Based on theory of mechanical dynamics, meshing characteristic as well as thedynamic model of double circular arc helical gearing, an analysis approach and a computer programhave been developed for studying the state ...Based on theory of mechanical dynamics, meshing characteristic as well as thedynamic model of double circular arc helical gearing, an analysis approach and a computer programhave been developed for studying the state of dynamic load and factor of dynamic load of thegearing, the changing situation of dynamic load and dynamic load factor vs some affecting factorssuch as gear width, helix angle and accuracy grade etc are investigated. A series of conclusions areobtained: ①With the increasing in the values of gear width, the dynamic load factor appears slowdecreasing tendency in most region of gear width. ②When the accuracy grades of the gearing areimproved, the values of dynamic load factor decrease. ③The value of dynamic load factor appears adecreasing tendency with the increasing of value of helix angle at the same ratio of criticalrotational speed.展开更多
There are differing views regarding the climatic factors that limit radial growth of Qilian juniper （Juniperus przewalskii Kom.） at the upper treelines on the northeastern Tibetan Plateau. In this study, trees from ...There are differing views regarding the climatic factors that limit radial growth of Qilian juniper （Juniperus przewalskii Kom.） at the upper treelines on the northeastern Tibetan Plateau. In this study, trees from an upper tree northeastern Tibetan Plateau were selected to limits the radial growth of Qilian juniper. Using ine site in the Anyemaqen Mountains of the present new evidence that low temperature a signal-free regional curve standardization （SF-RCS） method, a ring-width chronology for Qilian juniper was developed extending from AD 1082 to 2010. The results of correlation analysis between tree-ring index and instrumental climatic variables showed that both winter （December in the previous year and January in the current year） and summer （July and August in the current year） temperatures were signifi- cantly and positively correlated with the tree-ring index. Precipitation in June was also significantly and positively correlated with the tree-ring index, but was not as important as low temperature in controlling tree growth. To test the key limiting climatic factor for tree radial growth at different altitudes, an indicator termed the ＂relative distance to upper treeline＂ （RDUT） was developed to quantify the representativeness of collected samples for the forest＇s upper treeline. The RDUT showed that the upper 20% of the forest belt may be an important boundary in terms of capturing the temperature signal from tree-ring width at the upper treelines on the northeastern Tibetan Plateau. Our findings enhance the existing understanding that temperature is the limiting factor at upper treelines on the northeastern Tibetan Plateau, and will be useful in the reconstruction of past temperature in future studies.展开更多
A quasicrystal has a structure intermediate between crystals and compound glasses. The disorder in glass makes its diffraction diffuse, so it is surprising that quasicrystals diffract more sharply than crystals. The g...A quasicrystal has a structure intermediate between crystals and compound glasses. The disorder in glass makes its diffraction diffuse, so it is surprising that quasicrystals diffract more sharply than crystals. The greater sharpness is computed to be due to the hierarchic structure with unit cell alignment in 3-dimensional space. Electron microscope phase contrast images map the comparatively heavy Mn atoms in icosahedral Al<sub>6</sub>Mn, where the transition metal locates the centers of unit cells inside clusters and superclusters. Because the solid is aperiodic, each diffracted beam is a product of multiple interplanar spacings combined, and this contrasts with the unique relationship between spacing and incident angle in Bragg diffraction from crystals. Simulated quasi-structure factors add the relative phase shifts that are in geometric series from cell to cluster to superclusters of increasing order. The scattering becomes coherent in best fit, angular configuration between the aperiodic solid and a longitudinally periodic X-ray or electron probe. The quasi-structure factors express angular divergence in each diffracted beam from its corresponding Bragg condition, and the divergence provides a special metric, essential for atomic measurement in the geometric solids. The fit is reinforced at all levels from the unit cell to cluster to high order superclusters. The optics operates under a new quasi-Bragg law in a new geometric space. In this paper, we proceed to examine the effect of specimen size on line resolution in diffraction, first analytically and secondly in simulation. The line resolution follows a power law on the supercluster order, matching its atomic population.展开更多
The special case of a crack under mode Ⅲ conditions was treated, lying parallel to the edges of an infinite strip with finite width and with the shear modulus varying exponentially perpendicular to the edges. By usin...The special case of a crack under mode Ⅲ conditions was treated, lying parallel to the edges of an infinite strip with finite width and with the shear modulus varying exponentially perpendicular to the edges. By using Fourier transforms the problem was formulated in terms of a singular integral equation. It was numerically solved by representing the unknown dislocation density by a truncated series of Chebyshev polynomials leading to a linear system of equations. The stress intensity factor （SIF） results were discussed with respect to the influences of different geometric parameters and the strength of the non-homogeneity. It was indicated that the SIF increases with the increase of the crack length and decreases with the increase of the rigidity of the material in the vicinity of crack. The SIF of narrow strip is very sensitive to the change of the non-homogeneity parameter and its variation is complicated. With the increase of the non-homogeneity parameter, the stress intensity factor may increase, decrease or keep constant, which is mainly determined by the strip width and the relative crack location. If the crack is located at the midline of the strip or if the strip is wide, the stress intensity factor is not sensitive to the material non-homogeneity parameter.展开更多
Accumulated sand-belts refer to those formed along the oasis fringe,especially at the upwind location,due to the accumulation of sand blocked by farmland windbreak. In the 60 years since the foundation of new China,a ...Accumulated sand-belts refer to those formed along the oasis fringe,especially at the upwind location,due to the accumulation of sand blocked by farmland windbreak. In the 60 years since the foundation of new China,a lot of trees have been planted for desertification combating in northwest and north China,thus,accumulated sand-belts were formed at the upwind location. The formation and the ecological effects of the accumulated sand-belts along the oasis fringe is a new scientific concern. To study the formation causes of these belts in Hexi corridor,21 samples were selected,and the height / width of the belts,as well as the vegetation,soil,soil moisture and climatic factors were investigated. This paper analyzed the correlation between the height / width of the belts and the vegetation,soil,soil moisture and climatic factors using the methods of variance analysis,correlation analysis and canonical correlation analysis. The results indicate that: the accumulated sand-belts take a trend of being high and wide in the east whereas low and narrow in the west,and most of the parts tend to be stable; the species on the belts are dominated by Tamarix austromongolica,the vegetation cover and the pure vegetation cover of different dominant species on the leeward slope of the accumulated sand-belts vary significantly. The canonical correlation analysis shows that: the height and width of accumulated sand-belt is the interaction of precipitation,distance to the sand source,leeward vegetation cover and annual average wind speed. Moreover,the height of accumulated sand-belts are negatively correlated to the soil moisture at the depth of 30- 50 cm,air humidity and leeward vegetation cover,and the width of the belts is also negatively correlated with the distance to the sand source. The ecological effects of the accumulated sand-belts are both positive( stopping sands from moving into farmland,protective role as an obstacle)and negative( when the belts decay and activate one day,they will become the new sand sources). At present,there are no signs showing the negative effects of the belts. The ecological effects of the accumulated sand-belts are: firstly the protective role as an obstacle,and secondly to intercept and reduce the sands moving into farmlands.展开更多
Previous researches on the mixed mode fracture initiation criteria were mostly focused on opening mode fracture. In this study, the authors proposed a new criterion for mixed mode sliding fracture initiation, which is...Previous researches on the mixed mode fracture initiation criteria were mostly focused on opening mode fracture. In this study, the authors proposed a new criterion for mixed mode sliding fracture initiation, which is the maximum twin shear stress factor criterion. The authors studied a finite width plate with central slant crack, subject to a far field uniform uniaxial tensile or compressive stress.展开更多
致密储层地质特征及改造工艺是影响缝控改造效果的重要因素,直接决定缝控体积的大小,与油井产能呈正相关,但不同工程参数对储层改造效果的影响规律认识不清。为此,建立了缝网压裂模型,系统全面地分析段间干扰及无干扰条件下工程因素对...致密储层地质特征及改造工艺是影响缝控改造效果的重要因素,直接决定缝控体积的大小,与油井产能呈正相关,但不同工程参数对储层改造效果的影响规律认识不清。为此,建立了缝网压裂模型,系统全面地分析段间干扰及无干扰条件下工程因素对缝控体积的影响。结果表明,裂缝间距是影响缝控体积的关键因素,存在最优裂缝间距30 m;随着总液量的增加,段间干扰条件下缝网宽度增加幅度明显大于无干扰情况,两种情况下缝网长度增加均不明显;施工排量为10~12 m 3/min时,有利于提升缝控体积;前置液比例为40%时,既能提升改造体积,又能保证后续顺利携砂。研究结果为致密储层多簇压裂缝控体积研究提供思路,对致密砂岩储层压裂设计具有重要意义。展开更多
基金This project is supported by Science and Technology Developing Foundation of Tianjin Higher Education (No.20030703).
文摘Based on theory of mechanical dynamics, meshing characteristic as well as thedynamic model of double circular arc helical gearing, an analysis approach and a computer programhave been developed for studying the state of dynamic load and factor of dynamic load of thegearing, the changing situation of dynamic load and dynamic load factor vs some affecting factorssuch as gear width, helix angle and accuracy grade etc are investigated. A series of conclusions areobtained: ①With the increasing in the values of gear width, the dynamic load factor appears slowdecreasing tendency in most region of gear width. ②When the accuracy grades of the gearing areimproved, the values of dynamic load factor decrease. ③The value of dynamic load factor appears adecreasing tendency with the increasing of value of helix angle at the same ratio of criticalrotational speed.
基金The Strategic Priority Research Program from the Chinese Academy of Sciences No.XDA05080201 National Natural Science Foundation of China, No.41071061
文摘There are differing views regarding the climatic factors that limit radial growth of Qilian juniper （Juniperus przewalskii Kom.） at the upper treelines on the northeastern Tibetan Plateau. In this study, trees from an upper tree northeastern Tibetan Plateau were selected to limits the radial growth of Qilian juniper. Using ine site in the Anyemaqen Mountains of the present new evidence that low temperature a signal-free regional curve standardization （SF-RCS） method, a ring-width chronology for Qilian juniper was developed extending from AD 1082 to 2010. The results of correlation analysis between tree-ring index and instrumental climatic variables showed that both winter （December in the previous year and January in the current year） and summer （July and August in the current year） temperatures were signifi- cantly and positively correlated with the tree-ring index. Precipitation in June was also significantly and positively correlated with the tree-ring index, but was not as important as low temperature in controlling tree growth. To test the key limiting climatic factor for tree radial growth at different altitudes, an indicator termed the ＂relative distance to upper treeline＂ （RDUT） was developed to quantify the representativeness of collected samples for the forest＇s upper treeline. The RDUT showed that the upper 20% of the forest belt may be an important boundary in terms of capturing the temperature signal from tree-ring width at the upper treelines on the northeastern Tibetan Plateau. Our findings enhance the existing understanding that temperature is the limiting factor at upper treelines on the northeastern Tibetan Plateau, and will be useful in the reconstruction of past temperature in future studies.
文摘A quasicrystal has a structure intermediate between crystals and compound glasses. The disorder in glass makes its diffraction diffuse, so it is surprising that quasicrystals diffract more sharply than crystals. The greater sharpness is computed to be due to the hierarchic structure with unit cell alignment in 3-dimensional space. Electron microscope phase contrast images map the comparatively heavy Mn atoms in icosahedral Al<sub>6</sub>Mn, where the transition metal locates the centers of unit cells inside clusters and superclusters. Because the solid is aperiodic, each diffracted beam is a product of multiple interplanar spacings combined, and this contrasts with the unique relationship between spacing and incident angle in Bragg diffraction from crystals. Simulated quasi-structure factors add the relative phase shifts that are in geometric series from cell to cluster to superclusters of increasing order. The scattering becomes coherent in best fit, angular configuration between the aperiodic solid and a longitudinally periodic X-ray or electron probe. The quasi-structure factors express angular divergence in each diffracted beam from its corresponding Bragg condition, and the divergence provides a special metric, essential for atomic measurement in the geometric solids. The fit is reinforced at all levels from the unit cell to cluster to high order superclusters. The optics operates under a new quasi-Bragg law in a new geometric space. In this paper, we proceed to examine the effect of specimen size on line resolution in diffraction, first analytically and secondly in simulation. The line resolution follows a power law on the supercluster order, matching its atomic population.
基金Project supported by the National Natural Science Foundation of China (No.90305023)
文摘The special case of a crack under mode Ⅲ conditions was treated, lying parallel to the edges of an infinite strip with finite width and with the shear modulus varying exponentially perpendicular to the edges. By using Fourier transforms the problem was formulated in terms of a singular integral equation. It was numerically solved by representing the unknown dislocation density by a truncated series of Chebyshev polynomials leading to a linear system of equations. The stress intensity factor （SIF） results were discussed with respect to the influences of different geometric parameters and the strength of the non-homogeneity. It was indicated that the SIF increases with the increase of the crack length and decreases with the increase of the rigidity of the material in the vicinity of crack. The SIF of narrow strip is very sensitive to the change of the non-homogeneity parameter and its variation is complicated. With the increase of the non-homogeneity parameter, the stress intensity factor may increase, decrease or keep constant, which is mainly determined by the strip width and the relative crack location. If the crack is located at the midline of the strip or if the strip is wide, the stress intensity factor is not sensitive to the material non-homogeneity parameter.
基金Supported by the Pre-phase Project of the 973 Program(2014CB460611)
文摘Accumulated sand-belts refer to those formed along the oasis fringe,especially at the upwind location,due to the accumulation of sand blocked by farmland windbreak. In the 60 years since the foundation of new China,a lot of trees have been planted for desertification combating in northwest and north China,thus,accumulated sand-belts were formed at the upwind location. The formation and the ecological effects of the accumulated sand-belts along the oasis fringe is a new scientific concern. To study the formation causes of these belts in Hexi corridor,21 samples were selected,and the height / width of the belts,as well as the vegetation,soil,soil moisture and climatic factors were investigated. This paper analyzed the correlation between the height / width of the belts and the vegetation,soil,soil moisture and climatic factors using the methods of variance analysis,correlation analysis and canonical correlation analysis. The results indicate that: the accumulated sand-belts take a trend of being high and wide in the east whereas low and narrow in the west,and most of the parts tend to be stable; the species on the belts are dominated by Tamarix austromongolica,the vegetation cover and the pure vegetation cover of different dominant species on the leeward slope of the accumulated sand-belts vary significantly. The canonical correlation analysis shows that: the height and width of accumulated sand-belt is the interaction of precipitation,distance to the sand source,leeward vegetation cover and annual average wind speed. Moreover,the height of accumulated sand-belts are negatively correlated to the soil moisture at the depth of 30- 50 cm,air humidity and leeward vegetation cover,and the width of the belts is also negatively correlated with the distance to the sand source. The ecological effects of the accumulated sand-belts are both positive( stopping sands from moving into farmland,protective role as an obstacle)and negative( when the belts decay and activate one day,they will become the new sand sources). At present,there are no signs showing the negative effects of the belts. The ecological effects of the accumulated sand-belts are: firstly the protective role as an obstacle,and secondly to intercept and reduce the sands moving into farmlands.
文摘Previous researches on the mixed mode fracture initiation criteria were mostly focused on opening mode fracture. In this study, the authors proposed a new criterion for mixed mode sliding fracture initiation, which is the maximum twin shear stress factor criterion. The authors studied a finite width plate with central slant crack, subject to a far field uniform uniaxial tensile or compressive stress.
文摘致密储层地质特征及改造工艺是影响缝控改造效果的重要因素,直接决定缝控体积的大小,与油井产能呈正相关,但不同工程参数对储层改造效果的影响规律认识不清。为此,建立了缝网压裂模型,系统全面地分析段间干扰及无干扰条件下工程因素对缝控体积的影响。结果表明,裂缝间距是影响缝控体积的关键因素,存在最优裂缝间距30 m;随着总液量的增加,段间干扰条件下缝网宽度增加幅度明显大于无干扰情况,两种情况下缝网长度增加均不明显;施工排量为10~12 m 3/min时,有利于提升缝控体积;前置液比例为40%时,既能提升改造体积,又能保证后续顺利携砂。研究结果为致密储层多簇压裂缝控体积研究提供思路,对致密砂岩储层压裂设计具有重要意义。
