The fully nonlinear equations of gas dynamics are solved in the framework of a numerical approach in order to study the stability of the steady mode of Rayleigh-Bénard convection in compressible,viscous and heat-...The fully nonlinear equations of gas dynamics are solved in the framework of a numerical approach in order to study the stability of the steady mode of Rayleigh-Bénard convection in compressible,viscous and heat-conducting gases encapsulated in containers with no-slip boundaries and isothermal top and bottom walls.An initial linear temperature profile is assumed.A map of the possible convective modes is presented assuming the height of the region and the value of the temperature gradient as influential parameters.For a relatively small height,isobaric convection is found to take place,which is taken over by an adiabatic mode when the height exceeds the critical value,or by a super-adiabatic mode in case of a relatively high temperature gradient.In the adiabatic mode,convective flow develops due to adiabatic processes given a stable initial stratification.An analytic formula for the critical height of the region is derived taking into account and neglecting the dependence of the gas viscosity on the temperature.Moreover,an analytic formula is obtained for the upper boundary of the region of applicability of the Boussinesq approximation for incompressible gases.These models for compressible gases are relevant to practical situations such as the study of convective flows in spatially extended gas mixtures when dealing with safety issues related to hydrocarbons stored in gas stations.A dangerous situation arises when the tank is almost empty but some hydrocarbon is left at the bottom of the tank.In the presence of convective flows,the vaporized fuel is mixed with the oxidizer(air)forming a gas-vapor medium.However,if the volumetric concentration of fuel vapor(hydrocarbon)is in the interval between the lower and upper concentration limits of ignition,then the gas-vapor mixture becomes explosive and any accidental spark is sufficient to cause an emergency.展开更多
With rapid urbanization in recent years,severe air pollution has emerged as a major issue for many regions of China,especially in some metropolises.A persistent pollution case during 6 December 2016–8 January 2017 wa...With rapid urbanization in recent years,severe air pollution has emerged as a major issue for many regions of China,especially in some metropolises.A persistent pollution case during 6 December 2016–8 January 2017 was selected to investigate the relations between turbulent intermittency and frequent PM2.5(particulate matters with diameter less than 2.5μm)pollution events over the metropolitan region of Beijing,China.The accumulation of PM2.5 near the surface frequently occurred as a combined result of strong inversion layers,stagnant winds,high ambient humidity levels,and stable stratification during this case.Arbitrary-order Hilbert spectral analysis indicated that steep decreases in the PM2.5 concentration were simultaneous with the occurrence of intermittent turbulence and strong vertical mixing.A wind profiler observation revealed existence of low-level jets(LLJs)at the end of the polluted periods,suggesting that the upper-level turbulent mixing accompanied by the wind shear of LLJ was transported downward and enhanced the vertical mixing near the surface,which might have caused an abrupt reduction in PM2.5 and improvement in air conditions.展开更多
This paper investigates the consistency property of FC-normal logic program and presents an equivalent deciding condition whether a logic program P is an FC-normal program. The deciding condition describes the charact...This paper investigates the consistency property of FC-normal logic program and presents an equivalent deciding condition whether a logic program P is an FC-normal program. The deciding condition describes the characterizations of FC-normal program. By the Petri-net presentation of a logic program, the characterizations of stratification of FC-normal program are investigated. The stratification of FC-normal program motivates us to introduce a new kind of stratification, extended stratification, over logic program. It is shown that an extended (locally) stratified logic program is an FC-normal program. Thus, an extended (locally) stratified logic program has at least one stable model. Finally, we have presented algorithms about computation of consistency property and a few equivalent deciding methods of the finite FC-normal program.展开更多
Under the assumptions that the ground surface is an unbound inclined rough plane and that the wind direction changes less than 20 degrees below z/h<0.2, the universal function for the normalized shear over stably s...Under the assumptions that the ground surface is an unbound inclined rough plane and that the wind direction changes less than 20 degrees below z/h<0.2, the universal function for the normalized shear over stably stratified atmosphere at lower part of atmospheric boundary layer (ABL) is derived. The universal function is the sum of the contributions of that over the horizontal terrain and that over the terrain slope. Using the laws of geostrophic drag and heat transfer over a slightly inclined terrain, the effect of inclined terrain on the stable atmosphere at lower part of ABL is studied under the conditions of east Antarctic plateau. The result shows that the effect of inclined terrain is generally above 25%. The result of 38 cases of the Mizuho station in 1980 shows that the effect of the inclined terrain is in the range of 27%-49%.展开更多
Let X be a smooth projective curve of genus g 2 over an algebraically closed field k of characteristic p>0,and F:X→X(1)the relative Frobenius morphism.Let M s X(r,d)(resp.M ss X(r,d))be the moduli space of(resp.se...Let X be a smooth projective curve of genus g 2 over an algebraically closed field k of characteristic p>0,and F:X→X(1)the relative Frobenius morphism.Let M s X(r,d)(resp.M ss X(r,d))be the moduli space of(resp.semi-)stable vector bundles of rank r and degree d on X.We show that the set-theoretic map S ss Frob:M ss X(r,d)→M ss X(1)(rp,d+r(p-1)(g-1))induced by[E]→[F(E)]is a proper morphism.Moreover,the induced morphism S s Frob:M s X(r,d)→M s X(1)(rp,d+r(p-1)(g-1))is a closed immersion.As an application,we obtain that the locus of moduli space M s X(1)(p,d)consisting of stable vector bundles whose Frobenius pull backs have maximal Harder-Narasimhan polygons is isomorphic to the Jacobian variety Jac X of X.展开更多
文摘The fully nonlinear equations of gas dynamics are solved in the framework of a numerical approach in order to study the stability of the steady mode of Rayleigh-Bénard convection in compressible,viscous and heat-conducting gases encapsulated in containers with no-slip boundaries and isothermal top and bottom walls.An initial linear temperature profile is assumed.A map of the possible convective modes is presented assuming the height of the region and the value of the temperature gradient as influential parameters.For a relatively small height,isobaric convection is found to take place,which is taken over by an adiabatic mode when the height exceeds the critical value,or by a super-adiabatic mode in case of a relatively high temperature gradient.In the adiabatic mode,convective flow develops due to adiabatic processes given a stable initial stratification.An analytic formula for the critical height of the region is derived taking into account and neglecting the dependence of the gas viscosity on the temperature.Moreover,an analytic formula is obtained for the upper boundary of the region of applicability of the Boussinesq approximation for incompressible gases.These models for compressible gases are relevant to practical situations such as the study of convective flows in spatially extended gas mixtures when dealing with safety issues related to hydrocarbons stored in gas stations.A dangerous situation arises when the tank is almost empty but some hydrocarbon is left at the bottom of the tank.In the presence of convective flows,the vaporized fuel is mixed with the oxidizer(air)forming a gas-vapor medium.However,if the volumetric concentration of fuel vapor(hydrocarbon)is in the interval between the lower and upper concentration limits of ignition,then the gas-vapor mixture becomes explosive and any accidental spark is sufficient to cause an emergency.
基金Supported by the National Key Research and Development Program of China(2016YFC0203300)National Natural Science Foundation of China(91544216 and 41705003).
文摘With rapid urbanization in recent years,severe air pollution has emerged as a major issue for many regions of China,especially in some metropolises.A persistent pollution case during 6 December 2016–8 January 2017 was selected to investigate the relations between turbulent intermittency and frequent PM2.5(particulate matters with diameter less than 2.5μm)pollution events over the metropolitan region of Beijing,China.The accumulation of PM2.5 near the surface frequently occurred as a combined result of strong inversion layers,stagnant winds,high ambient humidity levels,and stable stratification during this case.Arbitrary-order Hilbert spectral analysis indicated that steep decreases in the PM2.5 concentration were simultaneous with the occurrence of intermittent turbulence and strong vertical mixing.A wind profiler observation revealed existence of low-level jets(LLJs)at the end of the polluted periods,suggesting that the upper-level turbulent mixing accompanied by the wind shear of LLJ was transported downward and enhanced the vertical mixing near the surface,which might have caused an abrupt reduction in PM2.5 and improvement in air conditions.
文摘This paper investigates the consistency property of FC-normal logic program and presents an equivalent deciding condition whether a logic program P is an FC-normal program. The deciding condition describes the characterizations of FC-normal program. By the Petri-net presentation of a logic program, the characterizations of stratification of FC-normal program are investigated. The stratification of FC-normal program motivates us to introduce a new kind of stratification, extended stratification, over logic program. It is shown that an extended (locally) stratified logic program is an FC-normal program. Thus, an extended (locally) stratified logic program has at least one stable model. Finally, we have presented algorithms about computation of consistency property and a few equivalent deciding methods of the finite FC-normal program.
基金Project supported by Meteorological Science Foundation of China,Meteorological Administration and National National Antarctic Key Project of China.
文摘Under the assumptions that the ground surface is an unbound inclined rough plane and that the wind direction changes less than 20 degrees below z/h<0.2, the universal function for the normalized shear over stably stratified atmosphere at lower part of atmospheric boundary layer (ABL) is derived. The universal function is the sum of the contributions of that over the horizontal terrain and that over the terrain slope. Using the laws of geostrophic drag and heat transfer over a slightly inclined terrain, the effect of inclined terrain on the stable atmosphere at lower part of ABL is studied under the conditions of east Antarctic plateau. The result shows that the effect of inclined terrain is generally above 25%. The result of 38 cases of the Mizuho station in 1980 shows that the effect of the inclined terrain is in the range of 27%-49%.
基金supported by National Natural Science Foundation of China (Grant No. 11271275)
文摘Let X be a smooth projective curve of genus g 2 over an algebraically closed field k of characteristic p>0,and F:X→X(1)the relative Frobenius morphism.Let M s X(r,d)(resp.M ss X(r,d))be the moduli space of(resp.semi-)stable vector bundles of rank r and degree d on X.We show that the set-theoretic map S ss Frob:M ss X(r,d)→M ss X(1)(rp,d+r(p-1)(g-1))induced by[E]→[F(E)]is a proper morphism.Moreover,the induced morphism S s Frob:M s X(r,d)→M s X(1)(rp,d+r(p-1)(g-1))is a closed immersion.As an application,we obtain that the locus of moduli space M s X(1)(p,d)consisting of stable vector bundles whose Frobenius pull backs have maximal Harder-Narasimhan polygons is isomorphic to the Jacobian variety Jac X of X.
