A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504 C camer...A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504 C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701 V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation,demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.展开更多
Suspended sediment particles contained in inflows of water systems of hydropower plants (HPPs) cause hydro-abrasive erosion of the hydraulic turbines and structures leading to significant maintenance costs, efficiency...Suspended sediment particles contained in inflows of water systems of hydropower plants (HPPs) cause hydro-abrasive erosion of the hydraulic turbines and structures leading to significant maintenance costs, efficiency reductions, and downtime. Relevant parameters such as suspended sediment concentration (SSC), particle size distribution (PSD), shape, and mineralogical composition were measured with an online multi-frequency acoustic instrument and based on manually taken samples from the end of the sand trap of the Toss HPP in the Himalayan region, India. In the laboratory, the samples were analyzed using the gravimetric method, laser diffraction, turbidity, dynamic digital image processing, scanning electron microscope, petrography analysis, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The online instrument and the samples provided measurement results at a single point. To investigate vertical gradients in concentration and particle sizes, additional samples were collected 9 times at 7 relative water depths. The SSC, most particle sizes, and particle shape were found to be evenly distributed over depth except d90, i.e. the diameter which is not exceeded by 90% of the particle mass. d90 measured at 76% of the water depth was in the range of fine sand and was multiplied by 1.05 to obtain an average value representative for the entire depth. Improved methodologies to quantify both particle shape and size in an analytical model for hydro-abrasive erosion are proposed. Also, the PSD measuring performance of laser diffraction and dynamic imaging was studied and similar values of the median particle sizes were obtained from both instruments. Further, multi-frequency acoustic, turbidity and laser diffraction techniques were found suitable for SSC measurement at the test case HPP.展开更多
Fine bubble technology has been applied in many fields, including semiconductor processing, waste water treatment, and agricultural development. Fine bubble technology has the advantages of being environ-mentally beni...Fine bubble technology has been applied in many fields, including semiconductor processing, waste water treatment, and agricultural development. Fine bubble technology has the advantages of being environ-mentally benign, and ease of production and use when compared with other processes that involve chemicals or biological agents. However, the mechanisms of fine bubble applications in many aspects still require exploration. The characterization of fine bubbles is one of the essential issues for better understanding the technology. This article reviews modern particle characterization technologies, espe-cially those that can be used to characterize fine bubbles, and briefly reports on some applications of fine bubbles.展开更多
Mountainous torrents often carry large amounts of loose materials into the rivers, thus causing strong sediment transport. Experimentally it was found for the first time that when the intensive sediment motion occurs ...Mountainous torrents often carry large amounts of loose materials into the rivers, thus causing strong sediment transport. Experimentally it was found for the first time that when the intensive sediment motion occurs downstream over a gentle slope, the siltation of the riverbed is induced and the sediment particles can move upstream rapidly in the form of a retrograde sand wave, resulting in a higher water level along the river. To further study the complex mechanisms of this problem, a sediment mass model in the framework of the Smoothed Particle Hydrodynamics(SPH) method was presented to simulate the riverbed evolution, sediment particle motion, and the generation and development of dynamic hydraulic jump under the condition of sufficient sediment supply over a steep slope with varying angles. Because the sediment is not a continuous medium, the marker particle tracking approach was proposed to represent a piece of sediment with a marked sediment particle. The twophase SPH model realizes the interaction between the sediment and fluid by moving the bed boundary particles up and down, so it can reasonably treat the fluid-sediment interfaces with high CPU efficiency. The critical triggering condition of sediment motion, the propagation of the hydraulic jump and the initial siltation position were all systematically studied. The experimental and numerical results revealed the extra disastrous sediment effect in a mountainous flood. The findings will be useful references to the disaster prevention and mitigation in mountainous rivers.展开更多
Insulators on high-voltage Uansmission lines are almost the only man-made structures on the Earth's surface intended for long-term operation under strong electric fields. After samples of natural contaminant parti...Insulators on high-voltage Uansmission lines are almost the only man-made structures on the Earth's surface intended for long-term operation under strong electric fields. After samples of natural contaminant particles were collected from insulator surfaces in China, it was found that the particle diameter distribution (PDD) was mainly concentrated in the 5-50 μm range. To analyze the statistical characteristics of these particles, this work studies the physical processes of particle collision and adhesion using the theories of hydrodynamics and collision dynamics. The physical model considers coupling of the fluid field and the electric field, introduces an adhesion criterion, and establishes a particle and surface collision model. The effects of relative humidity, wind speed, aerodynamic shape, electric field type, and electric field strength on particle adhesion were analyzed. The results show that the relative humidity and wind speed have very significant effects and the influences of the electric field type and the electric field strength are obvious, but the in fluence of the aerodynamic shape is relatively weak. The simulation results support the statistical characteristics determined in this work. The physical model established here provides reference values for study of the adhesion characteristics of particles on surfaces under electric fields.展开更多
The weighted-sum-of-gray-gas(WSGG)model and Mie theory are applied to study the influents of particle size on the radiative transfer in high temperature homogeneous gas-particle mixtures,such as the flame in aero-engi...The weighted-sum-of-gray-gas(WSGG)model and Mie theory are applied to study the influents of particle size on the radiative transfer in high temperature homogeneous gas-particle mixtures,such as the flame in aero-engine combustor.The radiative transfer equation is solved by the finite volume method.The particle size is assumed to obey uniform distribution and logarithmic normal(L-N)distribution,respectively.Results reveal that when particle size obeys uniform distribution,increasing particle size with total particle volume fraction fvunchanged will result in the decreasing of the absolute value of radiative heat transfer properties,and the effect of ignoring particle scattering will also be weakened.Opposite conclusions can be obtained when total particle number concentration N0 is unchanged.Moreover,if particle size obeys L-N distribution,increasing the narrowness indexσor decreasing the characteristic diameter Dˉwith the total particle volume fraction fvunchanged will increase the absolute value of radiative heat transfer properties.With total particle number concentration N0 unchanged,opposite conclusions for radiative heat source and incident radiation terms can be obtained except for radiative heat flux term.As a whole,the effects of particle size on the radiative heat transfer in the high-temperature homogeneous gas-particle mixtures are complicated,and the particle scattering cannot be ignoring just according to the particle size.展开更多
This paper considers fluid mixing driven by inflows connected to a circular shallow lake using a numerical framework consisting of a shallow water hydrodynamic model and a passive particle-tracking model.With the flow...This paper considers fluid mixing driven by inflows connected to a circular shallow lake using a numerical framework consisting of a shallow water hydrodynamic model and a passive particle-tracking model.With the flow field driven by alternate inflows predicted by a shallow water model,particle trajectories are traced out using a particle tracking model.The horizontal fluid mixing dynamics are then interpreted using dynamics system analysis approaches including finite-time Lyapunov exponent(FTLE)and Lagrangian coherent structure(LCS).From the simulation results,it is confirmed that periodic inflows are able to create a weak dynamic system in an idealised circular lake,with the particle dynamics controlled by a single dimensionless parameter associated with the inflow duration.The mixing and transport property of the lake changes from regular to chaotic as the value of the dimensionless parameter increases until global chaotic particle dynamics is achieved.By further analysing the advection of particles injected continuously to the inflows(freshwater),the fate of“freshwater”particles in a“polluted”lake is tracked and revealed.The results provide useful guidance for engineering applications,i.e.,transferring freshwater from rivers to improve the water quality in polluted water bodies such as lakes.The presented approach will be able to facilitate the design of‘optimised’schemes for such engineering implementation.展开更多
Recent theoretical and numerical models for the motion of saltating particles close to stream beds are constituted of three sub-models: a) a set of equations describing the particle “free flight”, b) a sub-model to ...Recent theoretical and numerical models for the motion of saltating particles close to stream beds are constituted of three sub-models: a) a set of equations describing the particle “free flight”, b) a sub-model to calculate the post-collision particle velocity, and c) a mathematical representation of the bed roughness. In this paper, a comprehensive three-dimensional (3-D), theoretical/numerical model for bedload motion at large Reynolds numbers is presented. By using geometric considerations and stochastic parameters to characterize collisions with the wall, five new sub-models for representation of bed roughness are, for the first time to the best of our knowledge, proposed and implemented. The emphasis of this paper is on the particle model, for which Basset, Magnus, drag, submerged weight, virtual mass, and lift forces are included. For the range of particle sizes (sands) analyzed herein, it is found that the stream-wise contribution of the Basset force, compared to other forces, may be as large as 60%. Whereas in the wall-normal direction, the Basset force is equally important as the drag force, and it is exceeded only by the submerged weight. It is also found that the best agreement between numerical and experimental results in terms of jump length, jump height, and stream-wise particle velocity is achieved for restitution and friction coefficients of 0.65 and 0.1, respectively. Important conclusions are obtained regarding the lack of realistic prediction with available “roughness” models with small ranges of angles.展开更多
Solid-particle settling occurs in many natural and industrial processes, such as in the transportation of drilling cuttings and fracturing proppant. Knowledge of the drag coefficient and settling velocity of cuttings ...Solid-particle settling occurs in many natural and industrial processes, such as in the transportation of drilling cuttings and fracturing proppant. Knowledge of the drag coefficient and settling velocity of cuttings and proppant is of significance to hydraulics design, wellbore cleanout, and fracture optimization. We conducted 553 tests to investigate the settling characteristics of spherical and non-spherical particles in power-law fluids. Three major particle shapes (spherical, cubic, and cylindrical) and eight different particle sphericities were used to simulate cuttings and proppant, and power-law fluids were applied to simulate drilling and fracturing fluids. Based on the data analysis, a new drag coefficient-particle Reynolds number correlation was developed to determine the drag coefficient in a power-law fluid for spherical and non-spherical particles. The drag coefficient increases as the sphericity decreases for the same particle Reynolds number. For a specific particle shape, the drag coefficient decreases as the particle Reynolds number increases, but the decreasing trend is reduced at high particle Reynolds number conditions. An explicit settling-velocity equation was proposed to calculate the settling velocity of spherical and non-spherical particles in power-law fluids by considering the effect of sphericity. A suitable range for the proposed model is 0.0001 < Re <200, 0.471 <φ< 1, and 0.505 < n < 1. An illustrative example is presented to show how to calculate the drag coefficient and settling velocity in power-law fluids with given particle and fluid properties.展开更多
A discrete element model based on super-ellipsoids was used to simulate cylindrical particle flow. The model can describe a cylindrical particle accurately provided the shape indices of the super-ellipsoids are set to...A discrete element model based on super-ellipsoids was used to simulate cylindrical particle flow. The model can describe a cylindrical particle accurately provided the shape indices of the super-ellipsoids are set to appropriate values. To achieve more rapid calculations, we implemented an "oriented bounding box algorithm"(OBBA) for the initial contact detection of cylindrical particles. Several types of Simulations were performed to validate the super-ellipsoid model and the contact-detection algorithm. First, the effect of shape index of the super-ellipsoids on model accuracy was investigated through three simulations: impact of a cylindrical particle on a flat wall, flow of cylindrical particles in a rotating tumbler, and segregation of cylindrical particles of different length flowing down a bounded heap. The simulation results show that the super-ellipsoids describe cylindrical particles accurately when the shape index that specifies the sharpness of the cylinder edges is sufficiently large. The efficiency of the OBBA is measured by simulations in which a box is filled with cylindrical particles and a tumbler that contains cylindrical particles is rotated. The simulation results show that the OBBA can accelerate the calculations significantly. The effect of particle shape (such as aspect ratio and shape index) on the calculation speed was obtained. The simulation of rod-like particles tended to take more calculation time than that of disk-like particles, and the simulation time increased with an increasing particle-shape index.展开更多
The fragment replacement method (FRM), a particle breakage simulation method, is often used in discrete element simulations to investigate the particle breakage effect on the mechanical behavior of granular materials....The fragment replacement method (FRM), a particle breakage simulation method, is often used in discrete element simulations to investigate the particle breakage effect on the mechanical behavior of granular materials. The fragment size distribution of the fragment replacement mode of FRM, which is generally generated based on the fragmentation characteristics of single particles after uniaxial compression, affects the breakage process and the mechanical behavior of the particle assembly. However, existing fragment replacement modes are seldom generated based on experimental data analysis. To capture the fragmentation process and investigate the breakage function for the construction of the fragment replacement mode, 60 numerical single particle compression tests were implemented by DEM. The bonded-particle model was applied to generate the crushable rock particles. The numerical simulations were qualitatively validated by experimental results, and the fragment size of broken single particles was analyzed. The fractal dimension was used to describe the fragmentation degree of single particles after compression. The fragmentation degree was random, and the fractal dimensions of the 60 tests at the same loading displacement fit the Weibull distribution well. The characteristic fractal dimension increased with increasing loading displacement, indicating that the fragmentation of single particles is a gradual process. According to the overall breakage function of the 60 tests at the first bulk breakage, a two-stage distribution model with 4 parameters was proposed and validated by the numerical and experimental results. The various fracture patterns of a single particle at the first bulk breakage under compression tests were well captured by the two-stage distribution model. Finally, an initial application strategy using the two-stage distribution model to construct fragment replacement modes was discussed and presented.展开更多
When using the beam scanning method for particle beam therapy, the target volume is divided into many iso-energy slices and is irradiated slice by slice. Each slice may comprise thousands of discrete scanning beam pos...When using the beam scanning method for particle beam therapy, the target volume is divided into many iso-energy slices and is irradiated slice by slice. Each slice may comprise thousands of discrete scanning beam positions. An optimized scanning path can decrease the transit dose and may bypass important organs. The minimization of the scanning path length can be considered as a variation of the traveling salesman problem;the simulated annealing algorithm is adopted to solve this problem. The initial scanning path is assumed as a simple zigzag path;subsequently, random searches for accepted new paths are performed through cost evaluation and criteria-based judging. To reduce the optimization time of a given slice, random searches are parallelized by employing thousands of threads. The simultaneous optimization of multiple slices is realized by using many thread blocks of generalpurpose computing on graphics processing units hardware. Running on a computer with an Intel i7-4790 CPU and NVIDIA K2200 GPU, our new method required only 1.3 s to obtain optimized scanning paths with a total of 40 slices in typically studied cases. The procedure and optimization results of this new method are presented in this work.展开更多
The significance of soil mineral properties and secondary environmental conditions such as pH, temperature, ionic strength and time in the partitioning of eight selected polychlorinated biphenyl (PCB) congeners betwee...The significance of soil mineral properties and secondary environmental conditions such as pH, temperature, ionic strength and time in the partitioning of eight selected polychlorinated biphenyl (PCB) congeners between aqueous solution and soil particles with different grain sizes was studied. The mineral properties of a model soil sample were determined, and Brunauer–Emmett–Teller (BET) adsorption–desorption isotherms were employed to observe the surface characteristics of the individual modeled soil particles. Batch adsorption experiments were conducted to determine the sorption of PCBs onto soil particles of different sizes. The results revealed that the sorption of PCB congeners onto the soil was dependent on the amount of soil organic matter, surface area, and pore size distribution of the various individual soil particles. Low pH favored the sorption of PCBs, with maximum sorption occurring between pH 6.5 and 7.5 with an equilibration period of 8 hr. Changes in the ionic strength were found to be less significant. Low temperature favored the sorption of PCBs onto the soil compared to high temperatures. Thermodynamic studies showed that the partition coefficient (Kd) decreased with increasing temperature, and negative and low values of ΔH° indicated an exothermic physisorption process. The data generated is critical and will help in further understanding remediation and cleanup strategies for polluted water.展开更多
To further extend knowledge about the detailed knowledge on the crossflow characteristics in a multi-jets system under a confined space,particle image velocimetry(PIV)was employed to investigate the flow structures to...To further extend knowledge about the detailed knowledge on the crossflow characteristics in a multi-jets system under a confined space,particle image velocimetry(PIV)was employed to investigate the flow structures together with the distributions of the mean velocity components for Reynolds numbers(Re)ranging from 6 213 to 13 418,nozzle-to-plate spacing(H/D)varying from 0.20 to 1.25,respectively.Results show that the crossflow configuration is significantly different from those of large nozzle-to-plate spacing.In addition,a turning point H/D=0.50 is revealed in the profile of the normalized maximum radial velocity which is associated with the heat transfer distribution on the impingement plate.展开更多
Glow discharge is introduced as an artificial disturbance to investigate the evolution of first-and second-mode instabilities in a hypersonic flat plate boundary layer.Experiments are conducted in a Mach 6.5 quiet win...Glow discharge is introduced as an artificial disturbance to investigate the evolution of first-and second-mode instabilities in a hypersonic flat plate boundary layer.Experiments are conducted in a Mach 6.5 quiet wind tunnel using Rayleigh scattering visualization and particle image velocimetry(PIV).Detailed analysis of the experimental observations is provided.It is found that the artificially introduced 17 kHz disturbance,which belongs to the first-mode frequency band,can effectively enhance first-mode waves.Moreover,it can enhance second-mode waves even more intensely.Possible mechanisms to explain this phenomenon are discussed.展开更多
Taking the Phyllostachys heterocycla forest in Qishan National Forest Park of Fuzhou for example,this study observed the diurnal variation of atmospheric aerosol particles in the forest in the growing season.The resul...Taking the Phyllostachys heterocycla forest in Qishan National Forest Park of Fuzhou for example,this study observed the diurnal variation of atmospheric aerosol particles in the forest in the growing season.The results showed that:① The diurnal variation curves of the particle concentration of the forest and the forest edge had “two peaks and two troughs”,but the peaks and troughs of the forest edge were advanced or delayed.The concentrations of the particles in the forest and at the forest edge had two peaks at 11:00–13:00 and 17:00–19:00 and two troughs at 7:00–9:00 and 15:00–17:00.② For the forest and the forest edge,the diurnal variation trends of the particles of different particle sizes were generally similar,except that the peaks and troughs of fine particles were slightly earlier or lagging than that of coarse particles.③ The concentrations of the particles were positively correlated with temperature,humidity and light,and negatively correlated with wind speed,and the concentrations of the particles at the forest edge were significantly negatively correlated with wind speed.展开更多
Alternating-structured polymers(ASPs), like alternating copolymers, regular multiblock copolymers and polycondensates, are very important polymer structures with broad applications in photoelectric materials. However,...Alternating-structured polymers(ASPs), like alternating copolymers, regular multiblock copolymers and polycondensates, are very important polymer structures with broad applications in photoelectric materials. However, their self-assembly behaviors,especially the self-assembly of alternating copolymers, have not been clearly studied up to now. Meanwhile, the unique characteristics therein have not been systematically disclosed yet by both experiments and theories. Herein, we have performed a systematic simulation study on the self-assembly of ASPs with two coil alternating segments in solution through dissipative particle dynamics(DPD) simulations. Several morphological phase diagrams were constructed as functions of different impact parameters. Diverse self-assemblies were observed, including spherical micelles, micelle networks, worm-like micelles, disklike micelles, multimicelle aggregates, bicontinuous micelles, vesicles, nanotubes and channelized micelles. Furthermore, a morphological evolutionary roadmap for all these self-assemblies was constructed, along with which the detailed molecular packing models and self-assembly mechanisms for each aggregate were disclosed. The ASPs were found to adopt a folded-chain mechanism in the self-assemblies. Finally, the unique characteristics for the self-assembly of alternating copolymers were revealed especially, including(1) ultra-fine and uniform feature sizes of the aggregates;(2) independence of self-assembled structures from molecular weight and molecular weight distribution;(3) ultra-small unimolecular aggregates. We believe the current work is beneficial for understanding the self-assembly of alternating structured polymers in solution and can serve as a guide for the further experiments.展开更多
基金the National Key Research and Development Program of China(Grant No.2017YFB0601900)the National Natural Science Foundation of China(Grant Nos.51827808,51821004,and 51406095)+1 种基金the Fundamental Research Funds for the Central Universities,China(Grant Nos.2018ZD03 and2017ZZD005)Science and Technology Partnership Program,Ministry of Science and Technology of China(Grant No.KY201401003).
文摘A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504 C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701 V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation,demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.
基金supported by the National Natural Science Foundation of China(21473029 and 51522302)the NSAF Foundation of China(U1530260)+3 种基金the Key Medical Projects of Jiangsu Province(BL2014078)Key Discipline of Jiangsu Province(2016-2020)the Natural Science Foundation of Jiangsu(BK20140028)the Scientific Research Foundation of Southeast University.
基金The financial support received from Ministry of Human Resource Development (MHRD), Government of India in the form of a Ph.D. scholarship is gratefully acknowledged. The help and cooperation from Mr. R.K. Verma, CEO of Sai Engineering Foundation and owner of the Toss HPPMr. Pankaj Thakur and Mr. Akhilesh, engineers in charge of the Toss HPPand staff Bidhi Chand are sincerely recognized. The authors acknowledge the suggestions and time of two anonymous reviewers to improve the manuscript.
文摘Suspended sediment particles contained in inflows of water systems of hydropower plants (HPPs) cause hydro-abrasive erosion of the hydraulic turbines and structures leading to significant maintenance costs, efficiency reductions, and downtime. Relevant parameters such as suspended sediment concentration (SSC), particle size distribution (PSD), shape, and mineralogical composition were measured with an online multi-frequency acoustic instrument and based on manually taken samples from the end of the sand trap of the Toss HPP in the Himalayan region, India. In the laboratory, the samples were analyzed using the gravimetric method, laser diffraction, turbidity, dynamic digital image processing, scanning electron microscope, petrography analysis, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The online instrument and the samples provided measurement results at a single point. To investigate vertical gradients in concentration and particle sizes, additional samples were collected 9 times at 7 relative water depths. The SSC, most particle sizes, and particle shape were found to be evenly distributed over depth except d90, i.e. the diameter which is not exceeded by 90% of the particle mass. d90 measured at 76% of the water depth was in the range of fine sand and was multiplied by 1.05 to obtain an average value representative for the entire depth. Improved methodologies to quantify both particle shape and size in an analytical model for hydro-abrasive erosion are proposed. Also, the PSD measuring performance of laser diffraction and dynamic imaging was studied and similar values of the median particle sizes were obtained from both instruments. Further, multi-frequency acoustic, turbidity and laser diffraction techniques were found suitable for SSC measurement at the test case HPP.
文摘Fine bubble technology has been applied in many fields, including semiconductor processing, waste water treatment, and agricultural development. Fine bubble technology has the advantages of being environ-mentally benign, and ease of production and use when compared with other processes that involve chemicals or biological agents. However, the mechanisms of fine bubble applications in many aspects still require exploration. The characterization of fine bubbles is one of the essential issues for better understanding the technology. This article reviews modern particle characterization technologies, espe-cially those that can be used to characterize fine bubbles, and briefly reports on some applications of fine bubbles.
基金supported by the National Key Research and Development Program of China(No.2016YFC0402302)the National Natural Science Foundation of China(No.51609161)the Open Funding SKHL1710 and SKHL1712 from the State Key Laboratory of Hydraulics and Mountain River Engineering in Sichuan University,China.
文摘Mountainous torrents often carry large amounts of loose materials into the rivers, thus causing strong sediment transport. Experimentally it was found for the first time that when the intensive sediment motion occurs downstream over a gentle slope, the siltation of the riverbed is induced and the sediment particles can move upstream rapidly in the form of a retrograde sand wave, resulting in a higher water level along the river. To further study the complex mechanisms of this problem, a sediment mass model in the framework of the Smoothed Particle Hydrodynamics(SPH) method was presented to simulate the riverbed evolution, sediment particle motion, and the generation and development of dynamic hydraulic jump under the condition of sufficient sediment supply over a steep slope with varying angles. Because the sediment is not a continuous medium, the marker particle tracking approach was proposed to represent a piece of sediment with a marked sediment particle. The twophase SPH model realizes the interaction between the sediment and fluid by moving the bed boundary particles up and down, so it can reasonably treat the fluid-sediment interfaces with high CPU efficiency. The critical triggering condition of sediment motion, the propagation of the hydraulic jump and the initial siltation position were all systematically studied. The experimental and numerical results revealed the extra disastrous sediment effect in a mountainous flood. The findings will be useful references to the disaster prevention and mitigation in mountainous rivers.
基金the Science-Technology Program of the State Grid Corporation of China (grant No. 521700140004)and the National Natural Science Foundation of China (grant No. 51777082)and the Fundamental Research Funds for the Central Universities (grant No. 2016YXZD069).
文摘Insulators on high-voltage Uansmission lines are almost the only man-made structures on the Earth's surface intended for long-term operation under strong electric fields. After samples of natural contaminant particles were collected from insulator surfaces in China, it was found that the particle diameter distribution (PDD) was mainly concentrated in the 5-50 μm range. To analyze the statistical characteristics of these particles, this work studies the physical processes of particle collision and adhesion using the theories of hydrodynamics and collision dynamics. The physical model considers coupling of the fluid field and the electric field, introduces an adhesion criterion, and establishes a particle and surface collision model. The effects of relative humidity, wind speed, aerodynamic shape, electric field type, and electric field strength on particle adhesion were analyzed. The results show that the relative humidity and wind speed have very significant effects and the influences of the electric field type and the electric field strength are obvious, but the in fluence of the aerodynamic shape is relatively weak. The simulation results support the statistical characteristics determined in this work. The physical model established here provides reference values for study of the adhesion characteristics of particles on surfaces under electric fields.
基金the National Natural Science Foundation of China(No:51806103)Jiangsu Provincial Natural Science Foundation(No:BK20170800)Open Funds of Aero‑engine Thermal Environment and Structure Key Laboratory of Ministry of Industry and Information Technology(No.CEPE2018005).
文摘The weighted-sum-of-gray-gas(WSGG)model and Mie theory are applied to study the influents of particle size on the radiative transfer in high temperature homogeneous gas-particle mixtures,such as the flame in aero-engine combustor.The radiative transfer equation is solved by the finite volume method.The particle size is assumed to obey uniform distribution and logarithmic normal(L-N)distribution,respectively.Results reveal that when particle size obeys uniform distribution,increasing particle size with total particle volume fraction fvunchanged will result in the decreasing of the absolute value of radiative heat transfer properties,and the effect of ignoring particle scattering will also be weakened.Opposite conclusions can be obtained when total particle number concentration N0 is unchanged.Moreover,if particle size obeys L-N distribution,increasing the narrowness indexσor decreasing the characteristic diameter Dˉwith the total particle volume fraction fvunchanged will increase the absolute value of radiative heat transfer properties.With total particle number concentration N0 unchanged,opposite conclusions for radiative heat source and incident radiation terms can be obtained except for radiative heat flux term.As a whole,the effects of particle size on the radiative heat transfer in the high-temperature homogeneous gas-particle mixtures are complicated,and the particle scattering cannot be ignoring just according to the particle size.
基金Project supported by the National Natural Science Foundation of China(Grant No.11371117).
文摘This paper considers fluid mixing driven by inflows connected to a circular shallow lake using a numerical framework consisting of a shallow water hydrodynamic model and a passive particle-tracking model.With the flow field driven by alternate inflows predicted by a shallow water model,particle trajectories are traced out using a particle tracking model.The horizontal fluid mixing dynamics are then interpreted using dynamics system analysis approaches including finite-time Lyapunov exponent(FTLE)and Lagrangian coherent structure(LCS).From the simulation results,it is confirmed that periodic inflows are able to create a weak dynamic system in an idealised circular lake,with the particle dynamics controlled by a single dimensionless parameter associated with the inflow duration.The mixing and transport property of the lake changes from regular to chaotic as the value of the dimensionless parameter increases until global chaotic particle dynamics is achieved.By further analysing the advection of particles injected continuously to the inflows(freshwater),the fate of“freshwater”particles in a“polluted”lake is tracked and revealed.The results provide useful guidance for engineering applications,i.e.,transferring freshwater from rivers to improve the water quality in polluted water bodies such as lakes.The presented approach will be able to facilitate the design of‘optimised’schemes for such engineering implementation.
基金the Chilean National Commission for Scientific and Technological Research (CONICYT), FONDECYT-Iniciación, Research Project no 11150511.
文摘Recent theoretical and numerical models for the motion of saltating particles close to stream beds are constituted of three sub-models: a) a set of equations describing the particle “free flight”, b) a sub-model to calculate the post-collision particle velocity, and c) a mathematical representation of the bed roughness. In this paper, a comprehensive three-dimensional (3-D), theoretical/numerical model for bedload motion at large Reynolds numbers is presented. By using geometric considerations and stochastic parameters to characterize collisions with the wall, five new sub-models for representation of bed roughness are, for the first time to the best of our knowledge, proposed and implemented. The emphasis of this paper is on the particle model, for which Basset, Magnus, drag, submerged weight, virtual mass, and lift forces are included. For the range of particle sizes (sands) analyzed herein, it is found that the stream-wise contribution of the Basset force, compared to other forces, may be as large as 60%. Whereas in the wall-normal direction, the Basset force is equally important as the drag force, and it is exceeded only by the submerged weight. It is also found that the best agreement between numerical and experimental results in terms of jump length, jump height, and stream-wise particle velocity is achieved for restitution and friction coefficients of 0.65 and 0.1, respectively. Important conclusions are obtained regarding the lack of realistic prediction with available “roughness” models with small ranges of angles.
基金The authors express their appreciation to the Science Fund for Creative Research Groups of the National Natural Science Foun-dation of China (No. 51521063)the National Natural Science Foundation of China (No. U1562212)+2 种基金the National Science and Technology Major Project of China (Grant No. 2016ZX05023-006)the National Key Research and Development Program of China (Grant No. 2016YFE0124600)the State Scholarship Fund (CSC file No. 201706440059).
文摘Solid-particle settling occurs in many natural and industrial processes, such as in the transportation of drilling cuttings and fracturing proppant. Knowledge of the drag coefficient and settling velocity of cuttings and proppant is of significance to hydraulics design, wellbore cleanout, and fracture optimization. We conducted 553 tests to investigate the settling characteristics of spherical and non-spherical particles in power-law fluids. Three major particle shapes (spherical, cubic, and cylindrical) and eight different particle sphericities were used to simulate cuttings and proppant, and power-law fluids were applied to simulate drilling and fracturing fluids. Based on the data analysis, a new drag coefficient-particle Reynolds number correlation was developed to determine the drag coefficient in a power-law fluid for spherical and non-spherical particles. The drag coefficient increases as the sphericity decreases for the same particle Reynolds number. For a specific particle shape, the drag coefficient decreases as the particle Reynolds number increases, but the decreasing trend is reduced at high particle Reynolds number conditions. An explicit settling-velocity equation was proposed to calculate the settling velocity of spherical and non-spherical particles in power-law fluids by considering the effect of sphericity. A suitable range for the proposed model is 0.0001 < Re <200, 0.471 <φ< 1, and 0.505 < n < 1. An illustrative example is presented to show how to calculate the drag coefficient and settling velocity in power-law fluids with given particle and fluid properties.
基金the China Scholarship Council and the National Natural Science Foundation of China (NSFC, Grant No. 51741608)thanks Austin Isner for help with data processing. The authors thank Guillaume Adoneth and Quentin Mougeot for acquiring experimental data on the seg-regation of cylindrical particles in a quasi-2D bounded heap.
文摘A discrete element model based on super-ellipsoids was used to simulate cylindrical particle flow. The model can describe a cylindrical particle accurately provided the shape indices of the super-ellipsoids are set to appropriate values. To achieve more rapid calculations, we implemented an "oriented bounding box algorithm"(OBBA) for the initial contact detection of cylindrical particles. Several types of Simulations were performed to validate the super-ellipsoid model and the contact-detection algorithm. First, the effect of shape index of the super-ellipsoids on model accuracy was investigated through three simulations: impact of a cylindrical particle on a flat wall, flow of cylindrical particles in a rotating tumbler, and segregation of cylindrical particles of different length flowing down a bounded heap. The simulation results show that the super-ellipsoids describe cylindrical particles accurately when the shape index that specifies the sharpness of the cylinder edges is sufficiently large. The efficiency of the OBBA is measured by simulations in which a box is filled with cylindrical particles and a tumbler that contains cylindrical particles is rotated. The simulation results show that the OBBA can accelerate the calculations significantly. The effect of particle shape (such as aspect ratio and shape index) on the calculation speed was obtained. The simulation of rod-like particles tended to take more calculation time than that of disk-like particles, and the simulation time increased with an increasing particle-shape index.
基金The authors gratefully acknowledge the financial support of the National Key R&D Program of China (grant No. 2017YFC0404801)the National Natural Science Foundation of China (grant No. 51579193).
文摘The fragment replacement method (FRM), a particle breakage simulation method, is often used in discrete element simulations to investigate the particle breakage effect on the mechanical behavior of granular materials. The fragment size distribution of the fragment replacement mode of FRM, which is generally generated based on the fragmentation characteristics of single particles after uniaxial compression, affects the breakage process and the mechanical behavior of the particle assembly. However, existing fragment replacement modes are seldom generated based on experimental data analysis. To capture the fragmentation process and investigate the breakage function for the construction of the fragment replacement mode, 60 numerical single particle compression tests were implemented by DEM. The bonded-particle model was applied to generate the crushable rock particles. The numerical simulations were qualitatively validated by experimental results, and the fragment size of broken single particles was analyzed. The fractal dimension was used to describe the fragmentation degree of single particles after compression. The fragmentation degree was random, and the fractal dimensions of the 60 tests at the same loading displacement fit the Weibull distribution well. The characteristic fractal dimension increased with increasing loading displacement, indicating that the fragmentation of single particles is a gradual process. According to the overall breakage function of the 60 tests at the first bulk breakage, a two-stage distribution model with 4 parameters was proposed and validated by the numerical and experimental results. The various fracture patterns of a single particle at the first bulk breakage under compression tests were well captured by the two-stage distribution model. Finally, an initial application strategy using the two-stage distribution model to construct fragment replacement modes was discussed and presented.
文摘When using the beam scanning method for particle beam therapy, the target volume is divided into many iso-energy slices and is irradiated slice by slice. Each slice may comprise thousands of discrete scanning beam positions. An optimized scanning path can decrease the transit dose and may bypass important organs. The minimization of the scanning path length can be considered as a variation of the traveling salesman problem;the simulated annealing algorithm is adopted to solve this problem. The initial scanning path is assumed as a simple zigzag path;subsequently, random searches for accepted new paths are performed through cost evaluation and criteria-based judging. To reduce the optimization time of a given slice, random searches are parallelized by employing thousands of threads. The simultaneous optimization of multiple slices is realized by using many thread blocks of generalpurpose computing on graphics processing units hardware. Running on a computer with an Intel i7-4790 CPU and NVIDIA K2200 GPU, our new method required only 1.3 s to obtain optimized scanning paths with a total of 40 slices in typically studied cases. The procedure and optimization results of this new method are presented in this work.
文摘The significance of soil mineral properties and secondary environmental conditions such as pH, temperature, ionic strength and time in the partitioning of eight selected polychlorinated biphenyl (PCB) congeners between aqueous solution and soil particles with different grain sizes was studied. The mineral properties of a model soil sample were determined, and Brunauer–Emmett–Teller (BET) adsorption–desorption isotherms were employed to observe the surface characteristics of the individual modeled soil particles. Batch adsorption experiments were conducted to determine the sorption of PCBs onto soil particles of different sizes. The results revealed that the sorption of PCB congeners onto the soil was dependent on the amount of soil organic matter, surface area, and pore size distribution of the various individual soil particles. Low pH favored the sorption of PCBs, with maximum sorption occurring between pH 6.5 and 7.5 with an equilibration period of 8 hr. Changes in the ionic strength were found to be less significant. Low temperature favored the sorption of PCBs onto the soil compared to high temperatures. Thermodynamic studies showed that the partition coefficient (Kd) decreased with increasing temperature, and negative and low values of ΔH° indicated an exothermic physisorption process. The data generated is critical and will help in further understanding remediation and cleanup strategies for polluted water.
基金National Natural Science Foundation of China(51335002).
文摘To further extend knowledge about the detailed knowledge on the crossflow characteristics in a multi-jets system under a confined space,particle image velocimetry(PIV)was employed to investigate the flow structures together with the distributions of the mean velocity components for Reynolds numbers(Re)ranging from 6 213 to 13 418,nozzle-to-plate spacing(H/D)varying from 0.20 to 1.25,respectively.Results show that the crossflow configuration is significantly different from those of large nozzle-to-plate spacing.In addition,a turning point H/D=0.50 is revealed in the profile of the normalized maximum radial velocity which is associated with the heat transfer distribution on the impingement plate.
基金the National Natural Science Foundation of China(Nos.10921202,11221061,11632002,11521091,and 11602005).
文摘Glow discharge is introduced as an artificial disturbance to investigate the evolution of first-and second-mode instabilities in a hypersonic flat plate boundary layer.Experiments are conducted in a Mach 6.5 quiet wind tunnel using Rayleigh scattering visualization and particle image velocimetry(PIV).Detailed analysis of the experimental observations is provided.It is found that the artificially introduced 17 kHz disturbance,which belongs to the first-mode frequency band,can effectively enhance first-mode waves.Moreover,it can enhance second-mode waves even more intensely.Possible mechanisms to explain this phenomenon are discussed.
基金Sponsored by the Science and Technology Project of Beijing Municipal Administration Center of Parks (ZX2019)Beijing Science and Technology Plan Project (D171100001817001).
文摘Taking the Phyllostachys heterocycla forest in Qishan National Forest Park of Fuzhou for example,this study observed the diurnal variation of atmospheric aerosol particles in the forest in the growing season.The results showed that:① The diurnal variation curves of the particle concentration of the forest and the forest edge had “two peaks and two troughs”,but the peaks and troughs of the forest edge were advanced or delayed.The concentrations of the particles in the forest and at the forest edge had two peaks at 11:00–13:00 and 17:00–19:00 and two troughs at 7:00–9:00 and 15:00–17:00.② For the forest and the forest edge,the diurnal variation trends of the particles of different particle sizes were generally similar,except that the peaks and troughs of fine particles were slightly earlier or lagging than that of coarse particles.③ The concentrations of the particles were positively correlated with temperature,humidity and light,and negatively correlated with wind speed,and the concentrations of the particles at the forest edge were significantly negatively correlated with wind speed.
基金The National Natural Science Foundation of China (21893073010,21404070,21474062, 51773115,21774077,91527304)the Program for Basic Research of Shanghai Science and Technology Commission (17JC1403400)Centre for High-Performance Computing,Shanghai Jiao Tong University.
文摘Alternating-structured polymers(ASPs), like alternating copolymers, regular multiblock copolymers and polycondensates, are very important polymer structures with broad applications in photoelectric materials. However, their self-assembly behaviors,especially the self-assembly of alternating copolymers, have not been clearly studied up to now. Meanwhile, the unique characteristics therein have not been systematically disclosed yet by both experiments and theories. Herein, we have performed a systematic simulation study on the self-assembly of ASPs with two coil alternating segments in solution through dissipative particle dynamics(DPD) simulations. Several morphological phase diagrams were constructed as functions of different impact parameters. Diverse self-assemblies were observed, including spherical micelles, micelle networks, worm-like micelles, disklike micelles, multimicelle aggregates, bicontinuous micelles, vesicles, nanotubes and channelized micelles. Furthermore, a morphological evolutionary roadmap for all these self-assemblies was constructed, along with which the detailed molecular packing models and self-assembly mechanisms for each aggregate were disclosed. The ASPs were found to adopt a folded-chain mechanism in the self-assemblies. Finally, the unique characteristics for the self-assembly of alternating copolymers were revealed especially, including(1) ultra-fine and uniform feature sizes of the aggregates;(2) independence of self-assembled structures from molecular weight and molecular weight distribution;(3) ultra-small unimolecular aggregates. We believe the current work is beneficial for understanding the self-assembly of alternating structured polymers in solution and can serve as a guide for the further experiments.