this paper, we briefly review the state-of-the-art studies on the dynamic defects in metallic glasses from the perspective of flow units. The characteristics, activation and evolution processes of flow units as well as their correlation with mechanical properties, including plasticity,strength, fracture, and dynamic relaxation, are introduced. We show that flow units that are similar to structural defects such as dislocations are crucial in the optimization and design of metallic glassy materials via the thermal, mechanical and high-pressure tailoring of these units. In this report, the relevant issues and open questions with regard to the flow unit model are also introduced and discussed.展开更多
Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orienta...Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number Reτ=100 given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wallnormal direction, which are consistent with earlier observation in low Reynolds number (Reτ=180)wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.展开更多
Due to its multiscale and multi-layer natures,the coherent structures of turbulent in the open channel flow is complex and difficult to be visualized for understanding its evolution.In this paper,five types of methods...Due to its multiscale and multi-layer natures,the coherent structures of turbulent in the open channel flow is complex and difficult to be visualized for understanding its evolution.In this paper,five types of methods for the vortical structure in the fluids,namely the Q-criterion,the vorticity,the Omega method,the velocity-vorticity correlation structures(VVCS)as well as the most recent Rortex method,are adopted to visualize the turbulent flow in the open channel.With the free surface modelled as a free slip boundary,a direct numerical simulation(DNS)is carried out to study the multi-layered flow structure characteristics under the free surface.The visualization results by the Q-criterion,the vorticity,the Omega method and the Rortex are firstly analyzed.Then the turbulent flow layers near the free surface are identified with corresponding anisotropy indices.Afterwards,the VVCS within various turbulence layers are visualized accordingly.This research indicates that the VVCS can straightforwardly show the geometry information of the cohere nt structures of turbulent in different layers for the ope n channel flow.展开更多
Conventionally,flexible barriers are rated based on their ability to resist a free-falling boulder with a particular input energy.However,there is still no well-accepted approach for evaluating performance of flexible...Conventionally,flexible barriers are rated based on their ability to resist a free-falling boulder with a particular input energy.However,there is still no well-accepted approach for evaluating performance of flexible barrier under debris flow impact.In this study,a large-nonlinear finite-element model was used to back-analyze centrifuge tests to discern the effects of impact material type,barrier stiffness,and flow aspect ratio(flow height to flow length)on the reaction force between the impacting medium and flexible barrier.Results show that,in contrast to flexible barriers for resisting rockfall,the normal impact force induced by the highly frictional and viscous debris is insensitive to barrier stiffness.This is because the elongated distributions of kinetic energy are mainly dissipated by the internal and boundary shearing,and only a small portion is forwarded to the barrier.Furthermore,a new stiffness number is proposed to characterize the equivalent stiffness between a debris flow or a boulder,and a flexible barrier.Under the circumstance of an extremely elongated debris flow event,i.e.,low aspect ratio,the load on a barrier is dominated by the static component and thus not sensitive to the barrier stiffness.展开更多
Calculating the flow accumulation matrix is an essential step for many hydrological and topographical analyses.This study gives an overview of the existing algorithms for flow accumulation calculations for singleflow ...Calculating the flow accumulation matrix is an essential step for many hydrological and topographical analyses.This study gives an overview of the existing algorithms for flow accumulation calculations for singleflow direction matrices.A fast and simple algorithm for calculating flow accumulation matrices is proposed in this study.The algorithm identifies three types of cells in a flow direction matrix: source cells,intersection cells,and interior cells.It traverses all source cells and traces the downstream interior cells of each source cell until an intersection cell is encountered.An intersection cell is treated as an interior cell when its last drainage path is traced and the tracing continues with its downstream cells.Experiments are conducted on thirty datasets with a resolution of 3 m.Compared with the existing algorithms for flow accumulation calculation,the proposed algorithm is easy to implement,runs much faster than existing algorithms,and generally requires less memory space.展开更多
The two-dimensional (2D) pseudo-steady isothermal flow, which is isentropic and irrotational, around a convex corner is studied. The self-similar solutions for the supersonic flow around the convex corner are construc...The two-dimensional (2D) pseudo-steady isothermal flow, which is isentropic and irrotational, around a convex corner is studied. The self-similar solutions for the supersonic flow around the convex corner are constructed, where the properties of the centered simple wave are used for the 2D isentropic irrotational pseudo-steady Euler equations. The geometric procedures of the center simple waves are given. It is proven that the supersonic flow turns the convex corner by an incomplete centered expansion wave or an incomplete centered compression wave, depending on the conditions of the downstream state.展开更多
文摘A great interest of using curved pipes was shown in the industrial applications due to many advantages such as developed agitation which can be obtained as an alternative to the conventional agitation at lower energy consumption and reduced cost.A computational investigations were conducted in this study to contemplate on the enhancement occur in the mixing flow upon using single and multi loops helical coil compared with a straight tube.The objective was to optimize the number of loops in the helical coil.The computations were achieved via“Fluent software”in which 3d,second order upwind were used for all cases studied.The study consisted of four parts in which the numerical error study was conducted in the first part to optimize the grid meshing and to make sure that the results are independent of it.The effect of using the helical coil on the pressure drop was conducted in the second part.The third and fourth parts of the study were devoted to envisage the enhancement of applying the curved loops in the coil on the heat and mass transfer,respectively.The main findings of the study were limited effect of single loop coil on the heat and mass transfer processes.Better effect was depicted upon increasing the number of loops in the helical coil into 2.However,applying 2 loops has resulted in building up higher pressure drops.Moreover,the pressure drop was shown slightly higher in the case of applying water when compared with air.
文摘The definition of the thixotropy is a decrease in viscosity with time in shear and a subsequent recovery of viscosity after the shear deformation is removed.We used therefore start-up experiments with constant shear rates and can speak consequently of shear stress instead of viscosity.The shear stress increases sharply at the beginning of the stress growth experiment,passes an overshoot and begins to decrease up to the steady state shear stress.We can assume that a stress growth curve with an overshoot points out to thixotropic behavior.After the steady state shear stress is reached,the shear deformation is stopped,and the stress relaxation begins.It is to be expected that the shear stress will recover after the stress relaxation.Systems with shear thinning and plastic flow behavior were examined.The thixotropy is discussed in relation to the flow behavior.Stress growth experiments were performed with shear rates from the first Newtonian region and from the shear thinning region of the systems with shear thinning flow behavior.Stress start-up experiments were carrying out with shear rates from the first and second yield stress regions and from the transition section of a system with plastic flow behavior.
基金Tianjin Municipal Science and Technology Commission,China (No.2009ZCKFGX01900).
文摘A circulating fluidized bed evaporator(including down-flow,horizontal,and up-flow beds) was constructed to study the effect of flow directions on multiphase flow boiling heat transfer.A range of experimental investigations were carried out by varying amount of added particles(0-2%),circulation flow rate(2.15-5.16 m^3/h) and heat flux(8-16 kW/m^2).The comparison of heat transfer performance in different vertical heights of the horizontal bed was also discussed.Results reveal that the glass bead particle can enhance heat transfer compared with vapor-liquid two-phase flow for all beds.At a low heat flux(q = 8 kW/m),the heat-transfer-enhancing factor of the horizontal bed is obviously greater than those of the up-flow and down-flow beds.With the increase in the amount of added particles,the heat-transfer-enhancing factors of the up-flow and down-flow beds increase,whereas that of the horizontal bed initially increases and then decreases.However,at a high heat flux(q=16 kW/m),the heat-transfer-enhancing factors of the three beds show an increasing tendency with the increase in the amount of added particles and become closer than those at a low heat flux.For all beds,the heat-transfer-enhancing factor generally increases with the circulation flow rate but decreases with the increase in heat flux.
基金supported by the National Nature Science Foundation of China (31670959, 81171422)the National Science and Technology Pillar Program of China (2012BAI05B03)+2 种基金the Key Projects in Science and Technology Program of Beijing Municipal Education Commission, China (KZ201210025022)Beijing Postdoctoral Research Foundation (2016ZZ-45)Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application.
文摘A better understanding of airflow characteristics in the upper airway(UA) is crucial in investigating obstructive sleep apnea(OSA), particle sedimentation, drug delivery, and many biomedical problems. Direct visualization of air flow patterns in in-vitro models with realistic anatomical structures is a big challenge. In this study, we constructed unique half-side transparent physical models of normal UA based on realistic anatomical structures. A smoke-wire method was developed to visualize the air flow in UA models directly. The results revealed that the airflow through the pharynx was laminar but not turbulent under normal inspiration, which suggested that compared with turbulent models, a laminar model should be more suitable in numerical simulations. The flow predicted numerically using the laminar model was consistent with the observations in the physical models. The comparison of the velocity fields predicted numerically using the half-side and complete models confirmed that it was reasonable to investigate the flow behaviors in UA using the half-side model. Using the laminar model, we simulated the flow and evaluated the effects of UA narrowing caused by rostral fluid shift on pharyngeal resistance. The results suggested that fluid shift could play an important role in the formation of hypopnea or OSA during sleep.
基金the National Natural Science Foundation of China (Nos.11772343 and 11402274)partly by the Beijing Institute of Technology Research Fund Program for Young Scholars.
文摘Impinging-jet injectors are widely used in liquid propulsion applications, since their simple configuration provides reliable and efficient atomization. The flowfield involves a series of complicated spatio-temporal evolutions. Much effort has been directed toward understanding the underlying physics and developing quantitative predictions of impinging-jet atomization. This paper summarizes the recent advances in this direction, including state-of-the-art theoretical, experimental, and numerical studies, along with representative results. Finally, concluding remarks address remaining challenges and highlight modeling capabilities of high-fidelity simulations.
基金Joint Key Funds of Zhejiang Provincial Natural Science Foundation of China and Powerchina Huadong Engineering Corporation Limited (Grant No. LHZ19E090004)the National Key R&D Program of China (Grant No. 2018YFD0900901).
文摘A series of three-dimensional numerical simulations is carried out to investigate the effect of inclined angle on flow behavior behind two side-by-side inclined cylinders at low Reynolds number Re=100 and small spacing ratio T/D=1.5 (T is the center-to-center distance between two side-by-side cylinders, D is the diameter of cylinder). The instantaneous and time-averaged flow fields, force coefficients and Strouhal numbers are analyzed. Special attention is focused on the axial flow characteristics with variation of the inclined angle. The results show that the inclined angle has a significant effect on the gap flow behaviors behind two inclined cylinders. The vortex shedding behind two cylinders is suppressed with the increase of the inclined angle as well as the flip-flop gap flow. Moreover, the mean drag coefficient, root-mean-square lift coefficient and Strouhal numbers decrease monotonously with the increase of the inclined angle, which follows the independent principle at small inclined angles.
基金the National Natural Science Foundation of China(Grant Nos.91852113,11772174,and 11672158)the National Key Research and Development Program of China(Grant No.2016YFA0401200).
文摘The improved delayed detached-eddy simulation(IDDES) method is used to simulate the reverse flows past an NACA0012 airfoil at medium(10°) and large(30°) angles of attack. The numerical results of the baseline configuration are compared with the available measurements. The effects of the undulating leading edge with four different amplitudes are compared and analyzed at angle of attack of 10°. Based on these analyses, the amplitude of A/C=0.04 yields the best performance. Compared with the uncontrolled case, the performances of the undulating leading edge are greatly improved with reducing of the aerodynamic fluctuations. Furthermore, the mechanisms of performance are explored by comparing the local flow structures near the undulations.
基金supported by the National Natural Science Foundation of China(Grant Nos.51839001,51679005 and 91752105)the Natural Science Foundation of Beijing(Grant No.3172029).
文摘The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street's forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented.
基金Educational Commission of Hubei Province of China(D20141302)Open Fund of Key Laboratory of Exploration Technologies for Oil and Gas Resources(Yangtze University),Ministry of Education(No.K2018-02)National Natural Science Foundation of China(41474115).
文摘The development of shale gas reservoir is mainly based on horizontal well production.Slug flow of gas-liquid two-phase is invariably encountered in inclined wells and horizontal wells of a producing environment.Due to gravitational differentiation,oil-water two-phase flow pattern,the local velocity and local phase holdup along the radial direction of pipe in near horizontal wells will perform complicatedly.This paper presented the results of an experimental study and a theoretical analysis of two-phase gas/water flow in horizontal and highly inclined systems.Extensive experiments were conducted using a test loop made of 124 mm diameter acrylic pipe with inclination angles from the horizontal of 0°,5°,15°,45°,°2°,°5°and°10°,and with the total flow rate ranging from 50 to 800 m3/day.Based on the research on the law of slug flow dynamics model for gas-water two-phase flow in near horizontal pipeline,the theoretical analysis and experimental researches were done to propose the expressions of stable and exact production logging interpretation model for two-phase flow in near horizontal pipeline.The performance of the proposed method for estimating water holdup and water superficial velocity is in good agreement with our measurements.As a result,the slug flow dynamics model of gas-water two-phase flow in near horizontal wellbore was developed.The application effect of production logging in near horizontal wells had been improved.
文摘Experimental studies on the flow pattern and the pressure gradient of gas-liquid co-current two-phase flow in a mini-channel were conducted.The test section was a transparent circular channel of 1.6 mm inner diameter.The working fluids were air and water.The superficial velocities of gas and liquid were in the range of 0.025-66.300 m/s and 0.033-4.935 m/s,respectively.In the present work,the flow pattern and the pressure gradient data were obtained by analyzing the flow images captured by a high-speed camera,and by using the pressure transducer,respectively.As a result,it was found that (1) the obtained flow patterns were bubbly,plug,slug-annular,annular,and chum flows,(2) new experimental correlations on the bubble and plug lengths were proposed,whereas the lengths are the function of the homogeneous void fraction,(3) both gas and liquid superficial velocities affect proportionally to the pressure gradient,whereas it increases with the increase both of JG,JL.In addition,the obtained flow patterns are in a good agreement with that of the available flow pattern maps in the open literatures,such as,Triplett et al.(1999) and Chung and Kawaji (2004).
基金the National Key Projects of China (Grant No.2018YFC0407603).
文摘Most unsteady channel flows in nature and practical engineering appear as gradually varied ones,and in the free surface,the deformation conforms to the long wave hypothesis.One-dimensional total flow models were usually used to for the numerical simulation of long-term and long-distance reaches to describe the water movements,however,the models lack a clear relationship between the three-dimensional flow field and the total flow field.Moreover,few studies of the variations of the roughness coefficient against the time in unsteady flows were conducted.The following results are obtained through the theoretical analysis and the numerical simulations in this paper.(1) One-dimensional total flow control equations of the unsteady gradually varied flow in open channels are obtained directly from the mathematical model of the viscous fluid motion,and can both reflect the influence of the turbulence and provide an explicit expression of the energy slope term.These equations establish a direct connection between the descriptions of the three-dimensional flow fields and the one-dimensional total flows.(2) Synchronous prototype observation data and planar two-dimensional numerical simulation results are used to extract the one-dimensional total flow information and discuss the total flow characteristics.(3)The orders of magnitude for terms in the total flow motion equation are compared,and the variation of the roughness coefficient against the time is analyzed.
基金supported by the National Natural Science Foundation of China(Grant Nos.11502236,51536008,and 91852203)the National Key Research and Development Program of China(Grant No.2016YFA0401200)+1 种基金Science Challenge Project(Grant No.TZ2016001)the Natural Science Foundation of Zhejiang Province(Grant No.LQ16E090005).
文摘Direct numerical simulations of temporally evolving supersonic turbulent channel flows of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800 for various values of the dimensional wall temperature to study the influence of the latter on the velocity-temperature correlations. The results show that in a fully developed turbulent channel flow, as the dimensional wall temperature increases, there is little change in the mean velocity, but the mean temperature decreases. The mean temperature is found to be a quadratic function of the mean velocity, the curvature of which increases with increasing dimensional wall temperature. The concept of 'recovery enthalpy' provides a connection between the mean velocity and the mean temperature, and is independent of dimensional wall temperature. The right tails of probability density function of the streamwise velocity fluctuation grows with increasing dimensional wall temperature. The dimensional wall temperature does not have a significant influence on the Reynolds analogy factor or strong Reynolds analogy(SRA). The modifications of SRA by Huang et al. and Zhang et al. provide reasonably good results, which are better than those of the modifications by Cebeci and Smith and by Rubesin.
基金National Key Research and Development Plan(2016YFB0300501 and 2017YFB0903902)National Basic Research Program of China(973 Program)(2015CB856800)+2 种基金National Natural Science Foundation of China (11790291, 51571209 and 51461165101)the Key Research Program of Frontier Sciences, CAS (QYZDYSSW- JSC017)the Strategic Priority Research of CAS (XDPB0601).
文摘In a crystalline material, structural defects such as dislocations or twins are well defined and largely determine the mechanical and other properties of the material. For metallic glass(MG) with unique properties in the absence of a long-range lattice, intensive efforts have focused on the search for similar ’defects’. The primary objective has been the elucidation of the flow mechanism of MGs. However,their atomistic mechanism of mechanical deformation and atomic flow response to stress, temperature, and failure, have proven to be challenging. In this paper, we briefly review the state-of-the-art studies on the dynamic defects in metallic glasses from the perspective of flow units. The characteristics, activation and evolution processes of flow units as well as their correlation with mechanical properties, including plasticity,strength, fracture, and dynamic relaxation, are introduced. We show that flow units that are similar to structural defects such as dislocations are crucial in the optimization and design of metallic glassy materials via the thermal, mechanical and high-pressure tailoring of these units. In this report, the relevant issues and open questions with regard to the flow unit model are also introduced and discussed.
基金the National Natural Science Foundation of China (91752205, 11702158 and 11490551)the Programme for Supercomputing(NN2649K).
文摘Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number Reτ=100 given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wallnormal direction, which are consistent with earlier observation in low Reynolds number (Reτ=180)wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.
基金the National Natural Science Foundation of China(Grant Nos.51809084,91852117).
文摘Due to its multiscale and multi-layer natures,the coherent structures of turbulent in the open channel flow is complex and difficult to be visualized for understanding its evolution.In this paper,five types of methods for the vortical structure in the fluids,namely the Q-criterion,the vorticity,the Omega method,the velocity-vorticity correlation structures(VVCS)as well as the most recent Rortex method,are adopted to visualize the turbulent flow in the open channel.With the free surface modelled as a free slip boundary,a direct numerical simulation(DNS)is carried out to study the multi-layered flow structure characteristics under the free surface.The visualization results by the Q-criterion,the vorticity,the Omega method and the Rortex are firstly analyzed.Then the turbulent flow layers near the free surface are identified with corresponding anisotropy indices.Afterwards,the VVCS within various turbulence layers are visualized accordingly.This research indicates that the VVCS can straightforwardly show the geometry information of the cohere nt structures of turbulent in different layers for the ope n channel flow.
基金the National Natural Science Foundation of China(Grant Nos.51809261,11672318,and 51709052)financial support from the Theme-based Research Grant T22-603/15N+2 种基金the General Research Fund 16209717 providedthe Research Grants Council of the Government of Hong Kong SAR,Chinathe Hong Kong Jockey Club Disaster Preparedness and Response Institute(HKJCDPRI18EG01).
文摘Conventionally,flexible barriers are rated based on their ability to resist a free-falling boulder with a particular input energy.However,there is still no well-accepted approach for evaluating performance of flexible barrier under debris flow impact.In this study,a large-nonlinear finite-element model was used to back-analyze centrifuge tests to discern the effects of impact material type,barrier stiffness,and flow aspect ratio(flow height to flow length)on the reaction force between the impacting medium and flexible barrier.Results show that,in contrast to flexible barriers for resisting rockfall,the normal impact force induced by the highly frictional and viscous debris is insensitive to barrier stiffness.This is because the elongated distributions of kinetic energy are mainly dissipated by the internal and boundary shearing,and only a small portion is forwarded to the barrier.Furthermore,a new stiffness number is proposed to characterize the equivalent stiffness between a debris flow or a boulder,and a flexible barrier.Under the circumstance of an extremely elongated debris flow event,i.e.,low aspect ratio,the load on a barrier is dominated by the static component and thus not sensitive to the barrier stiffness.
基金the National Natural Science Foundation of China (Grant No.41671427)the Fundamental Research Funds for the Central Universities (ZYGX2016J148).
文摘Calculating the flow accumulation matrix is an essential step for many hydrological and topographical analyses.This study gives an overview of the existing algorithms for flow accumulation calculations for singleflow direction matrices.A fast and simple algorithm for calculating flow accumulation matrices is proposed in this study.The algorithm identifies three types of cells in a flow direction matrix: source cells,intersection cells,and interior cells.It traverses all source cells and traces the downstream interior cells of each source cell until an intersection cell is encountered.An intersection cell is treated as an interior cell when its last drainage path is traced and the tracing continues with its downstream cells.Experiments are conducted on thirty datasets with a resolution of 3 m.Compared with the existing algorithms for flow accumulation calculation,the proposed algorithm is easy to implement,runs much faster than existing algorithms,and generally requires less memory space.
基金the National Natural Science Foundation of China (Nos. 11371240 and 11771274).
文摘The two-dimensional (2D) pseudo-steady isothermal flow, which is isentropic and irrotational, around a convex corner is studied. The self-similar solutions for the supersonic flow around the convex corner are constructed, where the properties of the centered simple wave are used for the 2D isentropic irrotational pseudo-steady Euler equations. The geometric procedures of the center simple waves are given. It is proven that the supersonic flow turns the convex corner by an incomplete centered expansion wave or an incomplete centered compression wave, depending on the conditions of the downstream state.