A family of unconditionally stable direct integration algorithm with controllable numerical dissipations is proposed. The numerical properties of the new algorithms are controlled by three parameters α,β and γ. By ...A family of unconditionally stable direct integration algorithm with controllable numerical dissipations is proposed. The numerical properties of the new algorithms are controlled by three parameters α,β and γ. By the consistent and stability analysis, the proposed algorithms achieve the second-order accuracy and are unconditionally stable under the condition that α≥-0.5,β≤ 0.5 and γ≥-(1+α)/2. Compared with other unconditionally stable algorithms, such as Chang’s algorithms and CR algorithm, the proposed algorithms are found to be superior in terms of the controllable numerical damping ratios. The unconditional stability and numerical damping ratios of the proposed algorithms are examined by three numerical examples. The results demonstrate that the proposed algorithms have a superior performance and can be used expediently in solving linear elastic dynamics problems.展开更多
Heat-driven thermoacoustic refrigeration has drawn extensive concern in the past decades due to its advantages of high reliability and external heat-driven mechanism.In such a system,heat can be firstly converted into...Heat-driven thermoacoustic refrigeration has drawn extensive concern in the past decades due to its advantages of high reliability and external heat-driven mechanism.In such a system,heat can be firstly converted into acoustic power and then the acoustical power drives a refrigerator to generate cooling effect without any moving mechanical components.So far,most of researches on heat-driven thermoacoustic refrigeration have focused on cryo- genic application,such as natural gas liquefaction [1,2].In addition, heat-driven thermoacoustic refrigeration also plays important roles in recovering waste heat and provides an environment- friendly alternative to the current abso'rption chiller especially in the small-scale power range [3-7].However,two main obstacles to use the thermoacoustic technology in practice are its relatively low cooling capacity and low cooling efficiency.Either the two-loop configuration proposed by Luo et al.[5]or the configuration of a one-unit refrigerator driven by a three-unit engine brought forward by Kees [6],they both suffer from phase-shifting tube and mismatch between the thermoacoustic engine (TAE)and thermoacoustic refrigerator (TAR).展开更多
In consideration of geometric parameters, several researches have already optimized the thermal efficiency of the cylindrical cavity receiver. However, most of the optimal results have been achieved at a fixed solar r...In consideration of geometric parameters, several researches have already optimized the thermal efficiency of the cylindrical cavity receiver. However, most of the optimal results have been achieved at a fixed solar radiation. At different direct normal irradiance (DNI), any single optimal result may not be suitable enough for different regions over the world. This study constructed a 3-D numerical model of cylindrical cavity receiver with DNI variation. In the model of a cylindrical cavity receiver containing a helical pipe, the heat losses of the cavity and heat transfer of working medium were also taken into account. The simulation results show that for a particular DNI in the range of 400 W/m2 to 800 W/m2, there exists a best design for achieving a highest thermal efficiency of the cavity receiver. Besides, for a receiver in constant geometric parameters, the total heat losses increases dramatically with the DNI increasing in that range, as well as the temperature of the working medium. The thermal efficiency presented a different variation tendency with the heat losses, which is 2.45% as a minimum decline. In summary, this paper proposed an optimization method in the form of a bunch of fitting curves which could be applied to receiver design in different DNI regions, with comparatively appropriate thermal performances.展开更多
Low methanol permeability of proton exchange membranes (PEMs) is greatly important for direct methanol fuel cells (DMFCs). Here, sulfonated poly (ether ether ketone) (SPEEK) based semiinterpenetrating polymer networks...Low methanol permeability of proton exchange membranes (PEMs) is greatly important for direct methanol fuel cells (DMFCs). Here, sulfonated poly (ether ether ketone) (SPEEK) based semiinterpenetrating polymer networks (semi-IPNs) are successfully prepared by interpenetrating SPEEK into the in-situ synthesized crosslinking networks. The polymeric networks are formed by the covalent bonds between bromobenzyl groups of bro mo methylated poly (phenylene oxide) and amine groups of diamine linkers as well as the ionic bonds between amine species and sulfonated groups. Two linkers without and with sulfonated groups are applied to fabricate the semi-IPNs. The core properties of the membranes, like phase separation, water uptake, proton conductivity and methanol permeability, are systematically studied and compared. The DMFCs assembled by using the semi-IPN membranes display better performance than Nafion 117 in high concentration methanol solutions. The present work provides a facile way to prepare PEMs with enhanced DMFC performance.展开更多
The compatibility between direct coal liquefaction residue(DCLR) and five kinds of pure bitumen(Shell-90,SK-90, ZSY-70, DM-70 and KLMY-50) was evaluated in this study. The rheological characteristics, glass transition...The compatibility between direct coal liquefaction residue(DCLR) and five kinds of pure bitumen(Shell-90,SK-90, ZSY-70, DM-70 and KLMY-50) was evaluated in this study. The rheological characteristics, glass transition temperatures(T_g), solubility parameters(SP) and SARA(saturates, aromatics, resins, and asphaltenes) fractions of DCLR,five kinds of pure bitumen and their blends(named as DCLR modified bitumen) were measured using the dynamic shear rheometer(DSR), differential scanning calorimetry(DSC), viscosity, and SARA tests, respectively. And the compatibility between DCLR and pure bitumen was characterized with three approaches, viz. the Cole-Cole plot,T_g, and the solubility parameter difference(SPD) method. Since each method has its own working mechanism, the compatibility ranking for the DCLR and five kinds of pure bitumen is slightly different according to the three approaches. However, the difference is pretty close and sometimes can be ignored. The general compatibility ranking decreases in the following order: Shell-90≈SK-90>DM-70≈ZSY-70>KLMY-50, which is affected by the asphaltenes content and the colloid index(I_c) value in the pure bitumen. Pure bitumen with lower asphaltenes content and colloid index(I_c) value has better compatibility with DCLR.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
The SeDeM Expert System was first known as a galenic pre-formulation system, which was based on the experimental research and quantitative determination of powdered substances. And the mathematical formula provided by...The SeDeM Expert System was first known as a galenic pre-formulation system, which was based on the experimental research and quantitative determination of powdered substances. And the mathematical formula provided by the SeDeM Expert System has plays an important role in the study of powder properties. The system can be used not only to evaluate the powder direct compression (DC) of excipients and active pharmaceutical ingredients (API’s), but also to predict the possible formulations, so it can reduce unnecessary research and trials, and shorten the time of development. In this paper, the research development and application of SeDeM Expert System in DC was summarized, and the results showed that with a few exceptions, the system was skilled in predicting acceptable tablet formulations. Finally, the new application prospect of the system is presented, including the application of the Internet traffic and content management (iTCM) database and the new co-processed excipients.展开更多
This paper calculates the China-U.S. trade balance from the national income perspective based on an input-output model that differentiates domestic and foreign-invested companies. The result shows that due to differen...This paper calculates the China-U.S. trade balance from the national income perspective based on an input-output model that differentiates domestic and foreign-invested companies. The result shows that due to different degrees of dependence of both countries on foreign production factors such as foreign capital for the manufacturing of export goods, only 87.7% of the domestic value-added created by China’s exports to the U.S. in 2012 was China’s national income, whereas 96.2% of value-added in U.S. exports to China was U.S. national income. In the comparison of total export volume and export value-added, the home country’s national income created by exports can more realistically reflect a country’s gains from trade. In 2012, China’s trade surplus with the U.S. stood at 102.8 billion US dollars in national income terms, which is 61% and 22% smaller than the results in gross and value-added terms, respectively. The implication is that the traditional trade balance accounting method seriously exaggerates the China-U.S. trade imbalance.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
A vortex is intuitively recognized as the rotational/swirling motion of fluids,but a rigorous and universally-accepted definition is still not available.Vorticity tube/filament has been regarded equivalent to a vortex...A vortex is intuitively recognized as the rotational/swirling motion of fluids,but a rigorous and universally-accepted definition is still not available.Vorticity tube/filament has been regarded equivalent to a vortex since Helmholtz proposed the concepts of vorticity tube/filament in 1858 and the vorticity-based methods can be categorized as the first generation of vortex identification methods.During the last three decades,a lot of vortex identification methods,including 0,A,and Aci criteria,have been proposed to overcome the problems associated with the vorticity-based methods.Most of these criteria are based on the Cauchy-Stokes decomposition and/or eigenvalues of the velocity gradient tensor and can be considered as the second generation of vortex identification methods.Starting from 2014,the Vortex and Turbulence Research Team at the University of Texas at Arlington(the UTA team)focus on the development of a new generation of vortex identification methods.The first fruit of this effort,a new Omega(/2)vortex identification method,which defined a vortex as a connected region where the vorticity overtakes the deformation,was published in 2016.In 2017 and 2018,a Liutex(previously called Rortex)vector was proposed to provide a mathematical definition of the local rigid rotation part of the fluid motion,including both the local rotational axis and the rotational strength.Liutex/Rortex is a new physical quantity with scalar,vector and tensor forms exactly representing the local rigid rotation of fluids.Meanwhile,a decomposition of the vorticity to a rotational part namely Liutex/Rortex and an anti-symmetric shear part(RS decomposition)was introduced in 2018,and a universal decomposition of the velocity gradient tensor to a rotation part(7?)and a non-rotation part(NR、was also given in 2018 as a counterpart of the traditional Cauchy-Stokes decomposition.Later in early 2019,a Liutex/Rortex based Omega method called Omega-Liutex,which combines the respective advantages of both Liutex/Rortex and Omega methods,was dev展开更多
Outwardforeign direct investment(OFDI)has increasingly become an important method for China to integrate into the world economy.This paper comprehensively reviews and analyzes policy development and the changing patte...Outwardforeign direct investment(OFDI)has increasingly become an important method for China to integrate into the world economy.This paper comprehensively reviews and analyzes policy development and the changing pattern of China's OFDI over the past 40 years.We divide the development into"restricted”(1978-1999),"relaxed"(2000-2016)and"regulated",(2017 onwards)stages.This paper also reviews literature on the impact of Chinese OFDI on China and host countries.Despite its generally positive effects,large-scale and unbalanced OFDI activities have alarmed Chinese policymakers.Both developing and developed host countries have expressed their concern over national security and the misbehavior of some Chinese overseas enterprises.Therefore,greater supervision and adjustment from quantity to quality growth is necessary for the future development of China's OFDI.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
Multi-mode fiber(MMF)links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks.Recently,mode division multiplexing(MDM)over MMF has been pr...Multi-mode fiber(MMF)links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks.Recently,mode division multiplexing(MDM)over MMF has been proposed,in which different modes in MMF are utilized as spatial channels for data transmission.Stronglycoupled MDM techniques utilizing coherent detection and multiplex-input-multiplex-output(MIMO)digital signal processing(DSP)are complex and expensive for shortreach transmission.So the weakly-coupled approach by significantly suppressing mode coupling in the fiber and optical components has been proposed.In this way,the signals in each mode can be independently transmitted and received using conventional intensity modulation and direct detection(IM-DD).In this paper,we elaborate the key technologies to realize weakly-coupled MDM transmission over conventional MMF,including mode characteristic in MMF and weakly-coupled mode multiplexer/ demultiplexer(MUX/DEMUX).We also present the upto-date experimental results for weakly-coupled MDM transmission over conventional OM3 MMF.We show that weakly-coupled MDM scheme is promising for high-speed optical interconnections and bandwidth upgrade of already-deployed MMF links.展开更多
As one solution to implement the largecapacity space division multiplexing(SDM)transmission systems,the mode division multiplexing(MDM)has gained much attention recently.The vector mode(VM),which is the eigenmode of t...As one solution to implement the largecapacity space division multiplexing(SDM)transmission systems,the mode division multiplexing(MDM)has gained much attention recently.The vector mode(VM),which is the eigenmode of the optical fiber,has also been adopted to realize the optical communications including the transmission over free-space optical(FSO)and optical fiber links.Considering the concerns on the short-reach optical interconnects,the low cost and high integration technologies should be developed.Direct detection(DD)with higher-order modulation formats in combination of MDM technologies could offer an available trade-off in system performance and complexity.We review demonstrations of FSO and fiber high-speed data transmission based on the VM MDM(VMDM)technologies.The special VMs,cylindrical vector beams(CVB),have been generated by the q-plate(QP)and characterized accordingly.And then they were used to implement the VMDM transmission with direct-detection orthogonal frequency division multiplexing(DD-OFDM).These demonstrations show the potential of VMDM-DD-OFDM technology in the large-capacity short-reach transmission links.展开更多
The paper presents a direct numerical simulation(DNS)for the drag-reducing channel flow using the Giesekus model with variable parameters.It is assumed that the relaxation time in the constitutive equation is varied d...The paper presents a direct numerical simulation(DNS)for the drag-reducing channel flow using the Giesekus model with variable parameters.It is assumed that the relaxation time in the constitutive equation is varied depending on the local shear rate.The maximal drag reduction rate is obtained when variable parameters are applied in the Giesekus model at a high Weissenberg number.The Reynolds shear stress is reduced when the Weissenberg number increases.In this case,the turbulence generation and transportation are further weakened and increasingly approach to the values in the experiments.展开更多
Analysis of the Direct Operating Cost(DOC) of aircraft is an important step towards achieving financially sustainable aviation operations. However, the value of the DOC for different aircraft types and flight scenario...Analysis of the Direct Operating Cost(DOC) of aircraft is an important step towards achieving financially sustainable aviation operations. However, the value of the DOC for different aircraft types and flight scenarios is not widely available. In this study, we perform a systematic analysis of the DOC of every wide-body passenger aircraft currently in production, using the method of the Association of European Airlines(AEA). The elements of the DOC, e.g. financial costs, maintenance costs, and flight costs, are evaluated individually. Several realistic flight scenarios are considered, each with differences in route distance, fuel price, passenger number, and seating arrangement. For each flight scenario, the most cost-efficient aircraft type is identified and evaluated in the context of operations from Hong Kong International Airport. The information provided in this study could be useful to airline operators and policy makers.展开更多
NbCrAl coatings in the presence or absence of Al outer layer were prepared on C103 Nb based alloy and alumina substrate by direct current magnetron sputtering technique. The oxidation performance of coating systems wa...NbCrAl coatings in the presence or absence of Al outer layer were prepared on C103 Nb based alloy and alumina substrate by direct current magnetron sputtering technique. The oxidation performance of coating systems was evaluated by isothermal oxidation tests. The element diffusion and oxidation behavior of the coating systems were investigated. The results indicate that the mass gains of NbCrAl and Al/NbCrAl coatings are 2.02 mg/cm2 and 0.79 mg/cm2 at 1 000 ℃ for short time. Al/NbCrAl coatings exhibit more effective protection than NbCrAl coatings. The addition of Al layer can improve the oxidation resistance of NbCrAl coatings, which is attributed to the Al layer offering enough Al content to react with oxygen to form a continuous and dense Al2O3 scale on NbCrAl coating and it can inhibit the further internal oxidation.展开更多
Downhole microseismic data has the significant advantages of high signal-to-noise ratio and well-developed P and S waves and the core component of microseismic monitoring is microseismic event location associated with...Downhole microseismic data has the significant advantages of high signal-to-noise ratio and well-developed P and S waves and the core component of microseismic monitoring is microseismic event location associated with hydraulic fracturing in a relatively high confidence level and accuracy.In this study,we present a multidimensional DIRECT inversion method for microseismic locations and applicability tests over modeling data based on a downhole microseismic monitoring system.Synthetic tests inidcate that the objective function of locations can be defined as a multi-dimensional matrix space by employing the global optimization DIRECT algorithm,because it can be run without the initial value and objective function derivation,and the discretely scattered objective points lead to an expeditious contraction of objective functions in each dimension.This study shows that the DIRECT algorithm can be extensively applied in real downhole microseismic monitoring data from hydraulic fracturing completions.Therefore,the methodology,based on a multidimensional DIRECT algorithm,can provide significant high accuracy and convergent efficiency as well as robust computation for interpretable spatiotemporal microseismic evolution,which is more suitable for real-time processing of a large amount of downhole microseismic monitoring data.展开更多
Based on the Fourier–Chebyshev spectral method,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS...Based on the Fourier–Chebyshev spectral method,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS) methods for different Reynolds numbers.A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient.With the application of electromagnetic force,the in-depth relations among the fluctuating velocities near the wall,Reynolds stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force.However,the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction.Therefore,the significant effect of drag reduction is obtained.Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.展开更多
基金National Natural Science Foundation of China under Grant No. 11372084.
文摘A family of unconditionally stable direct integration algorithm with controllable numerical dissipations is proposed. The numerical properties of the new algorithms are controlled by three parameters α,β and γ. By the consistent and stability analysis, the proposed algorithms achieve the second-order accuracy and are unconditionally stable under the condition that α≥-0.5,β≤ 0.5 and γ≥-(1+α)/2. Compared with other unconditionally stable algorithms, such as Chang’s algorithms and CR algorithm, the proposed algorithms are found to be superior in terms of the controllable numerical damping ratios. The unconditional stability and numerical damping ratios of the proposed algorithms are examined by three numerical examples. The results demonstrate that the proposed algorithms have a superior performance and can be used expediently in solving linear elastic dynamics problems.
基金the National Key Research and Development Program of China (2016YFB0901403)the National Natural Science Foundation of China (51506211 and 51876214)+1 种基金the Foundation of Director of Technical Institute of Physics and Chemistry,CAS (2017-ZLM)CAS Key Laboratory of Cryogenics,Technical Institute of Physics and Chemistry (Youth Innovation Fund CRYOQN201701).
文摘Heat-driven thermoacoustic refrigeration has drawn extensive concern in the past decades due to its advantages of high reliability and external heat-driven mechanism.In such a system,heat can be firstly converted into acoustic power and then the acoustical power drives a refrigerator to generate cooling effect without any moving mechanical components.So far,most of researches on heat-driven thermoacoustic refrigeration have focused on cryo- genic application,such as natural gas liquefaction [1,2].In addition, heat-driven thermoacoustic refrigeration also plays important roles in recovering waste heat and provides an environment- friendly alternative to the current abso'rption chiller especially in the small-scale power range [3-7].However,two main obstacles to use the thermoacoustic technology in practice are its relatively low cooling capacity and low cooling efficiency.Either the two-loop configuration proposed by Luo et al.[5]or the configuration of a one-unit refrigerator driven by a three-unit engine brought forward by Kees [6],they both suffer from phase-shifting tube and mismatch between the thermoacoustic engine (TAE)and thermoacoustic refrigerator (TAR).
基金the National Program of International Science and Technology Cooperation of China (Grant No. 20I4DFA60990).
文摘In consideration of geometric parameters, several researches have already optimized the thermal efficiency of the cylindrical cavity receiver. However, most of the optimal results have been achieved at a fixed solar radiation. At different direct normal irradiance (DNI), any single optimal result may not be suitable enough for different regions over the world. This study constructed a 3-D numerical model of cylindrical cavity receiver with DNI variation. In the model of a cylindrical cavity receiver containing a helical pipe, the heat losses of the cavity and heat transfer of working medium were also taken into account. The simulation results show that for a particular DNI in the range of 400 W/m2 to 800 W/m2, there exists a best design for achieving a highest thermal efficiency of the cavity receiver. Besides, for a receiver in constant geometric parameters, the total heat losses increases dramatically with the DNI increasing in that range, as well as the temperature of the working medium. The thermal efficiency presented a different variation tendency with the heat losses, which is 2.45% as a minimum decline. In summary, this paper proposed an optimization method in the form of a bunch of fitting curves which could be applied to receiver design in different DNI regions, with comparatively appropriate thermal performances.
基金the National Natural Science Foundation of China(Nos.21603197,21703212,21233006 and 21473164)Natural Science Foundation of Hubei Province of China(No.2016CFB181)+1 种基金Fundamental Research Funds for the Central University,China University of Geosciences(Wuhan)(No.CUGL180403)China University of Geosciences(Wuhan)for the program of Center for Advanced Energy Research and Technologies.
文摘Low methanol permeability of proton exchange membranes (PEMs) is greatly important for direct methanol fuel cells (DMFCs). Here, sulfonated poly (ether ether ketone) (SPEEK) based semiinterpenetrating polymer networks (semi-IPNs) are successfully prepared by interpenetrating SPEEK into the in-situ synthesized crosslinking networks. The polymeric networks are formed by the covalent bonds between bromobenzyl groups of bro mo methylated poly (phenylene oxide) and amine groups of diamine linkers as well as the ionic bonds between amine species and sulfonated groups. Two linkers without and with sulfonated groups are applied to fabricate the semi-IPNs. The core properties of the membranes, like phase separation, water uptake, proton conductivity and methanol permeability, are systematically studied and compared. The DMFCs assembled by using the semi-IPN membranes display better performance than Nafion 117 in high concentration methanol solutions. The present work provides a facile way to prepare PEMs with enhanced DMFC performance.
基金sponsored by the National Natural Science Foundation of China (51478028 and 51778038)the Program for Changjiang Scholars and Innovative Research Team in Universities(IRT-17R06).
文摘The compatibility between direct coal liquefaction residue(DCLR) and five kinds of pure bitumen(Shell-90,SK-90, ZSY-70, DM-70 and KLMY-50) was evaluated in this study. The rheological characteristics, glass transition temperatures(T_g), solubility parameters(SP) and SARA(saturates, aromatics, resins, and asphaltenes) fractions of DCLR,five kinds of pure bitumen and their blends(named as DCLR modified bitumen) were measured using the dynamic shear rheometer(DSR), differential scanning calorimetry(DSC), viscosity, and SARA tests, respectively. And the compatibility between DCLR and pure bitumen was characterized with three approaches, viz. the Cole-Cole plot,T_g, and the solubility parameter difference(SPD) method. Since each method has its own working mechanism, the compatibility ranking for the DCLR and five kinds of pure bitumen is slightly different according to the three approaches. However, the difference is pretty close and sometimes can be ignored. The general compatibility ranking decreases in the following order: Shell-90≈SK-90>DM-70≈ZSY-70>KLMY-50, which is affected by the asphaltenes content and the colloid index(I_c) value in the pure bitumen. Pure bitumen with lower asphaltenes content and colloid index(I_c) value has better compatibility with DCLR.
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 Science & Technology Major Project Key New Drug Creation and Manufacturing Program (2015ZX093001002007)R & D platform project of Chongqing science & Technology Commission (cstc2015yfpt-zdsys0042)+1 种基金Industrial key research and development project of Chongqing science & Technology Commission (cstc2017zdcycdyf193)the Postgraduate Science and Technology Innovation Program of Chongqing University of Science and Technology (YKJCX1820505).
文摘The SeDeM Expert System was first known as a galenic pre-formulation system, which was based on the experimental research and quantitative determination of powdered substances. And the mathematical formula provided by the SeDeM Expert System has plays an important role in the study of powder properties. The system can be used not only to evaluate the powder direct compression (DC) of excipients and active pharmaceutical ingredients (API’s), but also to predict the possible formulations, so it can reduce unnecessary research and trials, and shorten the time of development. In this paper, the research development and application of SeDeM Expert System in DC was summarized, and the results showed that with a few exceptions, the system was skilled in predicting acceptable tablet formulations. Finally, the new application prospect of the system is presented, including the application of the Internet traffic and content management (iTCM) database and the new co-processed excipients.
基金the National Natural Science Foundation of China (NSFC) projects (71473244, 61873261 and 71704195)the Fundamental Research Funds for the Central Universities, the University of International Business and Economics (CXTD7-06).
文摘This paper calculates the China-U.S. trade balance from the national income perspective based on an input-output model that differentiates domestic and foreign-invested companies. The result shows that due to different degrees of dependence of both countries on foreign production factors such as foreign capital for the manufacturing of export goods, only 87.7% of the domestic value-added created by China’s exports to the U.S. in 2012 was China’s national income, whereas 96.2% of value-added in U.S. exports to China was U.S. national income. In the comparison of total export volume and export value-added, the home country’s national income created by exports can more realistically reflect a country’s gains from trade. In 2012, China’s trade surplus with the U.S. stood at 102.8 billion US dollars in national income terms, which is 61% and 22% smaller than the results in gross and value-added terms, respectively. The implication is that the traditional trade balance accounting method seriously exaggerates the China-U.S. trade imbalance.
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.
文摘A vortex is intuitively recognized as the rotational/swirling motion of fluids,but a rigorous and universally-accepted definition is still not available.Vorticity tube/filament has been regarded equivalent to a vortex since Helmholtz proposed the concepts of vorticity tube/filament in 1858 and the vorticity-based methods can be categorized as the first generation of vortex identification methods.During the last three decades,a lot of vortex identification methods,including 0,A,and Aci criteria,have been proposed to overcome the problems associated with the vorticity-based methods.Most of these criteria are based on the Cauchy-Stokes decomposition and/or eigenvalues of the velocity gradient tensor and can be considered as the second generation of vortex identification methods.Starting from 2014,the Vortex and Turbulence Research Team at the University of Texas at Arlington(the UTA team)focus on the development of a new generation of vortex identification methods.The first fruit of this effort,a new Omega(/2)vortex identification method,which defined a vortex as a connected region where the vorticity overtakes the deformation,was published in 2016.In 2017 and 2018,a Liutex(previously called Rortex)vector was proposed to provide a mathematical definition of the local rigid rotation part of the fluid motion,including both the local rotational axis and the rotational strength.Liutex/Rortex is a new physical quantity with scalar,vector and tensor forms exactly representing the local rigid rotation of fluids.Meanwhile,a decomposition of the vorticity to a rotational part namely Liutex/Rortex and an anti-symmetric shear part(RS decomposition)was introduced in 2018,and a universal decomposition of the velocity gradient tensor to a rotation part(7?)and a non-rotation part(NR、was also given in 2018 as a counterpart of the traditional Cauchy-Stokes decomposition.Later in early 2019,a Liutex/Rortex based Omega method called Omega-Liutex,which combines the respective advantages of both Liutex/Rortex and Omega methods,was dev
文摘Outwardforeign direct investment(OFDI)has increasingly become an important method for China to integrate into the world economy.This paper comprehensively reviews and analyzes policy development and the changing pattern of China's OFDI over the past 40 years.We divide the development into"restricted”(1978-1999),"relaxed"(2000-2016)and"regulated",(2017 onwards)stages.This paper also reviews literature on the impact of Chinese OFDI on China and host countries.Despite its generally positive effects,large-scale and unbalanced OFDI activities have alarmed Chinese policymakers.Both developing and developed host countries have expressed their concern over national security and the misbehavior of some Chinese overseas enterprises.Therefore,greater supervision and adjustment from quantity to quality growth is necessary for the future development of China's OFDI.
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.61771024,61627814,61505002,61690194 and 61605004)Fundamental Research Project of Shenzhen Science and Technology Foundation(Nos.JCYJ 20170412153729436 and 20170307172513653).
文摘Multi-mode fiber(MMF)links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks.Recently,mode division multiplexing(MDM)over MMF has been proposed,in which different modes in MMF are utilized as spatial channels for data transmission.Stronglycoupled MDM techniques utilizing coherent detection and multiplex-input-multiplex-output(MIMO)digital signal processing(DSP)are complex and expensive for shortreach transmission.So the weakly-coupled approach by significantly suppressing mode coupling in the fiber and optical components has been proposed.In this way,the signals in each mode can be independently transmitted and received using conventional intensity modulation and direct detection(IM-DD).In this paper,we elaborate the key technologies to realize weakly-coupled MDM transmission over conventional MMF,including mode characteristic in MMF and weakly-coupled mode multiplexer/ demultiplexer(MUX/DEMUX).We also present the upto-date experimental results for weakly-coupled MDM transmission over conventional OM3 MMF.We show that weakly-coupled MDM scheme is promising for high-speed optical interconnections and bandwidth upgrade of already-deployed MMF links.
基金the National High Technology 863 Research and Development of China(No.2015AA017102)the National Natural Science Foundation of China(NSFC)(Grant Nos.61575082,61435006,61525502,and 61490715)+2 种基金the Youth Science and Technology Innovation Talents of Guangdong(No.2015TQ01X606)Guangdong Provincial Natural Science Foundation(GDSF)(No.2015A030313328)Pearl River S&T Nova Program of Guangzhou(No.201710010051).
文摘As one solution to implement the largecapacity space division multiplexing(SDM)transmission systems,the mode division multiplexing(MDM)has gained much attention recently.The vector mode(VM),which is the eigenmode of the optical fiber,has also been adopted to realize the optical communications including the transmission over free-space optical(FSO)and optical fiber links.Considering the concerns on the short-reach optical interconnects,the low cost and high integration technologies should be developed.Direct detection(DD)with higher-order modulation formats in combination of MDM technologies could offer an available trade-off in system performance and complexity.We review demonstrations of FSO and fiber high-speed data transmission based on the VM MDM(VMDM)technologies.The special VMs,cylindrical vector beams(CVB),have been generated by the q-plate(QP)and characterized accordingly.And then they were used to implement the VMDM transmission with direct-detection orthogonal frequency division multiplexing(DD-OFDM).These demonstrations show the potential of VMDM-DD-OFDM technology in the large-capacity short-reach transmission links.
基金by National Natural Science Foundation of China(Grant Nos.51106095,11575113).
文摘The paper presents a direct numerical simulation(DNS)for the drag-reducing channel flow using the Giesekus model with variable parameters.It is assumed that the relaxation time in the constitutive equation is varied depending on the local shear rate.The maximal drag reduction rate is obtained when variable parameters are applied in the Giesekus model at a high Weissenberg number.The Reynolds shear stress is reduced when the Weissenberg number increases.In this case,the turbulence generation and transportation are further weakened and increasingly approach to the values in the experiments.
文摘Analysis of the Direct Operating Cost(DOC) of aircraft is an important step towards achieving financially sustainable aviation operations. However, the value of the DOC for different aircraft types and flight scenarios is not widely available. In this study, we perform a systematic analysis of the DOC of every wide-body passenger aircraft currently in production, using the method of the Association of European Airlines(AEA). The elements of the DOC, e.g. financial costs, maintenance costs, and flight costs, are evaluated individually. Several realistic flight scenarios are considered, each with differences in route distance, fuel price, passenger number, and seating arrangement. For each flight scenario, the most cost-efficient aircraft type is identified and evaluated in the context of operations from Hong Kong International Airport. The information provided in this study could be useful to airline operators and policy makers.
基金the National Natural Science Foundation of China(51371059)the Guangxi Natural Science Foundation(2016GXNSFDA380022,AA17204100,AA18118030)+1 种基金the Project of Development of Science and Technology of Nanning(20181191-2)the High-Level Innovative Team and Outstanding School Program in Guangxi Colleges(the Second Batch).
文摘NbCrAl coatings in the presence or absence of Al outer layer were prepared on C103 Nb based alloy and alumina substrate by direct current magnetron sputtering technique. The oxidation performance of coating systems was evaluated by isothermal oxidation tests. The element diffusion and oxidation behavior of the coating systems were investigated. The results indicate that the mass gains of NbCrAl and Al/NbCrAl coatings are 2.02 mg/cm2 and 0.79 mg/cm2 at 1 000 ℃ for short time. Al/NbCrAl coatings exhibit more effective protection than NbCrAl coatings. The addition of Al layer can improve the oxidation resistance of NbCrAl coatings, which is attributed to the Al layer offering enough Al content to react with oxygen to form a continuous and dense Al2O3 scale on NbCrAl coating and it can inhibit the further internal oxidation.
基金the National Natural Science Foundation of China(Grant No.41807296 and No.41802006)Natural science found for universities of Anhui province(Grant No.KJ2017A036).
文摘Downhole microseismic data has the significant advantages of high signal-to-noise ratio and well-developed P and S waves and the core component of microseismic monitoring is microseismic event location associated with hydraulic fracturing in a relatively high confidence level and accuracy.In this study,we present a multidimensional DIRECT inversion method for microseismic locations and applicability tests over modeling data based on a downhole microseismic monitoring system.Synthetic tests inidcate that the objective function of locations can be defined as a multi-dimensional matrix space by employing the global optimization DIRECT algorithm,because it can be run without the initial value and objective function derivation,and the discretely scattered objective points lead to an expeditious contraction of objective functions in each dimension.This study shows that the DIRECT algorithm can be extensively applied in real downhole microseismic monitoring data from hydraulic fracturing completions.Therefore,the methodology,based on a multidimensional DIRECT algorithm,can provide significant high accuracy and convergent efficiency as well as robust computation for interpretable spatiotemporal microseismic evolution,which is more suitable for real-time processing of a large amount of downhole microseismic monitoring data.
基金the National Natural Science Foundation of China (Grant No.11672135)a Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant No.201461).
文摘Based on the Fourier–Chebyshev spectral method,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS) methods for different Reynolds numbers.A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient.With the application of electromagnetic force,the in-depth relations among the fluctuating velocities near the wall,Reynolds stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force.However,the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction.Therefore,the significant effect of drag reduction is obtained.Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.