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981.
箱体零件是矿井机械中重要的基础零件。它的加工质量直接关系矿井设备的寿命、机器的性能和安全性。针对煤矿井下煤尘和瓦斯存在的环境特性、煤矿井用箱体防爆面加工的特殊要求,采用控制理论的信息流图,对工艺过程中传统方块图进行简化,使繁杂的方块图解采用信息流图表示工艺过程的工序(变量)之间关系。进而讨论箱体零件加工工艺过程误差转换模型、形状精度特征、传递图、描述方程、误差转换结构图;提出防爆面磨削工艺系统的数学模型;求解出振动特性、幅值和相位,得出了形状误差与工艺参数的关系曲线。实践证明,合理选择磨削参数,能够保证煤矿井下工作机械具有高的防爆性能,达到矿井安全生产的技术要求。 相似文献
982.
Cohesive sediments besides their typical heterogeneity are characterised by structural discontinuity. Particularly, organic
consolidated muds are a good example of sediments that consist of vast aggregates, pore water and gaseous products. The texture
of a cohesive sediment bed is a result of a number of mutually affecting factors, such as deposition history, mineral and
organic composition, kind of biota and oxygen uptake. The presented work attempts to quantify the effect of sediment physical
properties and sediments structure on the sediment erosion potential, considering incipient motion and erosion rate. This
quantification is made on the basis of comparative testing of both unremoulded and remoulded samples of a river mud. Due attention
is paid to sediment handling to preserve the delicate structure of the sediment for the laboratory experiments. Mud with two
degrees of consolidation has been examined in a tilting flume under different flow situations. The test results show a typical
increase of erosion strength with dry matter concentration of the mud. It has also been found that the structural properties
increase the erosion strength for the less consolidated mud. An opposite effect has been recorded for a more consolidated
deposit. As a consequence, due to the sediment structure, the original beds differ much less in erosion resistance in relation
to the dry mass concentration than their disturbed analogues. Finally, the erosion resistance of the examined mud is compared
with data from the literature. 相似文献
983.
984.
985.
干旱河谷地区农业产业结构的调整对环境及经济的影响分析--以汉源地区为例 总被引:1,自引:1,他引:1
中国农业产业结构的调整,是中国改革开放、市场化发展的集中体现。本文以汉源地区为例,总结分析了典型的干旱河谷地带农业产业结构的调整情况,不仅促进了当地农村发展方式的根本转变、改善了农村经济、提高了农民收入,同时还解决了当地剩余劳动力的就地消化问题。本文指出了农业产业结构的调整可能对当地生态环境造成的影响,其中诸如不舍理用药用肥造成的水污染、土壤和农产品的硝酸盐污染以及土壤退化、大气污染等环境问题。需要在实践中更加审慎地调整农业产业结构。 相似文献
986.
987.
ABSTRACT: In‐reservoir thermal and ecological effects of releasing some flows over the surface spillway at Blue Mesa Reservoir, Colorado, rather than routing all releases through the hypolimnetic outlet were evaluated using a calibrated and validated one‐dimensional thermal model (CE‐THERM) with a set of ecological models. Thermal model output indicated that surface water temperatures were influenced primarily by atmospheric conditions, but the release of warmer water over the spillway resulted in a thinner epilimnion and cooler metalimnetic water temperatures. Ecological model predictions indicated that spillway releases and associated temperatures resulted in lower growth rates for young‐of‐year (YOY) kokanee salmon (Oncorhynchus nerka) in the reservoir by up to 9 percent when compared with growth rates under baseline operations with no releases over the spillway. Kokanee growth rates were reduced under spillway release scenarios because lower temperatures not only affected metabolic rates, but limited the productivity of the zooplankton as well. Thus, altering the release regime with spillway discharges could have deleterious effects on Blue Mesa's YOY kokanee. However, in other reservoirs, distributing discharges among different elevations may provide managers with a mechanism to regulate temperatures to benefit species of concern that are facing challenges imposed by environmental conditions such as global warming. 相似文献
988.
The results of phenetic studies on chum salmon populations from the rivers flowing into Taui and Yama bays, Sea of Okhotsk, are discussed. The heterogeneity of samples with respect to the frequency of a certain phene in females is not necessarily revealed when this frequency is analyzed in males. Statistically significant heterogeneity with respect to phene frequencies is observed in all chum salmon populations, whether the corresponding data are analyzed by years or throughout the observation period. This population heterogeneity is accounted for by specific phenetic features of each chum salmon population, although in different years it manifests itself in the frequencies of different phenes. The phenes distinguished on the head of fish contribute most significantly to interpopulation differences. 相似文献
989.
Clarence Richardson 《Journal of the American Water Resources Association》1978,14(5):1035-1047
ABSTRACT: A model was developed of the periodic-stochastic structure of daily precipitation over an area. The model is based ona multivariate normal distribution. The square roots of daily precipitation at a point were found to approximate a sample from a univariate normal distribution that had been truncated at zero. The zero daily precipitation amounts were considered negative amounts of unknown quantity. The multivariate normal distribution was used to describe the variation of daily precipitation over an area. The periodic fluctuations of the model parameters were described with Fourier series. The model was tested using data from two areas of different precipitation characteristics. Data generated with the model contained many of the statistical characteristics observed in the historical data. 相似文献
990.
火灾环境下具有弹性支座钢构件的稳定性能 总被引:5,自引:0,他引:5
根据弹性稳定理论原理研究了具有弹性支座钢结构构件在火灾环境下的静力特性问题 ,导出了高温下钢构件的静力稳定临界荷载和长度计算式 ,并讨论了温度、杆件的侧向弹簧刚度、杆长、保护层厚度等因素对杆件静力稳定临界荷载和长度的影响 ,分析表明 ,杆件的侧向弹簧刚度、杆长、保护层厚度对杆件的稳定承载力影响较大 ,并探讨了在实际工程中如何减小结构失稳的问题。 相似文献