The study aims to explore the damage characteristics and protection technologies of liquid-filled structures under high-speed projectile impact.A series of penetration impact experiments were conducted by focusing on ...The study aims to explore the damage characteristics and protection technologies of liquid-filled structures under high-speed projectile impact.A series of penetration impact experiments were conducted by focusing on different air layer configurations.By using high-speed camera and dynamic measurement systems,the effects of air layers on the projectile penetration,pressure wave propagation,cavitation evolution,and structural dynamic responses were analyzed.The results showed that the rarefaction wave reflected from the air-liquid interface significantly reduced the peak and specific impulse of the initial pressure wave,thereby diminishing the impact load on the structure.Additionally,the compressibility of air layers also attenuated the cavitation extrusion load.Both front and rear plates exhibited superimposed deformation modes,i.e.,local deformation or petal fracture with global deformation.Air layers effectively mitigated global deformation.However,when the air layer was positioned on the projectile's trajectory,it split the water-entry process and velocity attenuation of the projectile into two relatively independent phases.And the secondary water entry pressure wave caused more severe local deformation and petal fractures on the rear plate.展开更多
We describe a few mathematical tools which allow to investigate whether air-water interfaces exist(under prescribed conditions)and are mechanically stable and temporally persistent.In terms of physics,air-water interf...We describe a few mathematical tools which allow to investigate whether air-water interfaces exist(under prescribed conditions)and are mechanically stable and temporally persistent.In terms of physics,air-water interfaces are governed by the Young-Laplace equation.Mathematically they are surfaces of constant mean curvature which represent solutions of a nonlinear elliptic partial differential equation.Although explicit solutions of this equation can be obtained only in very special cases,it is -under moderately special circumstances-possible to establish the existence of a solution without actually solving the differential equation.We also derive criteria for mechanical stability and temporal persistence of an air layer.Furthermore we calculate the lifetime of a non-persistent air layer.Finally,we apply these tools to two examples which exhibit the symmetries of 2D lattices.These examples can be viewed as abstractions of the biological model represented by the aquatic fern Salvinia.展开更多
Atmospheric Infrared Sounder （AIRS） data show that the Saharan air layer （SAL） is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to ...Atmospheric Infrared Sounder （AIRS） data show that the Saharan air layer （SAL） is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 （TD14） were accompanied with outbreaks of SAL air during the period 1-12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1-12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic ？ows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.展开更多
Air lubrication by means of a bottom cavity is a promising method for ship drag reduction. The characteristics of the bottom cavity are sensitive to the flow field around the ship hull and the effect of drag reduction...Air lubrication by means of a bottom cavity is a promising method for ship drag reduction. The characteristics of the bottom cavity are sensitive to the flow field around the ship hull and the effect of drag reduction, especially the depth of the bottom cavity. In this study, a ship model experiment of a bulk carrier is conducted in a towing tank using the method of air layer drag reduction (ALDR) with different bottom cavity depths. The shape of the air layer is observed, and the changes in resistance are measured. The model experiments produce results of approximately 20% for the total drag reduction at the ship design speed for a 25-mm cavity continuously supplied with air at Cq = 0.224 in calm water, and the air layer covers the whole cavity when the air flow rate is suitable. In a regular head wave, the air layer is easily broken and reduces the drag reduction rate in short waves, particularly when λ/Lw1 is close to one;however, it still has a good drag reduction effect in the long waves.展开更多
Ship resistance issues are related to fuel economy,speed,and cost efficiency.Air lubrication is a promising technique for lowering hull frictional resistance as it is supposed to modify the energy in the turbulent bou...Ship resistance issues are related to fuel economy,speed,and cost efficiency.Air lubrication is a promising technique for lowering hull frictional resistance as it is supposed to modify the energy in the turbulent boundary layer and thereby reduce hull friction.In this paper,the objective is to identify the optimum type of air lubrication using microbubble drag reduction(MBDR)and air layer drag reduction(ALDR)techniques to reduce the resistance of a 56-m Indonesian self-propelled barge(SPB).A model with the following dimensions was constructed:length L=2000 mm,breadth B=521.60 mm,and draft T=52.50 mm.The ship model was towed using standard towing tank experimental parameters.The speed was varied over the Froude number range 0.11–0.31.The air layer flow rate was varied at 80,85,and 90 standard liters per minute(SLPM)and the microbubble injection coefficient over the range 0.20–0.60.The results show that the ship model using the air layer had the highest drag reduction up to a maximum of 90%.Based on the characteristics of the SPB,which operates at low speed,the optimum air lubrication type to reduce resistance in this instance is ALDR.展开更多
This study investigates the influence of air layer confinedunderwear on the heat and mass transfer of body-fabric-environment system. For a simple,a single-layer fabric in cotton is modeled.The study focuses on under-...This study investigates the influence of air layer confinedunderwear on the heat and mass transfer of body-fabric-environment system. For a simple,a single-layer fabric in cotton is modeled.The study focuses on under-standing the combined effect of convection,radiation and evaporation in thermal comfort condition.Based on the-oretical analysis,both sensible heat flux and latent heat flux have been mathematically modeled and quantified for different thickness of underclothing air layer.Thenumerical results have been compared with展开更多
The purpose of this paper is to study the energy efficiency of a local living space exposed to solar radiation in the subtropical climate of Casablanca. The study was mainly focused on the contribution of a phase chan...The purpose of this paper is to study the energy efficiency of a local living space exposed to solar radiation in the subtropical climate of Casablanca. The study was mainly focused on the contribution of a phase change material (PCM), inserted into a 7-cm thick air layer of a double brick wall, in two different locations. We note that the experimental study was conducted using two real-scale test cavities, located in the Faculty of Science Ain Chock-Casablanca. Two PCM mounting methods were used for the south and west walls, in order to test its energy efficiency as a storage and retrieval means of the solar flux coming from the outside. In the case of the southern wall, the PCM is put directly on the internal side of the outside part of the double wall (Case 1). For the west wall, the PCM is placed 1.2 cm away from the internal side of the outer part of the double wall (Case 2). The first result shows that the PCM placed to the wall allows storing the solar heat during the day and releasing it to the outside of the building at night. While in the second case, the PCM keeps the heat stored day and night.展开更多
The air layer drag reduction(ALDR)of an axisymmetric body in oscillatory motions is investigated in this paper with open source toolbox OpenFOAM.The unsteady Reynolds-averaged Navier-Stokes(URANS)equations are used to...The air layer drag reduction(ALDR)of an axisymmetric body in oscillatory motions is investigated in this paper with open source toolbox OpenFOAM.The unsteady Reynolds-averaged Navier-Stokes(URANS)equations are used to determine the viscous flow and the volume of fluid(VOF)model is adopted to capture the interface of the air-water two-phase flow.The k-e turbulence model is adopted to simulate the turbulence.The dynamic mesh technique is applied to model the movement of the axisymmetric body.Firstly,the ALDR results are validated by the experimental data.Then,the effects of the movements of the body on the drag reduction during the ALDR state are investigated.Two representative kinds of movements are considered,namely,the pitch and the heave.The numerical results show that the drag reduction varies during the movements and the average drag reduction rates will be reduced.The variation of the drag reduction is related to the morphological change of the air layer.The heave motion is more likely to reduce the effects of the ALDR than the pitch motion.For both oscillatory motions,the large motion amplitude and the low motion period are not conductive to improving the effects of the ALDR.The effects of the oscillatory motion on the ALDR are more sensitive at high water speeds than at low water speeds.Besides,increasing the air flow ratio can be considered as one way to improve the effects of the ALDR.展开更多
Air-layer drag reduction (ALDR) technology for ship energy saving is getting more and more attention in recent years because of the outstanding drag reduction effect. In order to promote practical application, it is n...Air-layer drag reduction (ALDR) technology for ship energy saving is getting more and more attention in recent years because of the outstanding drag reduction effect. In order to promote practical application, it is necessary to fully understand the two phase flow characteristics of the air layer. Recent experimental studies have shown that the surface of the air layer presents wave pattern, which has an important influence on its damage risk. However, it is difficult to measure the wave pattern quantificationally due to the interference of equipment. The main goal of the present paper is to investigate the wave pattern characteristic of air layer in cavity using numerical simulation method. On this basis, the effect of flow and geometric influence factors are discussed to understand the key control conditions. A computational fluid dynamics (CFD) numerical method based on Reynolds averaged Navier-Stokes (RANS) equations and volume of fluid (VOF) interface capturing method is established, and has been successfully applied in the simulation of air layer wave pattern. Both 2-D and 3-D simulations are carried out, aiming at analyzing air-water interface flow and vortex flow directly. Based on the simulation results, several important conclusions about the mechanism of air layer wave pattern can be obtained. Firstly, it is found to be an inherent characteristic that the wave height of the upstream air layer is higher than that of the downstream. The extremely high wave peak is easy to contact with the flat plate, leading to the breakup of air layer and a “central blank area” phenomenon. With the help of flow analysis, it is found that this characteristic is mainly caused by the strong counterclockwise vortex behind the bow wedge block. Secondly, the air layer stability is reduced with the increase of water flow velocity by affecting the wave height. There is a saturation point of air flow rate to reach maximum thickness of air layer. Thirdly, cavity configuration has obvious influence on air layer stability by influencing vortex flow field. The increase of cavity depth and width can aggravate the unsteady and nonlinear characteristics of air layer. Finally, comprehensive design criteria are concluded from the view of geometrical configuration and flow conditions. A cavity with the moderate depth and width can avoid the upstream damage of air layer. Longitudinal position of air nozzles should be set within the low pressure zone behind the wedge block for stable air layer formation.展开更多
Based on meteorological sounding data in 2011 -2014 and air pollution data in 2013 and 2014 from an environmental monitoring station in Xingtai City, the characteristics of inversion layer and air pollution and their ...Based on meteorological sounding data in 2011 -2014 and air pollution data in 2013 and 2014 from an environmental monitoring station in Xingtai City, the characteristics of inversion layer and air pollution and their correlation in Xingtai City were discussed. The results show that term perature inversion was very serious at 07：00 in Xingtai City, and days with temperature inversion accounted for above 90%. There were obvious seasonal variations in air pollution in Xingtai City, and air pollution was the most serious in winter but the slightest in summer. The primary air pollu- tants in Xingtai City are PM10 and PM2.5, and the primary air pollutants and AQI had close correlation with parameters of inversion layer. Stable ground inversion layer could hinder air convection and air capacity, so that air pollution become more serious with the discharge of pollutants. Inversion layer nearly appearing all the year around is an important meteorological reason for serious air pollution in Xingtai City.展开更多
Shea tree (Vitellaria paradoxa Gaertn. Family;Sapotaceae) indigenous to the Sudano-sahelian zone of Africa has great economic and ecological potential and attributes. Commercial cultivation of the tree is however, ham...Shea tree (Vitellaria paradoxa Gaertn. Family;Sapotaceae) indigenous to the Sudano-sahelian zone of Africa has great economic and ecological potential and attributes. Commercial cultivation of the tree is however, hampered by the poor rooting success of its planting material with adverse consequences on field establishment and total fruit yield. A 3 × 2 factorial experiment arranged in a randomized complete block design was carried out in 2012 at Bole in the Interior Savanna agro-ecological zone. The objective was to assess the rooting success of Shea shoots by the air-layering technique using two media types (palm fibre and Sphagnum moss) and three IBA hormone concentrations (0, 5000 and 10,000 ppm) under contrasting climatic (wet and dry) conditions. Layered shoots which were sprayed with 10,000 ppm IBA and wrapped with Sphagnum moss gave significantly (P < 0.05) higher rooting success in terms of more roots per cutting (73.3%) and longer roots per cutting (9.0 cm) than palm fibre at both 5000 ppm (30.0%;3.7 cm) and 10,000 ppm (46.7%;7.9 cm) concentrations. Higher rooting success was significantly achieved in the wet season than in the dry season. Sphagnum moss treated with 10,000 ppm IBA facilitated the translocation of higher levels of sugar and total free phenol (TFP) to the base of the layered shoots which resulted in significantly (P < 0.05) higher number of roots and better roots protection from fungal infection than the other treatments. Rooting of layered shoots was affected to a greater extent by low temperatures (Rooting = 836 – 34.2 Temp (low);R2 = 82%, p < 0.001) than by high temperatures (Rooting = 5250 – 175.0 Temp (high);R2 = 64.5%, p < 0.009). Rooting of layered shoots was also significantly and negatively affected by the spread of the canopy of the selected tree such that closed canopy trees resulted in higher and better rooting that open canopy trees (Rooting = 113.87-23.697(Canopy spread);R2 = 89%, p = 0.002;n = 9). Furthermore, significant and positive correlations were found between IBA concentration level and simple sugars (r = 0.99;P < 0.0001;n = 9) and also between IBA concentration level and total free phenols (r = 0.98;P < 0.0002;n = 9). The study concluded that to achieve high rooting success in the propagation of Shea nut tree by the air-layering technique, a combination of Sphagnum moss treated with 10,000 ppm of IBA hormone should be used and the whole set up carried out in the wet season.展开更多
The samples cut from U75V 60 kg/m heavy rail are heated to 900 ℃ in resistance furnace and then put into air spraying channel to be cooled for 80 s, and change air pressure from 0.16 MPa to 0.33 MPa, and observe the ...The samples cut from U75V 60 kg/m heavy rail are heated to 900 ℃ in resistance furnace and then put into air spraying channel to be cooled for 80 s, and change air pressure from 0.16 MPa to 0.33 MPa, and observe the effect of air pressure on hardened layer. The thickness and hardness of hardened layer increases with the increase of air pressure, and the thickness is more than 24 mm at the center and top fillets of rail head, and more than 15 mm at the blow fillets of rail head when air pressure is more than 0.26 MPa. During the tempering after heat treatment, tempering temperature of rail head is more than 570 ℃ when air pressure is separately 0.16 MPa, 0.20 MPa and 0.23 MPa, which is higher than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s according to CCT curve of U75V steel. When air pressure is separately 0.26, 0.30 and 0.33 MPa, the tempering temperature is 529 ℃ lower than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s. Under this condition, pearlite transformation is finished totally, so in order to reduce air consumption and control the cost, proper air pressure for U75V 60 kg/m heavy rail heat treating should be 0.26 MPa; the cooling rate increases with the increase of air pressure, and the average cooling rate on the surface of rail head is more than 3.21 ℃/s when air pressure is more than 0.26 MPa, and the largest cooling rate occurs at the top fillets of rail head.展开更多
In southern China, cold air is a common weather process during the winter season; it can cause strong wind, sharp temperature decreases, and even the snow or freezing rain events. However, the features of the atmosphe...In southern China, cold air is a common weather process during the winter season; it can cause strong wind, sharp temperature decreases, and even the snow or freezing rain events. However, the features of the atmospheric boundary layer during cold air passage are not clearly understood due to the lack of comprehensive observation data, especially regarding turbulence. In this study, four-layer gradient meteorological observation data and one-layer, 10-Hz ultrasonic anemometer-thermometer monitoring data from the northern side of Poyang Lake were employed to study the main features of the surface boundary layer during a strong cold-air passage over southern China. The results show that, with the passage of a cold air front, the wind speed exhibits low-frequency variations and that the wind systematically descends. During the strong wind period, the wind speed increases with height in the surface layer. Regular gust packets are superimposed on the basic strong wind flow. Before the passage of cold air, the wind gusts exhibit a coherent structure. The wind and turbulent momentum fluxes are small, although the gusty wind momentum flux is slightly larger than the turbulent momentum flux. However, during the invasion of cold air, both the gusty wind and turbulent momentum fluxes increase rapidly with wind speed, and the turbulent momentum flux is larger than the gusty wind momentum flux during the strong wind period. After the cold air invasion, this structure almost disappears.展开更多
The effects of air dissociation on ?at-plate hypersonic boundary-layer ?ow instability and transition prediction are studied. The air dissociation reactions are assumed to be in the chemical equilibrium. Based on the ...The effects of air dissociation on ?at-plate hypersonic boundary-layer ?ow instability and transition prediction are studied. The air dissociation reactions are assumed to be in the chemical equilibrium. Based on the ?at-plate boundary layer, the ?ow stability is analyzed for the Mach numbers from 8 to 15. The results reveal that the consideration of air dissociation leads to a decrease in the unstable region of the ?rst-mode wave and an increase in the maximum growth rate of the second mode. High frequencies appear earlier in the third mode than in the perfect gas model, and the unstable region moves to a lower frequency region. When the Mach number increases, the second-mode wave dominates the transition process, and the third-mode wave has little effect on the transition. Moreover, when the Mach number increases from 8 to 12, the N-factor envelope becomes higher, and the transition is promoted. However, when the Mach number exceeds 12, the N-factor envelope becomes lower, and the transition is delayed. The N-factor envelope decreases gradually with the increase in the altitude or Mach number.展开更多
The diurnal variations of gaseous pollutants and the dynamical and thermodynamic structures of the atmospheric boundary layer (ABL) in the Beijing area from January to March 2001 are analyzed in this study using data ...The diurnal variations of gaseous pollutants and the dynamical and thermodynamic structures of the atmospheric boundary layer (ABL) in the Beijing area from January to March 2001 are analyzed in this study using data from the Beijing City Air Pollution Observation Field Experiment (BECAPEX). A heavy pollution day (22 February) and a good air quality day (24 February) are selected and individually analyzed and compared to reveal the relationships between gaseous pollutants and the diurnal variations of the ABL. The results show that gaseous pollutant concentrations exhibit a double-peak-double-valley-type diurnal variation and have similar trends but with different magnitudes at different sites in Beijing. The diurnal variation of the gaseous pollutant concentrations is closely related to (with a 1-2 hour delay of) changes in the atmospheric stability and the mean kinetic energy in the ABL.展开更多
The thermal internal boundary layer（TIBL） is associated with coastal pollution dispersion,which can result in high concentrations of air pollutants near the surface of the Earth. In this study, boundary layer height...The thermal internal boundary layer（TIBL） is associated with coastal pollution dispersion,which can result in high concentrations of air pollutants near the surface of the Earth. In this study, boundary layer height data which were obtained using a ceilometer were used to assess the effect of the TIBL on atmospheric pollutants in Qinhuangdao, a coastal city in North China.A TIBL formed on 33% of summer days. When a TIBL formed, the sunshine duration was 2.4 hr longer, the wind speed was higher, the wind direction reflected a typical sea breeze, and the boundary layer height was lower from 9：00 LT to 20：00 LT compared to days without a TIBL. If no TIBL formed, the average concentrations of PM2.5 and PM10 decreased with increasing boundary layer height. However, when a TIBL was observed, the average concentrations of PM2.5 and PM10 increased with increasing boundary layer height. Because the air from the sea is clean, PM2.5 and PM10 concentrations reached minimums in the daytime at 16：00 LT. After16：00 LT, the PM2.5 and PM10 concentrations increased rapidly on days when a TIBL formed,which indicated that the TIBL leads to the rapid accumulation of atmospheric pollutants in the evening. Therefore, the maximum concentrations of particulate matters were larger when a TIBL formed compared to when no TIBL was present during the night. These results indicate that it is suitable for outdoor activities in the daytime on days with a TIBL in coastal cities.展开更多
To increase airspace capacity, alleviate flight delay,and improve network robustness, an optimization method of multi-layer air transportation networks is put forward based on Laplacian energy maximization. The effect...To increase airspace capacity, alleviate flight delay,and improve network robustness, an optimization method of multi-layer air transportation networks is put forward based on Laplacian energy maximization. The effectiveness of taking Laplacian energy as a measure of network robustness is validated through numerical experiments. The flight routes addition optimization model is proposed with the principle of maximizing Laplacian energy. Three methods including the depth-first search( DFS) algorithm, greedy algorithm and Monte-Carlo tree search( MCTS) algorithm are applied to solve the proposed problem. The trade-off between system performance and computational efficiency is compared through simulation experiments. Finally, a case study on Chinese airport network( CAN) is conducted using the proposed model. Through encapsulating it into multi-layer infrastructure via k-core decomposition algorithm, Laplacian energy maximization for the sub-networks is discussed which can provide a useful tool for the decision-makers to optimize the robustness of the air transportation network on different scales.展开更多
在高内压循环作用下地下储气库衬砌易产生裂缝与渗透通道,密封层受力与裂缝控制问题尤为关键。基于有限-离散元方法(finite-discrete element method,简称FDEM),建立了连续-非连续的围岩-衬砌-密封层整体模型,系统研究了平钢板与波拱两...在高内压循环作用下地下储气库衬砌易产生裂缝与渗透通道,密封层受力与裂缝控制问题尤为关键。基于有限-离散元方法(finite-discrete element method,简称FDEM),建立了连续-非连续的围岩-衬砌-密封层整体模型,系统研究了平钢板与波拱两类钢衬及预设缝设计参数对衬砌裂缝演化与密封性能的影响。结果表明,平钢板型密封层应力水平和裂缝数量较高,整体性差,预设缝改善衬砌开裂与密封结构受力作用有限;波拱型结构能显著降低峰值应力并改变裂缝分布,使得裂缝多集中在波拱底部,整体裂缝数量减少,但波拱处的裂缝开裂宽度更大。采用波拱+预设缝相结合的方案可进一步均匀化开裂,减少密封钢衬应力。随着波拱和预设缝数量增加,应力分布趋于均匀,裂缝扩展受控,但最大裂缝宽度呈先减后增趋势,钢衬应力模式由拉应力向弯-剪应力转变。当缝设于拱底时,裂缝可沿预设路径均匀扩展,并可结合防排水措施降低渗透风险。总体而言,波拱+预设缝复合设计在引导裂缝、释放应变和提升密封渗透性能方面具有显著优势,为储气库密封-衬砌协同优化设计提供参考。展开更多
基金the financial support provided by National Natural Science Foundation of China(Grant Nos.52271338,52371342 and 51979277).
文摘The study aims to explore the damage characteristics and protection technologies of liquid-filled structures under high-speed projectile impact.A series of penetration impact experiments were conducted by focusing on different air layer configurations.By using high-speed camera and dynamic measurement systems,the effects of air layers on the projectile penetration,pressure wave propagation,cavitation evolution,and structural dynamic responses were analyzed.The results showed that the rarefaction wave reflected from the air-liquid interface significantly reduced the peak and specific impulse of the initial pressure wave,thereby diminishing the impact load on the structure.Additionally,the compressibility of air layers also attenuated the cavitation extrusion load.Both front and rear plates exhibited superimposed deformation modes,i.e.,local deformation or petal fracture with global deformation.Air layers effectively mitigated global deformation.However,when the air layer was positioned on the projectile's trajectory,it split the water-entry process and velocity attenuation of the projectile into two relatively independent phases.And the secondary water entry pressure wave caused more severe local deformation and petal fractures on the rear plate.
基金funded by grants from the Deutsche Forschungsgemeinschaft,the Bundesministerium für Bildung und Forschung and the Landesgraduiertenfrderungsgesetz des Landes Baden-Württemberg
文摘We describe a few mathematical tools which allow to investigate whether air-water interfaces exist(under prescribed conditions)and are mechanically stable and temporally persistent.In terms of physics,air-water interfaces are governed by the Young-Laplace equation.Mathematically they are surfaces of constant mean curvature which represent solutions of a nonlinear elliptic partial differential equation.Although explicit solutions of this equation can be obtained only in very special cases,it is -under moderately special circumstances-possible to establish the existence of a solution without actually solving the differential equation.We also derive criteria for mechanical stability and temporal persistence of an air layer.Furthermore we calculate the lifetime of a non-persistent air layer.Finally,we apply these tools to two examples which exhibit the symmetries of 2D lattices.These examples can be viewed as abstractions of the biological model represented by the aquatic fern Salvinia.
基金supported by the Ty-phoon Research Project (2009CB421503) of the National Basic Research Program (the 973 Program) of Chinathe National Science Foundation of China (NSFC grant no 408750387)the Social Commonweal Research Program of the Ministry of Science and Technology of the People’s Republic of China (GYHY200806009)
文摘Atmospheric Infrared Sounder （AIRS） data show that the Saharan air layer （SAL） is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 （TD14） were accompanied with outbreaks of SAL air during the period 1-12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1-12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic ？ows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.
基金supported by the Ministry of Industry and High Technology Marine Scientific Research Projects(Grant No.2011530)the High Performance Marine Technology Key Laboratory of the Ministry of Education Open Foundation(Grant No.2013033102)
文摘Air lubrication by means of a bottom cavity is a promising method for ship drag reduction. The characteristics of the bottom cavity are sensitive to the flow field around the ship hull and the effect of drag reduction, especially the depth of the bottom cavity. In this study, a ship model experiment of a bulk carrier is conducted in a towing tank using the method of air layer drag reduction (ALDR) with different bottom cavity depths. The shape of the air layer is observed, and the changes in resistance are measured. The model experiments produce results of approximately 20% for the total drag reduction at the ship design speed for a 25-mm cavity continuously supplied with air at Cq = 0.224 in calm water, and the air layer covers the whole cavity when the air flow rate is suitable. In a regular head wave, the air layer is easily broken and reduces the drag reduction rate in short waves, particularly when λ/Lw1 is close to one;however, it still has a good drag reduction effect in the long waves.
文摘Ship resistance issues are related to fuel economy,speed,and cost efficiency.Air lubrication is a promising technique for lowering hull frictional resistance as it is supposed to modify the energy in the turbulent boundary layer and thereby reduce hull friction.In this paper,the objective is to identify the optimum type of air lubrication using microbubble drag reduction(MBDR)and air layer drag reduction(ALDR)techniques to reduce the resistance of a 56-m Indonesian self-propelled barge(SPB).A model with the following dimensions was constructed:length L=2000 mm,breadth B=521.60 mm,and draft T=52.50 mm.The ship model was towed using standard towing tank experimental parameters.The speed was varied over the Froude number range 0.11–0.31.The air layer flow rate was varied at 80,85,and 90 standard liters per minute(SLPM)and the microbubble injection coefficient over the range 0.20–0.60.The results show that the ship model using the air layer had the highest drag reduction up to a maximum of 90%.Based on the characteristics of the SPB,which operates at low speed,the optimum air lubrication type to reduce resistance in this instance is ALDR.
文摘This study investigates the influence of air layer confinedunderwear on the heat and mass transfer of body-fabric-environment system. For a simple,a single-layer fabric in cotton is modeled.The study focuses on under-standing the combined effect of convection,radiation and evaporation in thermal comfort condition.Based on the-oretical analysis,both sensible heat flux and latent heat flux have been mathematically modeled and quantified for different thickness of underclothing air layer.Thenumerical results have been compared with
文摘The purpose of this paper is to study the energy efficiency of a local living space exposed to solar radiation in the subtropical climate of Casablanca. The study was mainly focused on the contribution of a phase change material (PCM), inserted into a 7-cm thick air layer of a double brick wall, in two different locations. We note that the experimental study was conducted using two real-scale test cavities, located in the Faculty of Science Ain Chock-Casablanca. Two PCM mounting methods were used for the south and west walls, in order to test its energy efficiency as a storage and retrieval means of the solar flux coming from the outside. In the case of the southern wall, the PCM is put directly on the internal side of the outside part of the double wall (Case 1). For the west wall, the PCM is placed 1.2 cm away from the internal side of the outer part of the double wall (Case 2). The first result shows that the PCM placed to the wall allows storing the solar heat during the day and releasing it to the outside of the building at night. While in the second case, the PCM keeps the heat stored day and night.
基金supported by the National Natural Science Foundation of China(Grant Nos.of 51679037,51639003 and 51809122)supported by the Natural Science Foundation of Jiangsu Province(Grant No.BK20190966).
文摘The air layer drag reduction(ALDR)of an axisymmetric body in oscillatory motions is investigated in this paper with open source toolbox OpenFOAM.The unsteady Reynolds-averaged Navier-Stokes(URANS)equations are used to determine the viscous flow and the volume of fluid(VOF)model is adopted to capture the interface of the air-water two-phase flow.The k-e turbulence model is adopted to simulate the turbulence.The dynamic mesh technique is applied to model the movement of the axisymmetric body.Firstly,the ALDR results are validated by the experimental data.Then,the effects of the movements of the body on the drag reduction during the ALDR state are investigated.Two representative kinds of movements are considered,namely,the pitch and the heave.The numerical results show that the drag reduction varies during the movements and the average drag reduction rates will be reduced.The variation of the drag reduction is related to the morphological change of the air layer.The heave motion is more likely to reduce the effects of the ALDR than the pitch motion.For both oscillatory motions,the large motion amplitude and the low motion period are not conductive to improving the effects of the ALDR.The effects of the oscillatory motion on the ALDR are more sensitive at high water speeds than at low water speeds.Besides,increasing the air flow ratio can be considered as one way to improve the effects of the ALDR.
基金Project supported by the Ministry of Industry and Information Technology of China(Project No.CB01N20-05).
文摘Air-layer drag reduction (ALDR) technology for ship energy saving is getting more and more attention in recent years because of the outstanding drag reduction effect. In order to promote practical application, it is necessary to fully understand the two phase flow characteristics of the air layer. Recent experimental studies have shown that the surface of the air layer presents wave pattern, which has an important influence on its damage risk. However, it is difficult to measure the wave pattern quantificationally due to the interference of equipment. The main goal of the present paper is to investigate the wave pattern characteristic of air layer in cavity using numerical simulation method. On this basis, the effect of flow and geometric influence factors are discussed to understand the key control conditions. A computational fluid dynamics (CFD) numerical method based on Reynolds averaged Navier-Stokes (RANS) equations and volume of fluid (VOF) interface capturing method is established, and has been successfully applied in the simulation of air layer wave pattern. Both 2-D and 3-D simulations are carried out, aiming at analyzing air-water interface flow and vortex flow directly. Based on the simulation results, several important conclusions about the mechanism of air layer wave pattern can be obtained. Firstly, it is found to be an inherent characteristic that the wave height of the upstream air layer is higher than that of the downstream. The extremely high wave peak is easy to contact with the flat plate, leading to the breakup of air layer and a “central blank area” phenomenon. With the help of flow analysis, it is found that this characteristic is mainly caused by the strong counterclockwise vortex behind the bow wedge block. Secondly, the air layer stability is reduced with the increase of water flow velocity by affecting the wave height. There is a saturation point of air flow rate to reach maximum thickness of air layer. Thirdly, cavity configuration has obvious influence on air layer stability by influencing vortex flow field. The increase of cavity depth and width can aggravate the unsteady and nonlinear characteristics of air layer. Finally, comprehensive design criteria are concluded from the view of geometrical configuration and flow conditions. A cavity with the moderate depth and width can avoid the upstream damage of air layer. Longitudinal position of air nozzles should be set within the low pressure zone behind the wedge block for stable air layer formation.
基金Supported by the Government Science and Technology Planning Project of Xingtai City,Hebei Province,China(2014ZZ006-1)Scientific Research and Development Project of Hebei Meteorological Bureau(14ky29)
文摘Based on meteorological sounding data in 2011 -2014 and air pollution data in 2013 and 2014 from an environmental monitoring station in Xingtai City, the characteristics of inversion layer and air pollution and their correlation in Xingtai City were discussed. The results show that term perature inversion was very serious at 07：00 in Xingtai City, and days with temperature inversion accounted for above 90%. There were obvious seasonal variations in air pollution in Xingtai City, and air pollution was the most serious in winter but the slightest in summer. The primary air pollu- tants in Xingtai City are PM10 and PM2.5, and the primary air pollutants and AQI had close correlation with parameters of inversion layer. Stable ground inversion layer could hinder air convection and air capacity, so that air pollution become more serious with the discharge of pollutants. Inversion layer nearly appearing all the year around is an important meteorological reason for serious air pollution in Xingtai City.
文摘Shea tree (Vitellaria paradoxa Gaertn. Family;Sapotaceae) indigenous to the Sudano-sahelian zone of Africa has great economic and ecological potential and attributes. Commercial cultivation of the tree is however, hampered by the poor rooting success of its planting material with adverse consequences on field establishment and total fruit yield. A 3 × 2 factorial experiment arranged in a randomized complete block design was carried out in 2012 at Bole in the Interior Savanna agro-ecological zone. The objective was to assess the rooting success of Shea shoots by the air-layering technique using two media types (palm fibre and Sphagnum moss) and three IBA hormone concentrations (0, 5000 and 10,000 ppm) under contrasting climatic (wet and dry) conditions. Layered shoots which were sprayed with 10,000 ppm IBA and wrapped with Sphagnum moss gave significantly (P < 0.05) higher rooting success in terms of more roots per cutting (73.3%) and longer roots per cutting (9.0 cm) than palm fibre at both 5000 ppm (30.0%;3.7 cm) and 10,000 ppm (46.7%;7.9 cm) concentrations. Higher rooting success was significantly achieved in the wet season than in the dry season. Sphagnum moss treated with 10,000 ppm IBA facilitated the translocation of higher levels of sugar and total free phenol (TFP) to the base of the layered shoots which resulted in significantly (P < 0.05) higher number of roots and better roots protection from fungal infection than the other treatments. Rooting of layered shoots was affected to a greater extent by low temperatures (Rooting = 836 – 34.2 Temp (low);R2 = 82%, p < 0.001) than by high temperatures (Rooting = 5250 – 175.0 Temp (high);R2 = 64.5%, p < 0.009). Rooting of layered shoots was also significantly and negatively affected by the spread of the canopy of the selected tree such that closed canopy trees resulted in higher and better rooting that open canopy trees (Rooting = 113.87-23.697(Canopy spread);R2 = 89%, p = 0.002;n = 9). Furthermore, significant and positive correlations were found between IBA concentration level and simple sugars (r = 0.99;P < 0.0001;n = 9) and also between IBA concentration level and total free phenols (r = 0.98;P < 0.0002;n = 9). The study concluded that to achieve high rooting success in the propagation of Shea nut tree by the air-layering technique, a combination of Sphagnum moss treated with 10,000 ppm of IBA hormone should be used and the whole set up carried out in the wet season.
基金Sponsored by the National Key Technologies Research and Development Program(Grant No.2004BA317B-4-1)
文摘The samples cut from U75V 60 kg/m heavy rail are heated to 900 ℃ in resistance furnace and then put into air spraying channel to be cooled for 80 s, and change air pressure from 0.16 MPa to 0.33 MPa, and observe the effect of air pressure on hardened layer. The thickness and hardness of hardened layer increases with the increase of air pressure, and the thickness is more than 24 mm at the center and top fillets of rail head, and more than 15 mm at the blow fillets of rail head when air pressure is more than 0.26 MPa. During the tempering after heat treatment, tempering temperature of rail head is more than 570 ℃ when air pressure is separately 0.16 MPa, 0.20 MPa and 0.23 MPa, which is higher than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s according to CCT curve of U75V steel. When air pressure is separately 0.26, 0.30 and 0.33 MPa, the tempering temperature is 529 ℃ lower than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s. Under this condition, pearlite transformation is finished totally, so in order to reduce air consumption and control the cost, proper air pressure for U75V 60 kg/m heavy rail heat treating should be 0.26 MPa; the cooling rate increases with the increase of air pressure, and the average cooling rate on the surface of rail head is more than 3.21 ℃/s when air pressure is more than 0.26 MPa, and the largest cooling rate occurs at the top fillets of rail head.
基金supported by the National Natural Science Foundation of China (Grant Nos. 40965001 and 40875008)the open project of State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences (Grant No.2009LASW-A02)
文摘In southern China, cold air is a common weather process during the winter season; it can cause strong wind, sharp temperature decreases, and even the snow or freezing rain events. However, the features of the atmospheric boundary layer during cold air passage are not clearly understood due to the lack of comprehensive observation data, especially regarding turbulence. In this study, four-layer gradient meteorological observation data and one-layer, 10-Hz ultrasonic anemometer-thermometer monitoring data from the northern side of Poyang Lake were employed to study the main features of the surface boundary layer during a strong cold-air passage over southern China. The results show that, with the passage of a cold air front, the wind speed exhibits low-frequency variations and that the wind systematically descends. During the strong wind period, the wind speed increases with height in the surface layer. Regular gust packets are superimposed on the basic strong wind flow. Before the passage of cold air, the wind gusts exhibit a coherent structure. The wind and turbulent momentum fluxes are small, although the gusty wind momentum flux is slightly larger than the turbulent momentum flux. However, during the invasion of cold air, both the gusty wind and turbulent momentum fluxes increase rapidly with wind speed, and the turbulent momentum flux is larger than the gusty wind momentum flux during the strong wind period. After the cold air invasion, this structure almost disappears.
基金Project supported by the National Natural Science Foundation of China(Nos.11732011,11672205,and 11332007)the National Key Research and Development Program of China(No.2016YFA0401200)
文摘The effects of air dissociation on ?at-plate hypersonic boundary-layer ?ow instability and transition prediction are studied. The air dissociation reactions are assumed to be in the chemical equilibrium. Based on the ?at-plate boundary layer, the ?ow stability is analyzed for the Mach numbers from 8 to 15. The results reveal that the consideration of air dissociation leads to a decrease in the unstable region of the ?rst-mode wave and an increase in the maximum growth rate of the second mode. High frequencies appear earlier in the third mode than in the perfect gas model, and the unstable region moves to a lower frequency region. When the Mach number increases, the second-mode wave dominates the transition process, and the third-mode wave has little effect on the transition. Moreover, when the Mach number increases from 8 to 12, the N-factor envelope becomes higher, and the transition is promoted. However, when the Mach number exceeds 12, the N-factor envelope becomes lower, and the transition is delayed. The N-factor envelope decreases gradually with the increase in the altitude or Mach number.
文摘The diurnal variations of gaseous pollutants and the dynamical and thermodynamic structures of the atmospheric boundary layer (ABL) in the Beijing area from January to March 2001 are analyzed in this study using data from the Beijing City Air Pollution Observation Field Experiment (BECAPEX). A heavy pollution day (22 February) and a good air quality day (24 February) are selected and individually analyzed and compared to reveal the relationships between gaseous pollutants and the diurnal variations of the ABL. The results show that gaseous pollutant concentrations exhibit a double-peak-double-valley-type diurnal variation and have similar trends but with different magnitudes at different sites in Beijing. The diurnal variation of the gaseous pollutant concentrations is closely related to (with a 1-2 hour delay of) changes in the atmospheric stability and the mean kinetic energy in the ABL.
基金supported by the National Key R&D Program of China(No.2017YFC 0210000)the National Natural Science Foundation of China(Nos.41230642 and 41705113)the Beijing Municipal Science and Technology Project(No.ZL171100000617002)
文摘The thermal internal boundary layer（TIBL） is associated with coastal pollution dispersion,which can result in high concentrations of air pollutants near the surface of the Earth. In this study, boundary layer height data which were obtained using a ceilometer were used to assess the effect of the TIBL on atmospheric pollutants in Qinhuangdao, a coastal city in North China.A TIBL formed on 33% of summer days. When a TIBL formed, the sunshine duration was 2.4 hr longer, the wind speed was higher, the wind direction reflected a typical sea breeze, and the boundary layer height was lower from 9：00 LT to 20：00 LT compared to days without a TIBL. If no TIBL formed, the average concentrations of PM2.5 and PM10 decreased with increasing boundary layer height. However, when a TIBL was observed, the average concentrations of PM2.5 and PM10 increased with increasing boundary layer height. Because the air from the sea is clean, PM2.5 and PM10 concentrations reached minimums in the daytime at 16：00 LT. After16：00 LT, the PM2.5 and PM10 concentrations increased rapidly on days when a TIBL formed,which indicated that the TIBL leads to the rapid accumulation of atmospheric pollutants in the evening. Therefore, the maximum concentrations of particulate matters were larger when a TIBL formed compared to when no TIBL was present during the night. These results indicate that it is suitable for outdoor activities in the daytime on days with a TIBL in coastal cities.
基金The National Natural Science Foundation of China(No.61573098,71401072)the Natural Science Foundation of Jiangsu Province(No.BK20130814)
文摘To increase airspace capacity, alleviate flight delay,and improve network robustness, an optimization method of multi-layer air transportation networks is put forward based on Laplacian energy maximization. The effectiveness of taking Laplacian energy as a measure of network robustness is validated through numerical experiments. The flight routes addition optimization model is proposed with the principle of maximizing Laplacian energy. Three methods including the depth-first search( DFS) algorithm, greedy algorithm and Monte-Carlo tree search( MCTS) algorithm are applied to solve the proposed problem. The trade-off between system performance and computational efficiency is compared through simulation experiments. Finally, a case study on Chinese airport network( CAN) is conducted using the proposed model. Through encapsulating it into multi-layer infrastructure via k-core decomposition algorithm, Laplacian energy maximization for the sub-networks is discussed which can provide a useful tool for the decision-makers to optimize the robustness of the air transportation network on different scales.
文摘在高内压循环作用下地下储气库衬砌易产生裂缝与渗透通道,密封层受力与裂缝控制问题尤为关键。基于有限-离散元方法(finite-discrete element method,简称FDEM),建立了连续-非连续的围岩-衬砌-密封层整体模型,系统研究了平钢板与波拱两类钢衬及预设缝设计参数对衬砌裂缝演化与密封性能的影响。结果表明,平钢板型密封层应力水平和裂缝数量较高,整体性差,预设缝改善衬砌开裂与密封结构受力作用有限;波拱型结构能显著降低峰值应力并改变裂缝分布,使得裂缝多集中在波拱底部,整体裂缝数量减少,但波拱处的裂缝开裂宽度更大。采用波拱+预设缝相结合的方案可进一步均匀化开裂,减少密封钢衬应力。随着波拱和预设缝数量增加,应力分布趋于均匀,裂缝扩展受控,但最大裂缝宽度呈先减后增趋势,钢衬应力模式由拉应力向弯-剪应力转变。当缝设于拱底时,裂缝可沿预设路径均匀扩展,并可结合防排水措施降低渗透风险。总体而言,波拱+预设缝复合设计在引导裂缝、释放应变和提升密封渗透性能方面具有显著优势,为储气库密封-衬砌协同优化设计提供参考。
