The subgrid-scale(SGS)stress and SGS heat flux are modeled by using an artificial neural network(ANN)for large eddy simulation(LES)of compressible turbulence.The input features of ANN model are based on the first-orde...The subgrid-scale(SGS)stress and SGS heat flux are modeled by using an artificial neural network(ANN)for large eddy simulation(LES)of compressible turbulence.The input features of ANN model are based on the first-order and second-order derivatives of filtered velocity and temperature at different spatial locations.The proposed spatial artificial neural network(SANN)model gives much larger correlation coefficients and much smaller relative errors than the gradient model in an a priori analysis.In an a posteriori analysis,the SANN model performs better than the dynamic mixed model(DMM)in the prediction of spectra and statistical properties of velocity and temperature,and the instantaneous flow structures.展开更多
Controlling multiple multi-joint fish-like robots has long captivated the attention of engineers and biologists,for which a fundamental but challenging topic is to robustly track the postures of the individuals in rea...Controlling multiple multi-joint fish-like robots has long captivated the attention of engineers and biologists,for which a fundamental but challenging topic is to robustly track the postures of the individuals in real time.This requires detecting multiple robots,estimating multi-joint postures,and tracking identities,as well as processing fast in real time.To the best of our knowledge,this challenge has not been tackled in the previous studies.In this paper,to precisely track the planar postures of multiple swimming multi-joint fish-like robots in real time,we propose a novel deep neural network-based method,named TAB-IOL.Its TAB part fuses the top-down and bottom-up approaches for vision-based pose estimation,while the IOL part with long short-term memory considers the motion constraints among joints for precise pose tracking.The satisfying performance of our TAB-IOL is verified by testing on a group of freely swimming fish-like robots in various scenarios with strong disturbances and by a deed comparison of accuracy,speed,and robustness with most state-of-the-art algorithms.Further,based on the precise pose estimation and tracking realized by our TAB-IOL,several formation control experiments are conducted for the group of fish-like robots.The results clearly demonstrate that our TAB-IOL lays a solid foundation for the coordination control of multiple fish-like robots in a real working environment.We believe our proposed method will facilitate the growth and development of related fields.展开更多
This paper assesses the suitability of the inflow Reynolds number defined by Reo -- UoD/v (here Uo and D are respectively the initial jet velocity and diameter while v is kinematic viscosity) for a round air/air jet...This paper assesses the suitability of the inflow Reynolds number defined by Reo -- UoD/v (here Uo and D are respectively the initial jet velocity and diameter while v is kinematic viscosity) for a round air/air jet. Specifically an experimental investigation is performed for the influences of Uo, D and Reo on the mean-velocity decay and spread coefficients (Ku, Kr) in the far field of a circular air jet into air from a smoothly contracting nozzle. Present measurements agree well with those previously obtained under similar inflow conditions. The relations Ku ∝ Uo and Kr ∝ 1/Uo for Uo 〈 5 m/s appear to work, while each coefficient approaches asymptotically to a constant for Uo 〉 6 m/s, regardless of the magnitudes of Reo and D. It is revealed that Reo may not be an appropriate dimensionless parameter to characterize the entire flow of a free air/air jet. This paper is the first paper that has challenged the suitability of Reo for turbulent free jets.展开更多
This paper reports an investigation of Computational Fluid Dynamics(CFD)on the influence of injection momentum rate of premixed air and fuel on the flameless Moderate or Intense Low oxygen Dilution(MILD) combustion in...This paper reports an investigation of Computational Fluid Dynamics(CFD)on the influence of injection momentum rate of premixed air and fuel on the flameless Moderate or Intense Low oxygen Dilution(MILD) combustion in a recuperative furnace.Details of the furnace flow velocity,temperature,O2,CO2 and NOx concentrations are provided.Results obtained suggest that the flue gas recirculation plays a vital role in establishing the premixed MILD combustion.It is also revealed that there is a critical momentum rate of the fuel-air mixture below which MILD combustion does not occur.Moreover,the momentum rate appears to have less significant influence on conventional global combustion than on MILD combustion.展开更多
This paper is concerned with the determination of the normal force-displacement (NFD) relation for the contact problem of cylindrical joints with clearance. A simple formulation for this contact problem is developed...This paper is concerned with the determination of the normal force-displacement (NFD) relation for the contact problem of cylindrical joints with clearance. A simple formulation for this contact problem is developed by modeling the pin as a rigid wedge and the elastic plate as a simple Winkler elastic foundation. The numerical results show that the normal displacement relation based on Hertz theory is only valid for the case of large clearance with a small normal load, and the NFD relation based on Persson theory is only effective in the case of very small clearance. The proposed approximate model in this paper gives better results than Hertz theory and Persson theory in a large range of clearances as seen from the comparison with the results of FEM.展开更多
A series of direct numerical simulations of the fully developed plane Couette flow at a Reynolds number of 6000(based on the relative wall speed and half the channel height h) with different streamwise and spanwise ...A series of direct numerical simulations of the fully developed plane Couette flow at a Reynolds number of 6000(based on the relative wall speed and half the channel height h) with different streamwise and spanwise lengths are conducted to investigate the effects of the computational box sizes on the secondary flow(SF). Our focuses are the number of counter-rotating vortex pairs and its relationship to the statistics of the mean flow and the SF in the small and moderate computational box sizes. Our results show that the number of vortex pairs is sensitive to the computational box size, and so are the slope parameter, the rate of the turbulent kinetic energy contributed by the SF, and the ratio of the kinetic energy of the SF to the total kinetic energy. However, the averaged spanwise width of each counter-rotating vortex pair in the plane Couette flow is found, for the first time, within 4(1 ± 0.25)h despite the domain sizes.展开更多
We visualized the wake structure of circular disks falling vertically in quiescent water. The evolution of the wake was shown to be similar to the flow patterns behind a fixed disk. The Reynolds number, Re = Ud/v, is ...We visualized the wake structure of circular disks falling vertically in quiescent water. The evolution of the wake was shown to be similar to the flow patterns behind a fixed disk. The Reynolds number, Re = Ud/v, is in the range of 40 - 200. With the ascension of Reynolds numbers, a regular bifurcation occurred at the first critical Reynolds number Reel, leading to a transition from an axisymmetric wake structure to a plane symmetric one; A Hopf bifurcation took place at the second critical Reynolds number Rec2, as the wake structure became unsteady. Plane symmetry of the wake structure was first lost as periodic vortex shedding ap- peared, but recovered at higher Reynolds number. The differ- ence between the two critical Reynolds numbers was found to be shape-dependent, as we compared our results for thin discs with those for other falling bodies, such as spheres and cones. This observation could be understood in terms of the instability mechanism of the vortical structure.展开更多
Stokes' first problem has been investigated for a Maxwell fluid in a porous half-space for gaining insight into the effect of viscoelasticity on the start-up flow in a porous medium. An exact solution was obtained by...Stokes' first problem has been investigated for a Maxwell fluid in a porous half-space for gaining insight into the effect of viscoelasticity on the start-up flow in a porous medium. An exact solution was obtained by using the Fourier sine transform. It was found that at large values of the relaxation time the velocity overshoot occurs obviously and the system exhibits viscoelastic behaviours. On the other hand, for short relaxation time the velocity overshoot disappears and the system exhibits viscous behaviours. A critical value of the relaxation time was obtained for the emergence of the velocity overshoot. Furthermore, it was found that the velocity overshoot is caused by both the viscoelasticity of the Maxwell fluid and the Darcy resistance resulting from the structure of the micropore in the porous medium.展开更多
A self-learning fractal interpolation algorithm to construct synthetic fields with statistical properties close to real turbulence is proposed.Different from our previous work[Phys.Rev.E 84(2011)026328,82(2010)036311]...A self-learning fractal interpolation algorithm to construct synthetic fields with statistical properties close to real turbulence is proposed.Different from our previous work[Phys.Rev.E 84(2011)026328,82(2010)036311],the position mapping and stretching factors between the adjacent large and small scales are learned from the initial information.Using this method,a turbulence-like field with K41 spectra and without dissipation is constructed well through a coarse grid velocity signal from one experiment's data.After filtering the interpolated signal appropriately,the probability distribution of velocity,velocity structure functions and the anomalous scaling law of the synthetic field are close to those of the original signal.展开更多
The wind tunnel simulations of wind loading on a solid structure of revolution with one smooth and five rough surfaces were conducted using wind tunnel tests. Timemean and fluctuating pressure distributions on the sur...The wind tunnel simulations of wind loading on a solid structure of revolution with one smooth and five rough surfaces were conducted using wind tunnel tests. Timemean and fluctuating pressure distributions on the surface were obtained, and the relationships between the roughness Reynolds number and pressure distributions were analyzed and discussed. The results show that increasing the surface roughness can significantly affect the pressure distribution, and the roughness Reynolds numbers play an important role in the change of flow patterns. The three flow patterns of subcritical, critical and supercritical flows can be classified based on the changing patterns of both the mean and the fluctuating pressure distributions. The present study suggests that the wind tunnel results obtained in the supercritical pattern reflect more closely those of full-scale solid structure of revolution at the designed wind speed.展开更多
The global water resources network is simulated in the present work for the latest target year with statistical data available and with the most detailed data disaggregation. A top-down approach of systems inputoutput...The global water resources network is simulated in the present work for the latest target year with statistical data available and with the most detailed data disaggregation. A top-down approach of systems inputoutput simulation is employed to track the embodied water flows associated with economic flows for the globalized economy in 2004. The numerical simulation provides a database of embodied water intensities for all economic commodities from 4928 producers, based on which the differences between direct and indirect water using efficiencies at the global scale are discussed. The direct and embodied water uses are analyzed at continental level. Besides, the commodity demand in terms of monetary expenditure and the water demand in terms of embodied water use are compared for the world as well as for three major water using regions, i.e., India, China, and the United States. Results show that food product contributes to a significant fraction for water demand, despite the value varies significantly with respect to the economic status of region.展开更多
Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated. Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techni...Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated. Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques. The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods. Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated. It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids' surface, and such damage nucleation will be speeded up by the remote uniaxial compressive load.展开更多
In this paper,a design method for an acoustic cloak in the presence of background mean flows is proposed by using topology optimization,which enables the associated fabrication of the cloaking design.The density-based...In this paper,a design method for an acoustic cloak in the presence of background mean flows is proposed by using topology optimization,which enables the associated fabrication of the cloaking design.The density-based topology optimization method is used to allocate the designated materials,thus providing the structure of the cloak.The optimization problem is efficiently solved with the gradient-based globally convergent method of moving asymptotes,which utilizes the derivative information from the finite element simulation studies of the linearized acoustic potential equation.This paper introduces the whole design method first then numerically demonstrates the corresponding performance,which shall constitute the main contribution of the present work.展开更多
In this paper, new models of the density and modulus development of concrete under continued hydration were studied. Experimental study was performed for different mixes of concrete. To avoid considering the effect of...In this paper, new models of the density and modulus development of concrete under continued hydration were studied. Experimental study was performed for different mixes of concrete. To avoid considering the effect of variation of Poisson's ratio, the one-dimensional ultrasonic technique was adopted to detect the modulus development of concrete under continued hydration. The experimental results indicate the nonlinear characteristics of density and modulus evolution. At the initial stage of continued hydration, the density and modulus increase quickly, and then the increases slow down and finally tend to be constant. The mechanism of modulus enhancement is that the newly produced C-S-H gel in the continued hydration process not only leads to the decrease in porosity, but also repairs the initial defects of concrete. Based on this mechanism, simple differential equations for the density and modulus development of concrete were established by considering the chemical reactions of continued hydration, and new simple models for density and modulus development were proposed.展开更多
Using the incompressible isotropic turbulent fields obtained from direct numerical simulation and large-eddy simulation,we studied the statistics of oscillation structures based on local zero-crossings and their relat...Using the incompressible isotropic turbulent fields obtained from direct numerical simulation and large-eddy simulation,we studied the statistics of oscillation structures based on local zero-crossings and their relation with inertial-range intermittency for transverse velocity and passive scalar.Our results show that for both the velocity and passive scalar,the local oscillation structures are statistically scaleinvariant at high Reynolds number,and the inertial-range intermittency of the overall flow region is determined by the most intermittent structures characterized by one local zero-crossing.Local flow patterns conditioned on the oscillation structures are characterized by the joint probability density function of the invariants of the filtered velocity gradient tensor at inertial range.We demonstrate that the most intermittent regions for longitudinal velocity tend to lay at the saddle area,while those for the transverse velocity tend to locate at the vortex-dominated area.The connection between the ramp-cliff structures in passive scalar field and the corresponding saddle regions in the velocity field is also verified by the approach of oscillation structure classification.展开更多
Serrated leading edges are one of the most promising passive aerodynamic control methods for the reduction of aerofoil–turbulence interaction noise.To elucidate the possible physical mechanisms,the current paper stud...Serrated leading edges are one of the most promising passive aerodynamic control methods for the reduction of aerofoil–turbulence interaction noise.To elucidate the possible physical mechanisms,the current paper studies the simplified set-up with aerofoil–vortical gust interaction and proposes an analytical model by incorporating Fourier transform into the Wiener–Hopf method.The proposed model suggests that the serrations operate on the incident vortical gusts as convolution,which leads to the innovative concept that models serrations as transfer functions in the wavenumber domain.Overall,the current theoretical study could provide a unique insight of the inherent aerodynamic noise control mechanisms of leading-edge serrations.展开更多
Moderate or intense low oxygen dilution(MILD)combustion plays a significant role in the mitigation of combustion-generated pollutants and greenhouse gases whilst meeting thermal efficiency needs.However,due to the lac...Moderate or intense low oxygen dilution(MILD)combustion plays a significant role in the mitigation of combustion-generated pollutants and greenhouse gases whilst meeting thermal efficiency needs.However,due to the lack of the fundamental knowledge on this combustion,there is a misconception that MILD combustion should be established by high preheating of the air,which has limited its application.Our research and development on this combustion has been performed for several years. We have found that the requirements for establishing the MILD combustion are more relaxed than previously.It is also revealed that this combustion of different type,i.e.,non-premixed,partially premixed and fully premixed,can be achieved by firing various fuels(i.e.,gaseous,liquid and solid fuels).It is suggested that the application of the MILD combustion can be expanded significantly.The present review summarizes the progress and recent trend made in the R&D of this combustion and recommends further fundamental studies for improving our knowledge and widening its applications.展开更多
Experimental results about concrete under sulfate attack are summarized, which include the variation of mass density of samples and velocity of ultrasonic wave propagating in samples. The evolution damage is analyzed ...Experimental results about concrete under sulfate attack are summarized, which include the variation of mass density of samples and velocity of ultrasonic wave propagating in samples. The evolution damage is analyzed in terms of the experimental results, and close attention is paid to the effect of damage evolution on Poisson's ratio. This study shows that Poisson's ratio is significantly affected by the concentration of solution and water-cement ratio. Poisson's ratio of concrete changes very little when the water-cement ratio is selected as 0.6 or 0.8, so that such change may be neglected. If water-cement is 0.4, however, the Poisson's ratio of the sample significantly changes. When the concrete sample of 0.4 water-cement ratio is immersed in sodium sulfate solution of 8% concentration for 285 days, Poisson's ratio increase 10.14% compared with its initial value. There exist a sensitive region and a non-sensitive region for the change rate of Poisson's ratio with respect to corrosion time. The change rate of Poisson's ratio monotonously decreases with corrosion time in the sensitive region; in the non-sensitive region, the change rate of Poisson's ratio is almost equal to zero.展开更多
In this paper,a group consensus problem is investigated for multiple networked agents with parametric uncertainties where all the agents are governed by the Euler-Lagrange system with uncertain parameters.In the group...In this paper,a group consensus problem is investigated for multiple networked agents with parametric uncertainties where all the agents are governed by the Euler-Lagrange system with uncertain parameters.In the group consensus problem,the agents asymptotically reach several different states rather than one consistent state.A novel group consensus protocol and a time-varying estimator of the uncertain parameters are proposed for each agent in order to solve the couple-group consensus problem.It is shown that the group consensus is reachable even when the system contains the uncertain parameters.Furthermore,the multi-group consensus is discussed as an extension of the couple-group consensus,and then the group consensus with switching topology is considered.Simulation results are finally provided to validate the effectiveness of the theoretical analysis.展开更多
Concrete experiences expansive deformation during sulfate attack due to internal expansive stress. Sulfate ions enter concrete pores, react with the pore solution, and produce ettringite. The production of ettringite ...Concrete experiences expansive deformation during sulfate attack due to internal expansive stress. Sulfate ions enter concrete pores, react with the pore solution, and produce ettringite. The production of ettringite explains the internal expansive stress that reduces the durability of concrete. In this study, the model of internal expansive stress was achieved through the Eshelby's theory, as well as the experimental results for concrete erosion. Numerical simulation indicated that internal expansive stress is not only determined by the water-to-cement ratio of concrete and the concentration of sulfate solution, but also affected by the relaxation time of concrete.展开更多
基金This work was supported by the National Natural Science Foundation of China(Grants 91952104,11702127,and 91752201)the Technology and Innovation Commission of Shenzhen Municipality(Grants KQTD20180411143441009,JCYJ20170412151759222,and ZDSYS201802081843517).This work was also supported by Center for Computational Science and Engineering of Southern University of Science and Technology.J.Wang acknowledges the support from Young Elite Scientist Sponsorship Program by CAST(Grant 2016QNRC001).
文摘The subgrid-scale(SGS)stress and SGS heat flux are modeled by using an artificial neural network(ANN)for large eddy simulation(LES)of compressible turbulence.The input features of ANN model are based on the first-order and second-order derivatives of filtered velocity and temperature at different spatial locations.The proposed spatial artificial neural network(SANN)model gives much larger correlation coefficients and much smaller relative errors than the gradient model in an a priori analysis.In an a posteriori analysis,the SANN model performs better than the dynamic mixed model(DMM)in the prediction of spectra and statistical properties of velocity and temperature,and the instantaneous flow structures.
基金This work was supported in part by the National Natural Science Foundation of China(61973007,61633002).
文摘Controlling multiple multi-joint fish-like robots has long captivated the attention of engineers and biologists,for which a fundamental but challenging topic is to robustly track the postures of the individuals in real time.This requires detecting multiple robots,estimating multi-joint postures,and tracking identities,as well as processing fast in real time.To the best of our knowledge,this challenge has not been tackled in the previous studies.In this paper,to precisely track the planar postures of multiple swimming multi-joint fish-like robots in real time,we propose a novel deep neural network-based method,named TAB-IOL.Its TAB part fuses the top-down and bottom-up approaches for vision-based pose estimation,while the IOL part with long short-term memory considers the motion constraints among joints for precise pose tracking.The satisfying performance of our TAB-IOL is verified by testing on a group of freely swimming fish-like robots in various scenarios with strong disturbances and by a deed comparison of accuracy,speed,and robustness with most state-of-the-art algorithms.Further,based on the precise pose estimation and tracking realized by our TAB-IOL,several formation control experiments are conducted for the group of fish-like robots.The results clearly demonstrate that our TAB-IOL lays a solid foundation for the coordination control of multiple fish-like robots in a real working environment.We believe our proposed method will facilitate the growth and development of related fields.
基金supported by the National Natural Science Foundation of China (Grant Nos. 10921202 and 11072005)
文摘This paper assesses the suitability of the inflow Reynolds number defined by Reo -- UoD/v (here Uo and D are respectively the initial jet velocity and diameter while v is kinematic viscosity) for a round air/air jet. Specifically an experimental investigation is performed for the influences of Uo, D and Reo on the mean-velocity decay and spread coefficients (Ku, Kr) in the far field of a circular air jet into air from a smoothly contracting nozzle. Present measurements agree well with those previously obtained under similar inflow conditions. The relations Ku ∝ Uo and Kr ∝ 1/Uo for Uo 〈 5 m/s appear to work, while each coefficient approaches asymptotically to a constant for Uo 〉 6 m/s, regardless of the magnitudes of Reo and D. It is revealed that Reo may not be an appropriate dimensionless parameter to characterize the entire flow of a free air/air jet. This paper is the first paper that has challenged the suitability of Reo for turbulent free jets.
基金Supported by the National Natural Science Foundation of China(50936001)
文摘This paper reports an investigation of Computational Fluid Dynamics(CFD)on the influence of injection momentum rate of premixed air and fuel on the flameless Moderate or Intense Low oxygen Dilution(MILD) combustion in a recuperative furnace.Details of the furnace flow velocity,temperature,O2,CO2 and NOx concentrations are provided.Results obtained suggest that the flue gas recirculation plays a vital role in establishing the premixed MILD combustion.It is also revealed that there is a critical momentum rate of the fuel-air mixture below which MILD combustion does not occur.Moreover,the momentum rate appears to have less significant influence on conventional global combustion than on MILD combustion.
基金The project supported by the National Natural Science Foundation of China(10272002:60334030)
文摘This paper is concerned with the determination of the normal force-displacement (NFD) relation for the contact problem of cylindrical joints with clearance. A simple formulation for this contact problem is developed by modeling the pin as a rigid wedge and the elastic plate as a simple Winkler elastic foundation. The numerical results show that the normal displacement relation based on Hertz theory is only valid for the case of large clearance with a small normal load, and the NFD relation based on Persson theory is only effective in the case of very small clearance. The proposed approximate model in this paper gives better results than Hertz theory and Persson theory in a large range of clearances as seen from the comparison with the results of FEM.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11221061,11272013,and 11302006)
文摘A series of direct numerical simulations of the fully developed plane Couette flow at a Reynolds number of 6000(based on the relative wall speed and half the channel height h) with different streamwise and spanwise lengths are conducted to investigate the effects of the computational box sizes on the secondary flow(SF). Our focuses are the number of counter-rotating vortex pairs and its relationship to the statistics of the mean flow and the SF in the small and moderate computational box sizes. Our results show that the number of vortex pairs is sensitive to the computational box size, and so are the slope parameter, the rate of the turbulent kinetic energy contributed by the SF, and the ratio of the kinetic energy of the SF to the total kinetic energy. However, the averaged spanwise width of each counter-rotating vortex pair in the plane Couette flow is found, for the first time, within 4(1 ± 0.25)h despite the domain sizes.
基金supported by the National Natural Science Fundation of China (10910301062,11102197)the National Climb-B Plan (2009CB724100)+1 种基金the National Foundation for Distinguished Young Scholar of China (10525208)the Science Fund for Creative Research Groups (10921202)
文摘We visualized the wake structure of circular disks falling vertically in quiescent water. The evolution of the wake was shown to be similar to the flow patterns behind a fixed disk. The Reynolds number, Re = Ud/v, is in the range of 40 - 200. With the ascension of Reynolds numbers, a regular bifurcation occurred at the first critical Reynolds number Reel, leading to a transition from an axisymmetric wake structure to a plane symmetric one; A Hopf bifurcation took place at the second critical Reynolds number Rec2, as the wake structure became unsteady. Plane symmetry of the wake structure was first lost as periodic vortex shedding ap- peared, but recovered at higher Reynolds number. The differ- ence between the two critical Reynolds numbers was found to be shape-dependent, as we compared our results for thin discs with those for other falling bodies, such as spheres and cones. This observation could be understood in terms of the instability mechanism of the vortical structure.
基金Project supported by the National Natural Science Foundation of China (Grant Nos 10372007 and 10572006) and the New , Century Training Programme Foundation for the Talents by Chinese Ministry of Education.
文摘Stokes' first problem has been investigated for a Maxwell fluid in a porous half-space for gaining insight into the effect of viscoelasticity on the start-up flow in a porous medium. An exact solution was obtained by using the Fourier sine transform. It was found that at large values of the relaxation time the velocity overshoot occurs obviously and the system exhibits viscoelastic behaviours. On the other hand, for short relaxation time the velocity overshoot disappears and the system exhibits viscous behaviours. A critical value of the relaxation time was obtained for the emergence of the velocity overshoot. Furthermore, it was found that the velocity overshoot is caused by both the viscoelasticity of the Maxwell fluid and the Darcy resistance resulting from the structure of the micropore in the porous medium.
基金Supported by the National Basic Research Program of China under Grant No 2009CB724100.
文摘A self-learning fractal interpolation algorithm to construct synthetic fields with statistical properties close to real turbulence is proposed.Different from our previous work[Phys.Rev.E 84(2011)026328,82(2010)036311],the position mapping and stretching factors between the adjacent large and small scales are learned from the initial information.Using this method,a turbulence-like field with K41 spectra and without dissipation is constructed well through a coarse grid velocity signal from one experiment's data.After filtering the interpolated signal appropriately,the probability distribution of velocity,velocity structure functions and the anomalous scaling law of the synthetic field are close to those of the original signal.
文摘The wind tunnel simulations of wind loading on a solid structure of revolution with one smooth and five rough surfaces were conducted using wind tunnel tests. Timemean and fluctuating pressure distributions on the surface were obtained, and the relationships between the roughness Reynolds number and pressure distributions were analyzed and discussed. The results show that increasing the surface roughness can significantly affect the pressure distribution, and the roughness Reynolds numbers play an important role in the change of flow patterns. The three flow patterns of subcritical, critical and supercritical flows can be classified based on the changing patterns of both the mean and the fluctuating pressure distributions. The present study suggests that the wind tunnel results obtained in the supercritical pattern reflect more closely those of full-scale solid structure of revolution at the designed wind speed.
文摘The global water resources network is simulated in the present work for the latest target year with statistical data available and with the most detailed data disaggregation. A top-down approach of systems inputoutput simulation is employed to track the embodied water flows associated with economic flows for the globalized economy in 2004. The numerical simulation provides a database of embodied water intensities for all economic commodities from 4928 producers, based on which the differences between direct and indirect water using efficiencies at the global scale are discussed. The direct and embodied water uses are analyzed at continental level. Besides, the commodity demand in terms of monetary expenditure and the water demand in terms of embodied water use are compared for the world as well as for three major water using regions, i.e., India, China, and the United States. Results show that food product contributes to a significant fraction for water demand, despite the value varies significantly with respect to the economic status of region.
基金Project supported by the National Natural Science Foundation of China (Nos. 10932001,51079069,10972109,and 10802039)the National Basic Research Program of China (973 Program,No. 2009CB623203)+1 种基金the Ministry of Education of the People’s Republic of China (No. 20103305110001)K.C.Wong Magna Fund in Ningbo University
文摘Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated. Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques. The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods. Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated. It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids' surface, and such damage nucleation will be speeded up by the remote uniaxial compressive load.
基金This work was supported by the Beijing Municipal Science&Technology Commission(Grant Z181100001018030)the National Natural Science Foundation of China(Grant 11561130148)the Newton Advanced Fellowship from the Royal Society(Ref.NA14081).
文摘In this paper,a design method for an acoustic cloak in the presence of background mean flows is proposed by using topology optimization,which enables the associated fabrication of the cloaking design.The density-based topology optimization method is used to allocate the designated materials,thus providing the structure of the cloak.The optimization problem is efficiently solved with the gradient-based globally convergent method of moving asymptotes,which utilizes the derivative information from the finite element simulation studies of the linearized acoustic potential equation.This paper introduces the whole design method first then numerically demonstrates the corresponding performance,which shall constitute the main contribution of the present work.
基金the financial support by the National Natural Science Foundation of China (NSFC#11772164,#11272165,#11572163)the National Basic Research Program of China (973 Program,2009CB623203)+2 种基金the Key Research Program of Society Development of Ningbo (2013C51007)the K.C.Wong Magna Fund in Ningbo Universitysupported by the Research Project Foundation of Zhejiang Educational Department (Y201636745)
文摘In this paper, new models of the density and modulus development of concrete under continued hydration were studied. Experimental study was performed for different mixes of concrete. To avoid considering the effect of variation of Poisson's ratio, the one-dimensional ultrasonic technique was adopted to detect the modulus development of concrete under continued hydration. The experimental results indicate the nonlinear characteristics of density and modulus evolution. At the initial stage of continued hydration, the density and modulus increase quickly, and then the increases slow down and finally tend to be constant. The mechanism of modulus enhancement is that the newly produced C-S-H gel in the continued hydration process not only leads to the decrease in porosity, but also repairs the initial defects of concrete. Based on this mechanism, simple differential equations for the density and modulus development of concrete were established by considering the chemical reactions of continued hydration, and new simple models for density and modulus development were proposed.
文摘Using the incompressible isotropic turbulent fields obtained from direct numerical simulation and large-eddy simulation,we studied the statistics of oscillation structures based on local zero-crossings and their relation with inertial-range intermittency for transverse velocity and passive scalar.Our results show that for both the velocity and passive scalar,the local oscillation structures are statistically scaleinvariant at high Reynolds number,and the inertial-range intermittency of the overall flow region is determined by the most intermittent structures characterized by one local zero-crossing.Local flow patterns conditioned on the oscillation structures are characterized by the joint probability density function of the invariants of the filtered velocity gradient tensor at inertial range.We demonstrate that the most intermittent regions for longitudinal velocity tend to lay at the saddle area,while those for the transverse velocity tend to locate at the vortex-dominated area.The connection between the ramp-cliff structures in passive scalar field and the corresponding saddle regions in the velocity field is also verified by the approach of oscillation structure classification.
基金This work is partly supported by National Science Foundation of China(Grants 11772005,91852201)Ministry of Industry and Information Technology of China(Grant no.MJ-2015-F-012-03)the Research Grants Council of the Hong Kong Special Administrative Region(Grant no.16205317).
文摘Serrated leading edges are one of the most promising passive aerodynamic control methods for the reduction of aerofoil–turbulence interaction noise.To elucidate the possible physical mechanisms,the current paper studies the simplified set-up with aerofoil–vortical gust interaction and proposes an analytical model by incorporating Fourier transform into the Wiener–Hopf method.The proposed model suggests that the serrations operate on the incident vortical gusts as convolution,which leads to the innovative concept that models serrations as transfer functions in the wavenumber domain.Overall,the current theoretical study could provide a unique insight of the inherent aerodynamic noise control mechanisms of leading-edge serrations.
基金supported by the National Natural Science Foundation of China(Grant No.11072005,50936001)the Open Foundation of State Key Laboratory of Coal Combustion(Grant No.FSKLCC0801)
文摘Moderate or intense low oxygen dilution(MILD)combustion plays a significant role in the mitigation of combustion-generated pollutants and greenhouse gases whilst meeting thermal efficiency needs.However,due to the lack of the fundamental knowledge on this combustion,there is a misconception that MILD combustion should be established by high preheating of the air,which has limited its application.Our research and development on this combustion has been performed for several years. We have found that the requirements for establishing the MILD combustion are more relaxed than previously.It is also revealed that this combustion of different type,i.e.,non-premixed,partially premixed and fully premixed,can be achieved by firing various fuels(i.e.,gaseous,liquid and solid fuels).It is suggested that the application of the MILD combustion can be expanded significantly.The present review summarizes the progress and recent trend made in the R&D of this combustion and recommends further fundamental studies for improving our knowledge and widening its applications.
基金Project supported by the National Natural Science Foundation of China (Nos. 10932001,51079069 and 10572064)the National Basic Program of China (973 Program,2009CB623203)+1 种基金Ministry of Education of China (20103305110001)K.C. Wong Magna Fund in Ningbo University
文摘Experimental results about concrete under sulfate attack are summarized, which include the variation of mass density of samples and velocity of ultrasonic wave propagating in samples. The evolution damage is analyzed in terms of the experimental results, and close attention is paid to the effect of damage evolution on Poisson's ratio. This study shows that Poisson's ratio is significantly affected by the concentration of solution and water-cement ratio. Poisson's ratio of concrete changes very little when the water-cement ratio is selected as 0.6 or 0.8, so that such change may be neglected. If water-cement is 0.4, however, the Poisson's ratio of the sample significantly changes. When the concrete sample of 0.4 water-cement ratio is immersed in sodium sulfate solution of 8% concentration for 285 days, Poisson's ratio increase 10.14% compared with its initial value. There exist a sensitive region and a non-sensitive region for the change rate of Poisson's ratio with respect to corrosion time. The change rate of Poisson's ratio monotonously decreases with corrosion time in the sensitive region; in the non-sensitive region, the change rate of Poisson's ratio is almost equal to zero.
基金supported by the National Natural Science Foundation of China under Grant Nos.60974017.61273212,61322302,61104145,and 61004097Zhejiang Provincial Natural Science Foundation of China under Grant No.LQ14F030011+2 种基金the Natural Science Foundation of Jiangsu Province of China under Grant No.BK2011581the Research Fund for the Doctoral Program of Higher Education of China under Grant No.20110092120024the Fundamental Research Funds for the Central Universities of China
文摘In this paper,a group consensus problem is investigated for multiple networked agents with parametric uncertainties where all the agents are governed by the Euler-Lagrange system with uncertain parameters.In the group consensus problem,the agents asymptotically reach several different states rather than one consistent state.A novel group consensus protocol and a time-varying estimator of the uncertain parameters are proposed for each agent in order to solve the couple-group consensus problem.It is shown that the group consensus is reachable even when the system contains the uncertain parameters.Furthermore,the multi-group consensus is discussed as an extension of the couple-group consensus,and then the group consensus with switching topology is considered.Simulation results are finally provided to validate the effectiveness of the theoretical analysis.
基金supported by the National Natural Science Foundation of China(Nos.51079069,11272165 and 51378114)the Key Project of Ningbo City Society Development Funding(No.2013C51007)the K.C.Wong Magna Fund in Ningbo University
文摘Concrete experiences expansive deformation during sulfate attack due to internal expansive stress. Sulfate ions enter concrete pores, react with the pore solution, and produce ettringite. The production of ettringite explains the internal expansive stress that reduces the durability of concrete. In this study, the model of internal expansive stress was achieved through the Eshelby's theory, as well as the experimental results for concrete erosion. Numerical simulation indicated that internal expansive stress is not only determined by the water-to-cement ratio of concrete and the concentration of sulfate solution, but also affected by the relaxation time of concrete.