A new force is introduced in the social force model （SFM） for computing following behavior in pedestrian counterflow, whereby an individual tries to approach others in the same direction to avoid conflicts with pede...A new force is introduced in the social force model （SFM） for computing following behavior in pedestrian counterflow, whereby an individual tries to approach others in the same direction to avoid conflicts with pedestrians from the opposite direction. The force, like a kind of gravitation, is modeled based on the movement state and visual field of the pedestrian, and is added to the classical SFM. The modified model is presented to study the impact of following behavior on the process of lane formation, the conflict, the number of lanes formed, and the traffic efficiency in the simulations. Simulation results show that the following behavior has a significant effect on the phenomenon of lane formation and the traffic efficiency.展开更多
This paper investigates the effect of blending dimethyl ether(DME)and ethanol on the soot transition periods in ethylene counterflow diffusion flames by using a novel optical diagnostic method.The soot critical transi...This paper investigates the effect of blending dimethyl ether(DME)and ethanol on the soot transition periods in ethylene counterflow diffusion flames by using a novel optical diagnostic method.The soot critical transition point in different conditions is identified experimentally and numerically.Two kinds of flames are carried out to gain the soot critical transition point in counterflow diffusion flames by changing oxygen fraction(Xo)and changing volume flow rates of fuel and oxidizer(Qv).The red-green-blue(RGB)ratio method is used to precisely identify the soot critical transition point,and chemical kinetic simulations are performed to analyze the detailed reaction paths.The results show that compared to the ethylene flame,the soot critical transition point occurs at a higher Xoand a lower Qvwhen DME or ethanol is blended.The addition of DME and ethanol can inhibit soot formation,due to the degree of soot formation reaction being lower than the degree of the oxidation reaction in the blending flames.展开更多
The pedestrian counterflow through a bottleneck in a channel shows a variety of flow patterns due to self-organization.In order to reveal the underlying mechanism,a cellular automaton model was proposed by incorporati...The pedestrian counterflow through a bottleneck in a channel shows a variety of flow patterns due to self-organization.In order to reveal the underlying mechanism,a cellular automaton model was proposed by incorporating the floor field and the view field which reflects the global information of the studied area and local interactions with others.The presented model can well reproduce typical collective behaviors,such as lane formation.Numerical simulations were performed in the case of a wide bottleneck and typical flow patterns at different density ranges were identified as rarefied flow,laminar flow,interrupted bidirectional flow,oscillatory flow,intermittent flow,and choked flow.The effects of several parameters,such as the size of view field and the width of opening,on the bottleneck flow are also analyzed in detail.The view field plays a vital role in reproducing self-organized phenomena of pedestrian.Numerical results showed that the presented model can capture key characteristics of bottleneck flows.展开更多
Methane has a narrow range of flammable limits,low flame speed and poor ignition characteristics,which limit its utilization in internal combustion engines.However,this issue can be remedied through the use of CH_(4)/...Methane has a narrow range of flammable limits,low flame speed and poor ignition characteristics,which limit its utilization in internal combustion engines.However,this issue can be remedied through the use of CH_(4)/DME blends,because DME has better ignition and combustion characteristics.In this study,the effects of pressure and blending ratio on the combustion characteristics of CH_(4)/DME blended fuels were investigated by using a high-pressure diffusion counterflow system,a constant volume combustion bomb,and CHEMKIN software.The reaction pressures are 0.1 MPa,0.2 MPa,0.3 MPa,and the blending ratios are 100%DME,75%DME+25%CH_(4),50%DME+50%CH_(4)and 25%DME+75%CH_(4)(mol%).The results show that the laminar flame speed,flame temperature,and extinction limit of CH_(4)/DME blended fuel decrease as the CH_(4)blending ratio or pressure increases.CH_(4)addition and increasing pressure both lead to the competition for OH and H radicals between CH_(4)and DME.However,the increase of CH_(4)mole fraction can also increase the path flux of CH_(4)+H=CH_(3)+H_(2),while the increase of pressure can decrease this path flux.Moreover,increasing pressure can promote all reaction processes and reaction rates.展开更多
Direct numerical simulations are used to investigate the Open Von Kfirmfin Swirling Flow, a new type of unsteady three-dimensional flow that is formed between two counter-rotating coaxial disks with an axial extractio...Direct numerical simulations are used to investigate the Open Von Kfirmfin Swirling Flow, a new type of unsteady three-dimensional flow that is formed between two counter-rotating coaxial disks with an axial extraction enclosed by a cylinder chamber. Solution verification shows that monotonic convergence is achieved on three systematically refined grids for average pressure at the disk periphery with a small grid uncertainty at 3.5%. Effects of the rotational speeds and flow rates on the flow field are examined. When the disks are rotating at the lowest speed, ＋100 RPM, only circular vortices are formed regardless of the flow rates. When the disks are rotating at ＋300 RPM and ＋500 RPM, negative spiral vortex network is formed. The radial counterflow concept for such spiral vortex network is verified by examining various horizontal cuts and radial velocity component, which show radial outflows in two bands near the two disks and radial inflow in one band between them. Overall, the flow is similar to the Stewartson type flow but with significant differences for all three velocity components due to the axial suction at the upper disk center and gap between the disk periphery and chamber wall.展开更多
Three-dimensional quasi-direct numerical simulations have been performed to investigate a thermal plasma reactor with a counterflow jet. The effects of the momentum flux ratio and distance between the counterflow jet ...Three-dimensional quasi-direct numerical simulations have been performed to investigate a thermal plasma reactor with a counterflow jet. The effects of the momentum flux ratio and distance between the counterflow jet and the thermal plasma jet on the flow characteristics are addressed. The numerical results show that the dimensionless location of the stagnation layer is significantly affected by the momentum flux ratio, but it is not dependent on the distance.Specifically, the stagnation layer is closer to the plasma torch outlet with the increase of the momentum flux ratio. Furthermore, the flow regimes of the stagnation layer and the flow characteristics of the thermal plasma jet are closely related to the momentum flux ratio. The characteristic frequencies associated with the different regimes are identified. The deflecting oscillation flow regimes are found when the momentum flux ratio is low, which provokes axial velocity fluctuations inside the thermal plasma jet. By contrast, for cases with a high momentum flux ratio, flapping flow regimes are distinguished. The thermal plasma jets are very stable and the axial velocity fluctuations mainly exist in the stagnation layer.展开更多
A round jet into a counterflow under different jet-to-current velocity ratios was investigated using large eddy simulation.The results agree well with experimental measurements from laser-Doppler anemometry and laser-...A round jet into a counterflow under different jet-to-current velocity ratios was investigated using large eddy simulation.The results agree well with experimental measurements from laser-Doppler anemometry and laser-induced fluorescence that include velocity and mean concentrations along the centerline and radial direction.Vortex rings appear in the region near the jet exit and large-scale vortex structures still occur near the stagnation point.The flow becomes more chaotic and three-dimensional with the presence of these structures.In particular,their presence near the stagnation point results in large velocity fluctuations that enhance the mixing process and dilution.These fluctuations are described by probability density functions that deviate from Gaussian distribution.The three-dimensional streamlines indicate that the jet not only oscillates in three directions but also rotates about the jet axis and around the vortex.The second and third moments of the velocity or scalar fluctuations identify that the mixing processes are greater in the region before the stagnation point.展开更多
CH_(4)/DME mixtures can be used for engines and gas turbines,and have already been studied for many years.However,DME has a strong cool flame phenomenon,which will greatly influence the ignition and combustion charact...CH_(4)/DME mixtures can be used for engines and gas turbines,and have already been studied for many years.However,DME has a strong cool flame phenomenon,which will greatly influence the ignition and combustion characteristics of following hot flames.Therefore,the cool flame characteristics of CH_(4)/DME mixture are very important for their utilization.Recently,the inhibition effect of CH_(4)on DME cool flames has been discovered,but the mechanisms of the inhibition effects lack further verification and research.In this study,the inhibition effects were investigated via both experiments and simulations.In order to validate the inhibition effects,a comparison fuel of CH_(3)OH/DME was also used in this study.The extinction limits,flame temperatures and combustion products of the cool flames of the CH_(4)/DME and CH_(3)OH/DME mixtures were measured using a counterflow burner,and the reaction paths and heat release rate were derived from the HPMech-v3.3.The results indicate that CH_(4)and CH_(3)OH will both inhibit the cool flame of DME via competing with DME for OH and O radicals,and CH_(3)OH has stronger inhibition effects than CH_(4),because it is more competitive and produces more CH2O,which inhibits the oxidation of DME.The HPMech-v3.3 closely agrees with the experimental data,but still needs to be improved.展开更多
A relation between heat transferred and energy loss, for turbulent flow. In different tube arrangements, is made. The conditions are determined which decide the dimensions and velocities for a heat exchanger. Also, a ...A relation between heat transferred and energy loss, for turbulent flow. In different tube arrangements, is made. The conditions are determined which decide the dimensions and velocities for a heat exchanger. Also, a reference to the economic dimensioning of heat exchangers is presented. In this study, the conditions which a heat exchanger must satisfy represent the best balance between the amounts of material employed. The investigation is restricted to the case of turbulent flow.展开更多
This paper describes the use of particle tracking velocimetry to analyze the Lagrangian acceleration of small particles in superfluid helium with varying time increments, . The probability density of acceleration exhi...This paper describes the use of particle tracking velocimetry to analyze the Lagrangian acceleration of small particles in superfluid helium with varying time increments, . The probability density of acceleration exhibits Gaussian properties for <sub></sub><sub></sub>, but displays a lognormal distribution for , where <sub></sub><sub></sub> is the migration time characterizing the particle motion. The particle trajectories are well characterized by the Hurst exponent H. For smaller time scales than <sub></sub><sub></sub>, the trajectories exhibit linear motion (), but have certain fractal properties with for time scales larger than <sub></sub>.展开更多
An elliptic jet and a square jet flowing into a counterflow with different jet-to-current velocity ratios are investigated by using realizable Ice model. Some computed mean velocity and scalar features agree reasonabl...An elliptic jet and a square jet flowing into a counterflow with different jet-to-current velocity ratios are investigated by using realizable Ice model. Some computed mean velocity and scalar features agree reasonably well with experimental measurements, and more features are obtained by analyzing the computed results. After fluid issues from a nozzle, it entrains ambient fluid, and its velocity and concentration on the centerline decay with the distance downstream from the potential core （10）. The decay ratio increases with the decreasing jet-to-current velocity ratio a. For an elliptic jet, the evolution of the excess velocity half-width b and the concentration half-width be merely remains constant near the jet exit on major-axis plane while they increase linearly on the minor-axis plane. However, the half-widths on the major-axis and minor-axis plane become proportional to the axial distance downstream after equaling each other. For a square jet, b and bc increase linearly with the distance downstream from the jet exit, but the spread ratio is larger on the middle plane than that on the diagonal plane before they equal each other. The radial extent of the dividing streamline r~ or the mixing boundary rs~ increases linearly downstream, and decreases exponentially after reaching a peak at Xb. The ratio on the minor-axis plane is larger than that on the major-axis plane for an elliptic jet. The characteristics are the same for the square jet. b, be, rs, and rsc on two corresponding planes become equal to each other more rapidly for the square jet than for the elliptic jet, because the sharp comer of the square nozzle induces secondary structures that are more intense. The distributions of the excess axial velocity and scalar concentration exhibit self-similarity for either the elliptic jet or square jet in the region of 10 〈 x 〈 xb. On the cross section, four counter-rotating pairs of vortices, which enhance the entrainment between the jet and counterflow, form at the four comers of the square jet or at 展开更多
基金Project supported by the National Natural Science Foundation of China （Nos. 51278221 and 51378076）, the Chinese Postdoc- toral Science Foundation （Nos. 2015M571369 and 2012M511343）, and Jilin Science and Technology Development Program, China （Nos. 20140204027SF and 20170101155JC）
文摘A new force is introduced in the social force model （SFM） for computing following behavior in pedestrian counterflow, whereby an individual tries to approach others in the same direction to avoid conflicts with pedestrians from the opposite direction. The force, like a kind of gravitation, is modeled based on the movement state and visual field of the pedestrian, and is added to the classical SFM. The modified model is presented to study the impact of following behavior on the process of lane formation, the conflict, the number of lanes formed, and the traffic efficiency in the simulations. Simulation results show that the following behavior has a significant effect on the phenomenon of lane formation and the traffic efficiency.
基金supported by the National Natural Science Foundation of China(Grant Nos.52106160 and 52076110)the Natural Science Foundation of Jiangsu Province(Grant No.BK20200490)。
文摘This paper investigates the effect of blending dimethyl ether(DME)and ethanol on the soot transition periods in ethylene counterflow diffusion flames by using a novel optical diagnostic method.The soot critical transition point in different conditions is identified experimentally and numerically.Two kinds of flames are carried out to gain the soot critical transition point in counterflow diffusion flames by changing oxygen fraction(Xo)and changing volume flow rates of fuel and oxidizer(Qv).The red-green-blue(RGB)ratio method is used to precisely identify the soot critical transition point,and chemical kinetic simulations are performed to analyze the detailed reaction paths.The results show that compared to the ethylene flame,the soot critical transition point occurs at a higher Xoand a lower Qvwhen DME or ethanol is blended.The addition of DME and ethanol can inhibit soot formation,due to the degree of soot formation reaction being lower than the degree of the oxidation reaction in the blending flames.
基金Project supported by the National Basic Research Program of China(Grant No.2012CB725404)the National Natural Science Foundation of China(Grant Nos.11172164 and 11572184)
文摘The pedestrian counterflow through a bottleneck in a channel shows a variety of flow patterns due to self-organization.In order to reveal the underlying mechanism,a cellular automaton model was proposed by incorporating the floor field and the view field which reflects the global information of the studied area and local interactions with others.The presented model can well reproduce typical collective behaviors,such as lane formation.Numerical simulations were performed in the case of a wide bottleneck and typical flow patterns at different density ranges were identified as rarefied flow,laminar flow,interrupted bidirectional flow,oscillatory flow,intermittent flow,and choked flow.The effects of several parameters,such as the size of view field and the width of opening,on the bottleneck flow are also analyzed in detail.The view field plays a vital role in reproducing self-organized phenomena of pedestrian.Numerical results showed that the presented model can capture key characteristics of bottleneck flows.
基金the support from the Fundamental Research Funds for the Central Universities(30919012104)the National Key R&D Program of China(2016YFB0600100)。
文摘Methane has a narrow range of flammable limits,low flame speed and poor ignition characteristics,which limit its utilization in internal combustion engines.However,this issue can be remedied through the use of CH_(4)/DME blends,because DME has better ignition and combustion characteristics.In this study,the effects of pressure and blending ratio on the combustion characteristics of CH_(4)/DME blended fuels were investigated by using a high-pressure diffusion counterflow system,a constant volume combustion bomb,and CHEMKIN software.The reaction pressures are 0.1 MPa,0.2 MPa,0.3 MPa,and the blending ratios are 100%DME,75%DME+25%CH_(4),50%DME+50%CH_(4)and 25%DME+75%CH_(4)(mol%).The results show that the laminar flame speed,flame temperature,and extinction limit of CH_(4)/DME blended fuel decrease as the CH_(4)blending ratio or pressure increases.CH_(4)addition and increasing pressure both lead to the competition for OH and H radicals between CH_(4)and DME.However,the increase of CH_(4)mole fraction can also increase the path flux of CH_(4)+H=CH_(3)+H_(2),while the increase of pressure can decrease this path flux.Moreover,increasing pressure can promote all reaction processes and reaction rates.
文摘Direct numerical simulations are used to investigate the Open Von Kfirmfin Swirling Flow, a new type of unsteady three-dimensional flow that is formed between two counter-rotating coaxial disks with an axial extraction enclosed by a cylinder chamber. Solution verification shows that monotonic convergence is achieved on three systematically refined grids for average pressure at the disk periphery with a small grid uncertainty at 3.5%. Effects of the rotational speeds and flow rates on the flow field are examined. When the disks are rotating at the lowest speed, ＋100 RPM, only circular vortices are formed regardless of the flow rates. When the disks are rotating at ＋300 RPM and ＋500 RPM, negative spiral vortex network is formed. The radial counterflow concept for such spiral vortex network is verified by examining various horizontal cuts and radial velocity component, which show radial outflows in two bands near the two disks and radial inflow in one band between them. Overall, the flow is similar to the Stewartson type flow but with significant differences for all three velocity components due to the axial suction at the upper disk center and gap between the disk periphery and chamber wall.
基金supported by National Natural Science Foundation of China (Nos. 12035015 and 12105282)。
文摘Three-dimensional quasi-direct numerical simulations have been performed to investigate a thermal plasma reactor with a counterflow jet. The effects of the momentum flux ratio and distance between the counterflow jet and the thermal plasma jet on the flow characteristics are addressed. The numerical results show that the dimensionless location of the stagnation layer is significantly affected by the momentum flux ratio, but it is not dependent on the distance.Specifically, the stagnation layer is closer to the plasma torch outlet with the increase of the momentum flux ratio. Furthermore, the flow regimes of the stagnation layer and the flow characteristics of the thermal plasma jet are closely related to the momentum flux ratio. The characteristic frequencies associated with the different regimes are identified. The deflecting oscillation flow regimes are found when the momentum flux ratio is low, which provokes axial velocity fluctuations inside the thermal plasma jet. By contrast, for cases with a high momentum flux ratio, flapping flow regimes are distinguished. The thermal plasma jets are very stable and the axial velocity fluctuations mainly exist in the stagnation layer.
基金supported by the National Natural Science Foundation of China (Grant No. 11172218)academic award for excellent Ph.D.Candidates funded by the Ministry of Education of China
文摘A round jet into a counterflow under different jet-to-current velocity ratios was investigated using large eddy simulation.The results agree well with experimental measurements from laser-Doppler anemometry and laser-induced fluorescence that include velocity and mean concentrations along the centerline and radial direction.Vortex rings appear in the region near the jet exit and large-scale vortex structures still occur near the stagnation point.The flow becomes more chaotic and three-dimensional with the presence of these structures.In particular,their presence near the stagnation point results in large velocity fluctuations that enhance the mixing process and dilution.These fluctuations are described by probability density functions that deviate from Gaussian distribution.The three-dimensional streamlines indicate that the jet not only oscillates in three directions but also rotates about the jet axis and around the vortex.The second and third moments of the velocity or scalar fluctuations identify that the mixing processes are greater in the region before the stagnation point.
基金The authors gratefully acknowledge the support from the National Key R&D Program of China(2016YFB0600100)the Fundamental Research Funds for the Central Universities(30919012104).
文摘CH_(4)/DME mixtures can be used for engines and gas turbines,and have already been studied for many years.However,DME has a strong cool flame phenomenon,which will greatly influence the ignition and combustion characteristics of following hot flames.Therefore,the cool flame characteristics of CH_(4)/DME mixture are very important for their utilization.Recently,the inhibition effect of CH_(4)on DME cool flames has been discovered,but the mechanisms of the inhibition effects lack further verification and research.In this study,the inhibition effects were investigated via both experiments and simulations.In order to validate the inhibition effects,a comparison fuel of CH_(3)OH/DME was also used in this study.The extinction limits,flame temperatures and combustion products of the cool flames of the CH_(4)/DME and CH_(3)OH/DME mixtures were measured using a counterflow burner,and the reaction paths and heat release rate were derived from the HPMech-v3.3.The results indicate that CH_(4)and CH_(3)OH will both inhibit the cool flame of DME via competing with DME for OH and O radicals,and CH_(3)OH has stronger inhibition effects than CH_(4),because it is more competitive and produces more CH2O,which inhibits the oxidation of DME.The HPMech-v3.3 closely agrees with the experimental data,but still needs to be improved.
文摘A relation between heat transferred and energy loss, for turbulent flow. In different tube arrangements, is made. The conditions are determined which decide the dimensions and velocities for a heat exchanger. Also, a reference to the economic dimensioning of heat exchangers is presented. In this study, the conditions which a heat exchanger must satisfy represent the best balance between the amounts of material employed. The investigation is restricted to the case of turbulent flow.
文摘This paper describes the use of particle tracking velocimetry to analyze the Lagrangian acceleration of small particles in superfluid helium with varying time increments, . The probability density of acceleration exhibits Gaussian properties for <sub></sub><sub></sub>, but displays a lognormal distribution for , where <sub></sub><sub></sub> is the migration time characterizing the particle motion. The particle trajectories are well characterized by the Hurst exponent H. For smaller time scales than <sub></sub><sub></sub>, the trajectories exhibit linear motion (), but have certain fractal properties with for time scales larger than <sub></sub>.
基金supported by the National Natural Science Foundation of China(Grant Nos.51239003,51125034,11172218 and 51409085)China Postdoctoral Science Foundation funded projectJiangsu Postdoctoral Science Foundation funded project(Grant No.1302047B)
文摘An elliptic jet and a square jet flowing into a counterflow with different jet-to-current velocity ratios are investigated by using realizable Ice model. Some computed mean velocity and scalar features agree reasonably well with experimental measurements, and more features are obtained by analyzing the computed results. After fluid issues from a nozzle, it entrains ambient fluid, and its velocity and concentration on the centerline decay with the distance downstream from the potential core （10）. The decay ratio increases with the decreasing jet-to-current velocity ratio a. For an elliptic jet, the evolution of the excess velocity half-width b and the concentration half-width be merely remains constant near the jet exit on major-axis plane while they increase linearly on the minor-axis plane. However, the half-widths on the major-axis and minor-axis plane become proportional to the axial distance downstream after equaling each other. For a square jet, b and bc increase linearly with the distance downstream from the jet exit, but the spread ratio is larger on the middle plane than that on the diagonal plane before they equal each other. The radial extent of the dividing streamline r~ or the mixing boundary rs~ increases linearly downstream, and decreases exponentially after reaching a peak at Xb. The ratio on the minor-axis plane is larger than that on the major-axis plane for an elliptic jet. The characteristics are the same for the square jet. b, be, rs, and rsc on two corresponding planes become equal to each other more rapidly for the square jet than for the elliptic jet, because the sharp comer of the square nozzle induces secondary structures that are more intense. The distributions of the excess axial velocity and scalar concentration exhibit self-similarity for either the elliptic jet or square jet in the region of 10 〈 x 〈 xb. On the cross section, four counter-rotating pairs of vortices, which enhance the entrainment between the jet and counterflow, form at the four comers of the square jet or at
