Reinforcement learning(RL)algorithms are expected to become the next generation of wind farm control methods.However,as wind farms continue to grow in size,the computational complexity of collective wind farm control ...Reinforcement learning(RL)algorithms are expected to become the next generation of wind farm control methods.However,as wind farms continue to grow in size,the computational complexity of collective wind farm control will exponentially increase with the growth of action and state spaces,limiting its potential in practical applications.In this Letter,we employ a RL-based wind farm control approach with multi-agent deep deterministic policy gradient to optimize the yaw manoeuvre of grouped wind turbines in wind farms.To reduce the computational complexity,the turbines in the wind farm are grouped according to the strength of the wake interaction.Meanwhile,to improve the control efficiency,each subgroup is treated as a whole and controlled by a single agent.Optimized results show that the proposed method can not only increase the power production of the wind farm but also significantly improve the control efficiency.展开更多
The yaw effect of the side flow around a high-speed train is studied by means of large eddy simulation at two typical yaw angles of φ= 30° and 60°, respectively. Both the mean and fluctuating values of lift...The yaw effect of the side flow around a high-speed train is studied by means of large eddy simulation at two typical yaw angles of φ= 30° and 60°, respectively. Both the mean and fluctuating values of lift force and side force coefficients increase obviously as the yaw angle increases. The spectral analysis indicates that the time-dependent aerodynamic forces are dominated by several energetic frequencies and the frequency range is broadened to a higher extent for the large yaw angle. To have a better understanding of the train aerodynamic behaviors, the dedicate three-dimensional vortical structures are analyzed for the flow at the two yaw angles. Moreover, the time-averaged flow patterns, turbulent statistics, and the surface forces are also studied on sectional planes along the train.展开更多
以探寻合理的滴头埋深与流量为目的,开展浅埋式滴灌下土壤水分分布规律的试验研究。试验土箱净尺寸为30cm×30cm×60cm(长×宽×高),供试土壤取自新疆青河县阿苇灌区试验站。试验设置5、10、15 cm 3个滴头埋深,各埋深...以探寻合理的滴头埋深与流量为目的,开展浅埋式滴灌下土壤水分分布规律的试验研究。试验土箱净尺寸为30cm×30cm×60cm(长×宽×高),供试土壤取自新疆青河县阿苇灌区试验站。试验设置5、10、15 cm 3个滴头埋深,各埋深条件下设置不同流量的处理。结果表明:不同滴头埋深的情况下均存在土体破坏的临界流量,滴头埋深越大临界流量也越大,滴头埋深为5、10和15 cm对应的临界流量分别为1.0、1.7和2.5 L/h;随着流量的增大,滴头埋深过浅时,水量向湿润体上部聚集,当埋深超过一定深度时,水量向湿润体下部聚集;在临界流量情况下,湿润锋前60 min运移速率较快,随着滴头埋深的增大,灌水结束后湿润体的垂向湿润长度越长,土壤平均含水率值越小;滴头埋深为10 cm、流量1.7 L/h时,湿润体水分分布较为合理。展开更多
In this work we extend the method of the constrained large-eddy simulation(CLES)to simulate the tur-bulent flow over inhomogeneous rough walls.In the original concept of CLES,the subgrid-scale(SGS)stress is constraine...In this work we extend the method of the constrained large-eddy simulation(CLES)to simulate the tur-bulent flow over inhomogeneous rough walls.In the original concept of CLES,the subgrid-scale(SGS)stress is constrained so that the mean part and the fluctuation part of the SGS stress can be modelled separately to improve the accuracy of the simulation result.Here in the simulation of the rough-wall flows,we propose to interpret the extra stress terms in the CLES formulation as the roughness-induced stress so that the roughness inhomogeneity can be incorporated by modifying the formulation of the constrained SGS stress.This is examined with the simulations of the channel flow with the spanwise alternating high/low roughness strips.Then the CLES method is employed to investigate the temporal response of the turbulence to the change of the wall condition from rough to smooth.We demonstrate that the temporal development of the internal boundary layer is just similar to that in a spatial rough-to-smooth transition process,and the spanwise roughness inhomogeneity has little impact on the transition process.展开更多
The Richtmyer-Meshkov instability of interfaces separating elastic-plastic materials from vacuum is investigated by numerical simulation using a multi-material solid mechanics algorithm based on an Eulerian framework....The Richtmyer-Meshkov instability of interfaces separating elastic-plastic materials from vacuum is investigated by numerical simulation using a multi-material solid mechanics algorithm based on an Eulerian framework.The research efforts are directed to reveal the influence of the initial perturbation and material strength on the deformation of the perturbed interface impacted by an initial shock.By varying the initial amplitude(kx0)of the perturbed interface and the yield stress(sY),three typical modes of interface deformation have been identified as the broken mode,the stable mode and the oscillating mode.For the broken mode,the interface width(i.e.,the bubble position with respect to that of the spike)increases continuously resulting in a final separation of the spike from the perturbed interface.For the stable mode,the interface width grows to saturation and then maintains a nearly constant value in the long term.For the oscillating mode,the wavy-like interface moving forward obtains an aperiodic oscillation of small amplitude,namely,the interface width varies in time slightly around zero.The intriguing difference of the typical modes is interpreted qualitatively by comparing the early-stage wave motion and the commensurate pressure and effective stress.Further,the subsequent interface deformation is illustrated quantitatively via the time series of the interface positions and velocities of these three typical modes.展开更多
The effects of electron nonlocal heat transport (NLHT) on the two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) upto the highly nonlinear phase are reported for the first time through numerical ...The effects of electron nonlocal heat transport (NLHT) on the two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) upto the highly nonlinear phase are reported for the first time through numerical simulations with a multigroup diffusion model. It is found thatas well as its role in the linear stabilization of ARTI growth, NLHT can also mitigate ARTI bubble nonlinear growth after the first saturationto the classical terminal velocity, compared with what is predicted by the local Spitzer–Härm model. The key factor affecting the reductionin the linear growth rate is the enhancement of the ablation velocity Va by preheating. It is found that NLHT mitigates nonlinear bubblegrowth through a mechanism involving reduction of vorticity generation. NLHT enhances ablation near the spike tip and slows down thespike, leading to weaker vortex generation as the pump of bubble reacceleration in the nonlinear stage. NLHT more effectively reduces thenonlinear growth of shorter-wavelength ARTI modes seeded by the laser imprinting phase in direct-drive laser fusion.展开更多
This paper numerically and analytically studies the onset of instability of a flag in uniform flow. The three-dimensional （3D） simulation is performed by using an immersed-boundary method coupled with a nonlinear fi...This paper numerically and analytically studies the onset of instability of a flag in uniform flow. The three-dimensional （3D） simulation is performed by using an immersed-boundary method coupled with a nonlinear finite element method. The global stability, bistability and instability are identified in the 3D simulations. The Squire＇s theorem is extended to analyze the stability of the fluid-flag system with 3D initial perturbations. It is found that if a parallel flow around the flag admits an unstable 3D disturbance for a certain value of the flutter speed, then a two-dimensional （2D） disturbance at a lower flutter speed is also adnfitted. In addition, the growth rate of 2D disturbance is larger than that of the 3D disturbance.展开更多
Abstract A vortex ring impacting a three-dimensional circular cylinder is studied using large eddy simulation （LES） for a Reynolds number Re = 4 × 10^4 based on the initial translation speed and diameter of the...Abstract A vortex ring impacting a three-dimensional circular cylinder is studied using large eddy simulation （LES） for a Reynolds number Re = 4 × 10^4 based on the initial translation speed and diameter of the vortex ring. We have investigated the evolution of vortical structures and identified three typical evolution phases. When the primary vortex closely approaches to the cylinder, a secondary vortex is generated and its segment parts move inward to the primary vortex ring. Then two large-scale loop-like vortices are formed to evolve in opposite directions. Thirdly, the two loop-like vortices collide with each other to form complicated small-scale vortical structures. Moreover, a series of hair-pin vortices are generated due to the stretching and deformation of the tertiary vortex. The trajectories of vortical structures and the relevant evolution speeds are analyzed. The total kinetic energy and enstrophy are investigated to reveal their properties relevant to the three evolution phases.展开更多
The propulsive performance of two- and three-dimensional （2D and 3D） flapping flexible plates in a fluid at rest is investigated by a finite element method for the plate motion and an immersed boundary-lattice Boltz...The propulsive performance of two- and three-dimensional （2D and 3D） flapping flexible plates in a fluid at rest is investigated by a finite element method for the plate motion and an immersed boundary-lattice Boltzmann method for the fluid flow. We consider a model that as the leading-edge of the plate takes a vertical oscillation, the entire plate moves freely due to the fluid-structure interaction. The effects of flexibility on the dynamics of the 2D and 3D flapping plates are investigated. It is found that a suitable flexibility is benefit for improving the propulsive performance. The results obtained in this study provide physical insight into the understanding of the dynamics of the flapping-based locomotion.展开更多
The two-winged insect hovering flight is investigated numerically using the lattice Boltzmann method(LBM).A virtual model of two elliptic foils with flapping motion is used to study the aerodynamic performance of the ...The two-winged insect hovering flight is investigated numerically using the lattice Boltzmann method(LBM).A virtual model of two elliptic foils with flapping motion is used to study the aerodynamic performance of the insect hovering flight with and without the effect of ground surface.Systematic studies have been carried out by changing some parameters of the wing kinematics,including the stroke amplitude,attack angle,and the Reynolds number for the insect hovering flight without ground effect,as well as the distance between the flapping foils and the ground surface when the ground effect is considered.The influence of the wing kinematic parameters and the effect of the ground surface on the unsteady forces and vortical structures are analyzed.The unsteady forces acting on the flapping foils are verified to be closely associated with the time evolution of the vortex structures,foil translational and rotational accelerations,and interaction between the flapping foils and the existed vortical flow.Typical unsteady mechanisms of lift production are identified by examining the vortical structures around the flapping foils.The results obtained in this study provide some physical insight into the understanding of the aerodynamics and flow structures for the insect hovering flight.展开更多
Interaction of vortex rings with solid is an important research topic of hydrodynamic.In this study,a multiple-relaxation time(MRT)lattice Boltzmann method(LBM)is used to investigate the flow of a vortex ring impactin...Interaction of vortex rings with solid is an important research topic of hydrodynamic.In this study,a multiple-relaxation time(MRT)lattice Boltzmann method(LBM)is used to investigate the flow of a vortex ring impacting spheroidal particles.The MRT-LBM is validated through the cases of vortex ring impacting a flat wall.The vortex evolution due to particle size,the aspect ratio of a prolate particle,as well as Reynolds(Re)number are discussed in detail.When the vortex ring impacting a stationary sphere,the primary and secondary vortex rings wrap around each other,which is different from the situation of the vortex ring impacting a plate.For the vortex ring impacting with a prolate spheroid,the secondary vortex ring stretches mainly along the long axis of the ellipsoid particle.However,it is found that after the vortex wrapping stage,the primary vortex recovers along the short axis of the particle faster than that in the long axis,i.e.,the primary vortex ring stretches mainly along the short axis of the particle.That has never been address in the literature.展开更多
To investigate the relationship of milk line position with grain weight and mechanized harvest of summer corn in Huang-Huai-Hai Region, 8 varieties （A, B, C, D, E, F, G, H） with large planting areas were selected to...To investigate the relationship of milk line position with grain weight and mechanized harvest of summer corn in Huang-Huai-Hai Region, 8 varieties （A, B, C, D, E, F, G, H） with large planting areas were selected to measure the grain filling rate, 100-grain weight, water content and milk line position, and the correlation was analyzed. Results showed that when the milk line position was 90%, the grain filling of all the 8 varieties finished and 100-grain weight reached the highest value, which was 43.02 g. The grain filling time was in positive correlation with 100-grain weight. However, when the milk line position completely disappeared, the 100-grain weight was reduced by 8.66% at most. There was no significant difference during the periods of grain weight rising, but in the periods of grain weight falling, the traits of D, E, H were significantly different with the other varieties, and water loss rate of C and A showed significant difference with the other six varieties. The water content of grain was negatively correlated with milk line position. When the milk line percentage was 90% , the grain water content was less than 30% . The key factor influencing the mechanized harvest of summer corn is harvesting time, rather than the varieties. Moreover, milk-line position of 90% is the best time for harvest; if the harvest is too late, the yield will be reduced with varying degrees.展开更多
The investigation of supersonic channel flow with periodic oscillatory backpressures at the outlet of the channel was performed using large-eddy simulation for the inlet free-stream Mach number 4 and the Reynolds numb...The investigation of supersonic channel flow with periodic oscillatory backpressures at the outlet of the channel was performed using large-eddy simulation for the inlet free-stream Mach number 4 and the Reynolds number approximately 5.2104 based on the height of the channel.Results have been validated carefully against our experimental data.Three typical backpressures are considered for constant backpressure and both periodic oscillatory backpressures with low and high frequency.The oscillatory backpressure can obviously influence the flow features occurring up to the middle region of the channel for the low frequency case and the downstream region for the high frequency case.Obvious differences of phase-averaged quantities at different phases are observed for the low frequency backpressure while the differences are relatively small for the high frequency backpressure.The spectral analysis reveals that the flow field experiences a periodic-like evolution of flow structures including shocks and vortices for the low frequency backpressure,resulting in the enhancement of turbulence fluctuations due to the complicated interaction of shocks and vortices.展开更多
基金supported by the National Natural Science Foundation of China (Grant No.12388101)the Science Challenge Project+1 种基金the Anhui NARI Jiyuan Electric Power Grid Technology Co.Ltd.through the Joint Laboratory of USTC-NARIthe advanced computing resources provided by the Supercomputing Center of the USTC
文摘Reinforcement learning(RL)algorithms are expected to become the next generation of wind farm control methods.However,as wind farms continue to grow in size,the computational complexity of collective wind farm control will exponentially increase with the growth of action and state spaces,limiting its potential in practical applications.In this Letter,we employ a RL-based wind farm control approach with multi-agent deep deterministic policy gradient to optimize the yaw manoeuvre of grouped wind turbines in wind farms.To reduce the computational complexity,the turbines in the wind farm are grouped according to the strength of the wake interaction.Meanwhile,to improve the control efficiency,each subgroup is treated as a whole and controlled by a single agent.Optimized results show that the proposed method can not only increase the power production of the wind farm but also significantly improve the control efficiency.
基金supported by the Knowledge Innovation Project of the Chinese Academy of Sciencesthe National Natural Science Foundation of China (11402261)the Fundamental Research Funds for the Central Universities (2013)
文摘The yaw effect of the side flow around a high-speed train is studied by means of large eddy simulation at two typical yaw angles of φ= 30° and 60°, respectively. Both the mean and fluctuating values of lift force and side force coefficients increase obviously as the yaw angle increases. The spectral analysis indicates that the time-dependent aerodynamic forces are dominated by several energetic frequencies and the frequency range is broadened to a higher extent for the large yaw angle. To have a better understanding of the train aerodynamic behaviors, the dedicate three-dimensional vortical structures are analyzed for the flow at the two yaw angles. Moreover, the time-averaged flow patterns, turbulent statistics, and the surface forces are also studied on sectional planes along the train.
基金supported by the National Natural Science Foundation of China (Grants 11988102, 91752201, and 11822208)the Department of Science and Technology of Guangdong Province (Grant 2019B21203001)+3 种基金Key Special Project for Introduced Tal ents Team of Southern Marine Science and Engineering Guang dong Laboratory (Guangzhou) (Grant GML2019ZD0103)Shenzhen Science & Technology Program (Grant KQTD2018 0411143441009)supported by Center for Computational Science and Engineering of Southern University of Science and Technologythe support from Centers for Mechanical Engineering Research and Education at MIT and SUSTech
文摘In this work we extend the method of the constrained large-eddy simulation(CLES)to simulate the tur-bulent flow over inhomogeneous rough walls.In the original concept of CLES,the subgrid-scale(SGS)stress is constrained so that the mean part and the fluctuation part of the SGS stress can be modelled separately to improve the accuracy of the simulation result.Here in the simulation of the rough-wall flows,we propose to interpret the extra stress terms in the CLES formulation as the roughness-induced stress so that the roughness inhomogeneity can be incorporated by modifying the formulation of the constrained SGS stress.This is examined with the simulations of the channel flow with the spanwise alternating high/low roughness strips.Then the CLES method is employed to investigate the temporal response of the turbulence to the change of the wall condition from rough to smooth.We demonstrate that the temporal development of the internal boundary layer is just similar to that in a spatial rough-to-smooth transition process,and the spanwise roughness inhomogeneity has little impact on the transition process.
基金supported by the National Natural Science Foundation of China(Nos.12172353,92052301,11621202 and 12202436)the Science Challenge Project(No.TZ2016001)the National Science Foundation(No.CBET0755269).
文摘The Richtmyer-Meshkov instability of interfaces separating elastic-plastic materials from vacuum is investigated by numerical simulation using a multi-material solid mechanics algorithm based on an Eulerian framework.The research efforts are directed to reveal the influence of the initial perturbation and material strength on the deformation of the perturbed interface impacted by an initial shock.By varying the initial amplitude(kx0)of the perturbed interface and the yield stress(sY),three typical modes of interface deformation have been identified as the broken mode,the stable mode and the oscillating mode.For the broken mode,the interface width(i.e.,the bubble position with respect to that of the spike)increases continuously resulting in a final separation of the spike from the perturbed interface.For the stable mode,the interface width grows to saturation and then maintains a nearly constant value in the long term.For the oscillating mode,the wavy-like interface moving forward obtains an aperiodic oscillation of small amplitude,namely,the interface width varies in time slightly around zero.The intriguing difference of the typical modes is interpreted qualitatively by comparing the early-stage wave motion and the commensurate pressure and effective stress.Further,the subsequent interface deformation is illustrated quantitatively via the time series of the interface positions and velocities of these three typical modes.
基金This research was supported by Science Challenge Project No.TZ2016001by the Strategic Priority Research Program of Chinese Academy of Sciences under Grant Nos.XDA25050400,XDA25010200,and XDB16000000by the National Natural Science Foundation of China(NSFC)under Grant Nos.U1530261,12175229,and 11621202,and by the Fundamental Research Funds for the Central Universities.Some of the numerical calculations in this paper were performed on the supercomputing system at the Supercomputing Center of the University of Science and Technology of China.
文摘The effects of electron nonlocal heat transport (NLHT) on the two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) upto the highly nonlinear phase are reported for the first time through numerical simulations with a multigroup diffusion model. It is found thatas well as its role in the linear stabilization of ARTI growth, NLHT can also mitigate ARTI bubble nonlinear growth after the first saturationto the classical terminal velocity, compared with what is predicted by the local Spitzer–Härm model. The key factor affecting the reductionin the linear growth rate is the enhancement of the ablation velocity Va by preheating. It is found that NLHT mitigates nonlinear bubblegrowth through a mechanism involving reduction of vorticity generation. NLHT enhances ablation near the spike tip and slows down thespike, leading to weaker vortex generation as the pump of bubble reacceleration in the nonlinear stage. NLHT more effectively reduces thenonlinear growth of shorter-wavelength ARTI modes seeded by the laser imprinting phase in direct-drive laser fusion.
基金supported by the National Natural Science Foundation of China (10832010)the Innovation Project of the Chinese Academy of Sciences (KJCX2-YW-L05)the United States National Science Foundation(CBET-0954381)
文摘This paper numerically and analytically studies the onset of instability of a flag in uniform flow. The three-dimensional （3D） simulation is performed by using an immersed-boundary method coupled with a nonlinear finite element method. The global stability, bistability and instability are identified in the 3D simulations. The Squire＇s theorem is extended to analyze the stability of the fluid-flag system with 3D initial perturbations. It is found that if a parallel flow around the flag admits an unstable 3D disturbance for a certain value of the flutter speed, then a two-dimensional （2D） disturbance at a lower flutter speed is also adnfitted. In addition, the growth rate of 2D disturbance is larger than that of the 3D disturbance.
基金supported by the National Natural Science Foundation of China (11132010 and 11072236)the 111 Project (B07033)
文摘Abstract A vortex ring impacting a three-dimensional circular cylinder is studied using large eddy simulation （LES） for a Reynolds number Re = 4 × 10^4 based on the initial translation speed and diameter of the vortex ring. We have investigated the evolution of vortical structures and identified three typical evolution phases. When the primary vortex closely approaches to the cylinder, a secondary vortex is generated and its segment parts move inward to the primary vortex ring. Then two large-scale loop-like vortices are formed to evolve in opposite directions. Thirdly, the two loop-like vortices collide with each other to form complicated small-scale vortical structures. Moreover, a series of hair-pin vortices are generated due to the stretching and deformation of the tertiary vortex. The trajectories of vortical structures and the relevant evolution speeds are analyzed. The total kinetic energy and enstrophy are investigated to reveal their properties relevant to the three evolution phases.
基金supported by the National Natural Science Foundation of China(11372304)the 111 Project(B07033)
文摘The propulsive performance of two- and three-dimensional （2D and 3D） flapping flexible plates in a fluid at rest is investigated by a finite element method for the plate motion and an immersed boundary-lattice Boltzmann method for the fluid flow. We consider a model that as the leading-edge of the plate takes a vertical oscillation, the entire plate moves freely due to the fluid-structure interaction. The effects of flexibility on the dynamics of the 2D and 3D flapping plates are investigated. It is found that a suitable flexibility is benefit for improving the propulsive performance. The results obtained in this study provide physical insight into the understanding of the dynamics of the flapping-based locomotion.
基金supported by the Innovation Project of the Chinese Academy of Sciences(Contract Nos.KJCX2-YW-L05 and CXJJ-237)the National Natural Science Foundation of China(Contract Nos.10832010 and 10772173)the Anhui Province Excellent Young Scholars Foundation(No.08040106826).
文摘The two-winged insect hovering flight is investigated numerically using the lattice Boltzmann method(LBM).A virtual model of two elliptic foils with flapping motion is used to study the aerodynamic performance of the insect hovering flight with and without the effect of ground surface.Systematic studies have been carried out by changing some parameters of the wing kinematics,including the stroke amplitude,attack angle,and the Reynolds number for the insect hovering flight without ground effect,as well as the distance between the flapping foils and the ground surface when the ground effect is considered.The influence of the wing kinematic parameters and the effect of the ground surface on the unsteady forces and vortical structures are analyzed.The unsteady forces acting on the flapping foils are verified to be closely associated with the time evolution of the vortex structures,foil translational and rotational accelerations,and interaction between the flapping foils and the existed vortical flow.Typical unsteady mechanisms of lift production are identified by examining the vortical structures around the flapping foils.The results obtained in this study provide some physical insight into the understanding of the aerodynamics and flow structures for the insect hovering flight.
基金the National Science Foundation of China(NSFC,No.11172297).
文摘Interaction of vortex rings with solid is an important research topic of hydrodynamic.In this study,a multiple-relaxation time(MRT)lattice Boltzmann method(LBM)is used to investigate the flow of a vortex ring impacting spheroidal particles.The MRT-LBM is validated through the cases of vortex ring impacting a flat wall.The vortex evolution due to particle size,the aspect ratio of a prolate particle,as well as Reynolds(Re)number are discussed in detail.When the vortex ring impacting a stationary sphere,the primary and secondary vortex rings wrap around each other,which is different from the situation of the vortex ring impacting a plate.For the vortex ring impacting with a prolate spheroid,the secondary vortex ring stretches mainly along the long axis of the ellipsoid particle.However,it is found that after the vortex wrapping stage,the primary vortex recovers along the short axis of the particle faster than that in the long axis,i.e.,the primary vortex ring stretches mainly along the short axis of the particle.That has never been address in the literature.
基金Supported by the"Corn Industry Technology System of Henan Province-Shangqiu Comprehensive Test Station"of the Special Fund for Modern Agricultural Technology System of Henan Province(Z2015-02-02)the"Research and Application of Full Mechanization and Supporting High-Yield Cultivation Technology of Summer Corn"of the Key Science and Technology Project of Shangqiu City(153026)~~
文摘To investigate the relationship of milk line position with grain weight and mechanized harvest of summer corn in Huang-Huai-Hai Region, 8 varieties （A, B, C, D, E, F, G, H） with large planting areas were selected to measure the grain filling rate, 100-grain weight, water content and milk line position, and the correlation was analyzed. Results showed that when the milk line position was 90%, the grain filling of all the 8 varieties finished and 100-grain weight reached the highest value, which was 43.02 g. The grain filling time was in positive correlation with 100-grain weight. However, when the milk line position completely disappeared, the 100-grain weight was reduced by 8.66% at most. There was no significant difference during the periods of grain weight rising, but in the periods of grain weight falling, the traits of D, E, H were significantly different with the other varieties, and water loss rate of C and A showed significant difference with the other six varieties. The water content of grain was negatively correlated with milk line position. When the milk line percentage was 90% , the grain water content was less than 30% . The key factor influencing the mechanized harvest of summer corn is harvesting time, rather than the varieties. Moreover, milk-line position of 90% is the best time for harvest; if the harvest is too late, the yield will be reduced with varying degrees.
基金This work was supported by the National Natural Science Foundation of China(Nos.11572312,11621202,11472268,and 91752110)by Science Challenge Project(No.TZ2016001).
文摘The investigation of supersonic channel flow with periodic oscillatory backpressures at the outlet of the channel was performed using large-eddy simulation for the inlet free-stream Mach number 4 and the Reynolds number approximately 5.2104 based on the height of the channel.Results have been validated carefully against our experimental data.Three typical backpressures are considered for constant backpressure and both periodic oscillatory backpressures with low and high frequency.The oscillatory backpressure can obviously influence the flow features occurring up to the middle region of the channel for the low frequency case and the downstream region for the high frequency case.Obvious differences of phase-averaged quantities at different phases are observed for the low frequency backpressure while the differences are relatively small for the high frequency backpressure.The spectral analysis reveals that the flow field experiences a periodic-like evolution of flow structures including shocks and vortices for the low frequency backpressure,resulting in the enhancement of turbulence fluctuations due to the complicated interaction of shocks and vortices.
