Plough pans （PPs）, common in the black-soil region of northeastern China, have significant effects on the root systems of crops. We conducted a field experiment to study the distribution of soybean roots under the i...Plough pans （PPs）, common in the black-soil region of northeastern China, have significant effects on the root systems of crops. We conducted a field experiment to study the distribution of soybean roots under the influence of PPs. The soybean roots showed compensatory growth above the PP, with higher root length and weight in soil with a PP compared to those without a PP. Roots were heavier and longer in the 15-75 cm soil layer without a PP than with a PP. Soil porosity was lower in the PP and the soil below the PP, which likely decreased the oxygen content of the soil and induced more growth of roots above the PP. The PP is also likely to decrease infiltration of rain-water and hinder the migration of nitrate downward, which in turn increased the density and length of soybean root hairs, which, hence, promoted growth.展开更多
2006—2014年夏季,使用PANs在线监测仪对北京大气中PANs浓度进行监测,期间NOx浓度以每年1.7 n L/L(约4%/a)的速率下降,PAN浓度以每年0.03 n L/L(约3%/a)的速率下降,O3浓度却以每年1.5 n L/L(约4%/a)的速率上升,表明这些年采取的NOx治理...2006—2014年夏季,使用PANs在线监测仪对北京大气中PANs浓度进行监测,期间NOx浓度以每年1.7 n L/L(约4%/a)的速率下降,PAN浓度以每年0.03 n L/L(约3%/a)的速率下降,O3浓度却以每年1.5 n L/L(约4%/a)的速率上升,表明这些年采取的NOx治理措施对控制PAN浓度有效,治理O3需要在控制NOx的同时加强对VOCs排放的控制。展开更多
Reed pans are a very uncommon type of endorheic wetland, and as such the amount of information available is very limited. Thus, they are being impacted on by various agricultural, livestock and other anthropogenic act...Reed pans are a very uncommon type of endorheic wetland, and as such the amount of information available is very limited. Thus, they are being impacted on by various agricultural, livestock and other anthropogenic activities. The objectives of this study were to determine the spatial and temporal variations of macroinvertebrate community structures in reed pans and the environmental factors (i.e., water quality) responsible for the maintenance of these structures. Reed pans were studied over four different seasons, during which time subsurface water, sediment and macroinvertebrate samples were collected and analyzed. The reed pans studied showed that the macroinvertebrates were able to reflect various changes in reed pans with regard to seasonal variability and anthropogenic impacts on water quality. These anthropogenic impacts caused the disappearance of sensitive macroinvertebrate taxa and the increase of tolerant macroinvertebrate taxa.展开更多
In the present paper,the unsteady cavitating turbulent flow over the twisted NACA66 hydrofoil is investigated based on an modified shear stress transfer k-ωpartially averaged Navier-Stokes(MSST PANS)model,i.e.,new MS...In the present paper,the unsteady cavitating turbulent flow over the twisted NACA66 hydrofoil is investigated based on an modified shear stress transfer k-ωpartially averaged Navier-Stokes(MSST PANS)model,i.e.,new MSST PANS(NMSST PANS)model,where the production term of kinetic energy in the turbulence model is modified with helicity.Compared with the experimental data,cavitation evolution and its characteristic frequency are satisfactorily predicted by the proposed NMSST PANS model.It is revealed that the interaction among the main flow,the reentrant jets,and sheet cavitation causes the formation of the primary shedding cavity near the mid-span and the secondary shedding cavity at each side of the twisted hydrofoil,and further induces the remarkable pressure gradient around shedding cavities.Along with the development of the primary and the secondary shedding cavities,the great pressure gradient associated with large cavity volume variation promotes the vortical flow generation and the spatial deformation of vortex structure during cavitation evolution,and results in the primary and the secondary U-type vortices.Further,dynamic mode decomposition(DMD)analysis is utilized to confirm the interaction among the main flow,the main reentrant jet and two side reentrant jets,and cavitation.These results indicate that the proposed NMSST PANS model is suitable to simulate the complicated cavitating turbulent flow for various engineering applications.展开更多
A novel Omega(Ω)-driven dynamic partially-averaged Navier-Stokes(PANS)model is proposed in this paper.The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity rat...A novel Omega(Ω)-driven dynamic partially-averaged Navier-Stokes(PANS)model is proposed in this paper.The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity ratio(the ratio of the rigid vorticity to the total vorticity),the key parameter of the Ω vortex identification method.Three classical flow cases with rotation and curvature are used to test the model.The results show that the turbulent viscosity is effectively adjusted by the new dynamic fk and the LES-like mode is activated,which can help the revelation of more turbulence information and improve the prediction accuracy.The new PANS model does not contain any explicit dependency on the grid size and enjoys good adaptability to the flow fields,and can be used for efficient engineering computations of the turbulent flows in the hydraulic machinery.展开更多
In most of partially averaged Navier-Stokes（PANS）methods,the Reynolds stress is solved by a linear hypothesis isotropic model.They could not capture all kinds of vortexes in tubomachineries.In this paper,a PANS mode...In most of partially averaged Navier-Stokes（PANS）methods,the Reynolds stress is solved by a linear hypothesis isotropic model.They could not capture all kinds of vortexes in tubomachineries.In this paper,a PANS model is modified from the RNG k-？turbulence model and is used to investigate the influence of the nonlinear shear stress on the simulation of the high pressure gradient flows and the large curvature flows.Comparisons are made between the result obtained by using the PANS model modified from the RNG k-？model and that obtained by using the nonlinear PANS methods.The flow past a curved rectangular duct is calculated by using the PANS methods.The obtained nonlinear shear stress agrees well with the experimental results,especially in the high pressure gradient region.The calculation results show that the nonlinear PANS methods are more reliable than the linear PANS methods for the high pressure gradient flows,the large curvature flows,and they can be used to capture complex vortexes in a turbomachinary.展开更多
基金supported by the National Natural Science Foundation of China (51239009,41301288,51479032 and 51409213)the Postdoctoral Science Foundation of China (2014M561317)+2 种基金the Heilongjiang Postdoctoral Financial Assistance,China (LBH-Z13023)the Heilongjiang Provincial Department of Education Science and Technology Research Project,China (12541039)the Young Talents Project of Northeast Agricultural University,China (14QC48)
文摘Plough pans （PPs）, common in the black-soil region of northeastern China, have significant effects on the root systems of crops. We conducted a field experiment to study the distribution of soybean roots under the influence of PPs. The soybean roots showed compensatory growth above the PP, with higher root length and weight in soil with a PP compared to those without a PP. Roots were heavier and longer in the 15-75 cm soil layer without a PP than with a PP. Soil porosity was lower in the PP and the soil below the PP, which likely decreased the oxygen content of the soil and induced more growth of roots above the PP. The PP is also likely to decrease infiltration of rain-water and hinder the migration of nitrate downward, which in turn increased the density and length of soybean root hairs, which, hence, promoted growth.
文摘2006—2014年夏季,使用PANs在线监测仪对北京大气中PANs浓度进行监测,期间NOx浓度以每年1.7 n L/L(约4%/a)的速率下降,PAN浓度以每年0.03 n L/L(约3%/a)的速率下降,O3浓度却以每年1.5 n L/L(约4%/a)的速率上升,表明这些年采取的NOx治理措施对控制PAN浓度有效,治理O3需要在控制NOx的同时加强对VOCs排放的控制。
文摘Reed pans are a very uncommon type of endorheic wetland, and as such the amount of information available is very limited. Thus, they are being impacted on by various agricultural, livestock and other anthropogenic activities. The objectives of this study were to determine the spatial and temporal variations of macroinvertebrate community structures in reed pans and the environmental factors (i.e., water quality) responsible for the maintenance of these structures. Reed pans were studied over four different seasons, during which time subsurface water, sediment and macroinvertebrate samples were collected and analyzed. The reed pans studied showed that the macroinvertebrates were able to reflect various changes in reed pans with regard to seasonal variability and anthropogenic impacts on water quality. These anthropogenic impacts caused the disappearance of sensitive macroinvertebrate taxa and the increase of tolerant macroinvertebrate taxa.
基金Project supported by the National Natural Science Foundation of China (Grant No.52336001).
文摘In the present paper,the unsteady cavitating turbulent flow over the twisted NACA66 hydrofoil is investigated based on an modified shear stress transfer k-ωpartially averaged Navier-Stokes(MSST PANS)model,i.e.,new MSST PANS(NMSST PANS)model,where the production term of kinetic energy in the turbulence model is modified with helicity.Compared with the experimental data,cavitation evolution and its characteristic frequency are satisfactorily predicted by the proposed NMSST PANS model.It is revealed that the interaction among the main flow,the reentrant jets,and sheet cavitation causes the formation of the primary shedding cavity near the mid-span and the secondary shedding cavity at each side of the twisted hydrofoil,and further induces the remarkable pressure gradient around shedding cavities.Along with the development of the primary and the secondary shedding cavities,the great pressure gradient associated with large cavity volume variation promotes the vortical flow generation and the spatial deformation of vortex structure during cavitation evolution,and results in the primary and the secondary U-type vortices.Further,dynamic mode decomposition(DMD)analysis is utilized to confirm the interaction among the main flow,the main reentrant jet and two side reentrant jets,and cavitation.These results indicate that the proposed NMSST PANS model is suitable to simulate the complicated cavitating turbulent flow for various engineering applications.
基金Supported by the National Natural Science simulating the unsteady eddying motions⑴.Foundation of China(Grant Nos.51836010,51779258 and 51839001)the National Key Research and Development Program of China(Grant No.2018YFB0606103)the Nature Science Foundation of Beijing(Gmat No.3182018).
文摘A novel Omega(Ω)-driven dynamic partially-averaged Navier-Stokes(PANS)model is proposed in this paper.The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity ratio(the ratio of the rigid vorticity to the total vorticity),the key parameter of the Ω vortex identification method.Three classical flow cases with rotation and curvature are used to test the model.The results show that the turbulent viscosity is effectively adjusted by the new dynamic fk and the LES-like mode is activated,which can help the revelation of more turbulence information and improve the prediction accuracy.The new PANS model does not contain any explicit dependency on the grid size and enjoys good adaptability to the flow fields,and can be used for efficient engineering computations of the turbulent flows in the hydraulic machinery.
基金supported by the National Natural Science Foundation of China(Grant Nos.51406010,51479166)
文摘In most of partially averaged Navier-Stokes（PANS）methods,the Reynolds stress is solved by a linear hypothesis isotropic model.They could not capture all kinds of vortexes in tubomachineries.In this paper,a PANS model is modified from the RNG k-？turbulence model and is used to investigate the influence of the nonlinear shear stress on the simulation of the high pressure gradient flows and the large curvature flows.Comparisons are made between the result obtained by using the PANS model modified from the RNG k-？model and that obtained by using the nonlinear PANS methods.The flow past a curved rectangular duct is calculated by using the PANS methods.The obtained nonlinear shear stress agrees well with the experimental results,especially in the high pressure gradient region.The calculation results show that the nonlinear PANS methods are more reliable than the linear PANS methods for the high pressure gradient flows,the large curvature flows,and they can be used to capture complex vortexes in a turbomachinary.
