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241.
The temporal and spatial variations in the rate of sedimentation and sediment resuspension in Lake Rehtijärvi (southern Finland) were studied by sediment traps deployed in the stratifying and non‐stratifying regions of the lake. Both the gross sedimentation rate and the resuspension rate recorded by near‐bottom traps were significantly higher in the stratifying region (>20 m depth) than in the shallow part of the lake (2 m depth). These rate changes were caused by temporarily elevated hypolimnetic resuspension rates exceeding 40 g dry weight m?2 day?1 in July 2005, whereas in the shallow station no such peak was observed and the resuspension rate remained below 30 g dry weight m?2 day?1 throughout the study. The elevated resuspension rate in the hypolimnion could not be attributed to surface wave action or sediment disturbance by fish, but was due to the occurrence of internal seiches. The seiche amplitude was on average the highest in July, although the wind velocities were low; this was because winds temporarily were blowing along the longitudinal axis of the lake. Due to the steep slopes, the sediments of Lake Rehtijärvi are prone to redistribution and sediment focusing towards the accumulation areas also affected trap catches in the deep stations. 相似文献
242.
The work presents a theoretical and experimental assessment of the mechanical power absorption by a prototype horizontal ring mill, during the continuous comminution of olivine with feed rates for a wide range of operating conditions. The forces acting on a ring element of a prototype ring mill under continuous operating conditions have been theoretically calculated and it has been experimentally confirmed that they are proportional to the square of the rotational speed. A linear variation of power with feed rate has been established and a functional relationship between consumed power and rotational speed is also proposed. Results demonstrate that there exists a feed rate for which the value of the friction coefficient between the rings and the powdered material is minimized, independently of the rotational speed. It was further found that the product size depends on the absorbed mechanical power and it decreases with increasing power values. 相似文献
243.
244.
本文在前期统计工作的基础上,选取了一次典型的中尺度对流涡旋(MCV)个例,利用NCEP/NCAR再分析资料分析其背景场特征,并利用WRF数值模拟结果分析其成因及其触发"二次对流"的可能机制。结果表明:MCV发生前,江淮地区处于200 h Pa强辐散场中,高层抽吸作用明显,500 h Pa江淮西北部短波槽槽后不断有冷空气南下,加强该地区大气层结不稳定,850 h Pa湖北至安徽中部有切变线活动,这种高低层配置十分有利于MCV生成及对流发生;MCV生命史各阶段垂直输送项和涡管倾斜项呈反位相分布,而水平平流项和辐合辐散项的作用基本是相互抵消的,垂直输送项和辐合辐散项是MCV生成阶段中低层涡度的主要来源;MCV引发的"二次对流"出现在其生成阶段,且位于其南侧,MCV发展成熟后,对流迅速减弱;MCV的生成使南侧西南低空急流加强,伴随水平涡度的变化,"二次对流"的发生发展与水平涡度对应的垂直环流上升支有直接联系。 相似文献
245.
A semi-analytical method of the stress-strain analysis of buried steel pipelines under submarine landslides was proposed, considering the nonlinearities of the pipe-soil interaction and mechanical properties of the pipe steel. The pipeline was divided into three parts according to different loading conditions, and the corresponding differential equations were established based on a combination of the beam-on-elastic foundation and elastoplastic-beam theories. According to the second-order central difference method, the transverse horizontal displacement was calculated, and then the bending strain was obtained based on the relation between bending strains and curvatures. Considering the interaction between the axial and bending strains, the axial strain can be derived from the equilibrium condition by equating the axial force. The proposed method was verified through the comparison of obtained solutions to ANSYS results, with minor deviations which do not exceed about 4.6%. Additionally, the effects of the slide width, the buried depth of pipelines, the internal friction angle of soils, the cohesion of soils and the bulk density of soils are investigated through parametric studies. 相似文献
246.
介绍采用两种不同投影系统的卫星资料制作1:50万TM卫星影像图的基本工艺及关键技术,(10对TM卫星底片进行密度测定和反差分析,(2)卫星像片平面位置误差赋和控制;(3)卫星像片彩色合成色调协调一致控制,在此基础上分析论述了TM卫星影像图的特点及其在地理学上的应用。 相似文献
247.
提出一种由GPS独立观测边组成闭合环的各坐标分量闭合差计算GPS平面控制全中误差的公式。给出了根据地心直角坐标系各坐标分量计算平面上环闭合差的简化公式。根据《工程测量规范》各等级三角网、三边网的主要精度指标,计算了相应等级GPS平面控制网的全中误差指标。 相似文献
248.
Sea surface temperature (SST) is a result of multiple interactions in air-sea processes. During days with strong insolation and low wind speed, there may be uneven net heating of the water layer near the surface of the ocean, when there are horizontal temperature gradients at the sea surface. Cooling of the water caused by evaporation, sensible, or longwave radiative, heat loss would be greater from warm water compared to that from relatively cold water. As a result, under low wind speed conditions and clear skies, the horizontal SST discontinuities, occurring at fronts, eddies, or in storm wakes, may diminish or even vanish. This phenomenon is illustrated here with some field and modelling results. The dependence on latitude and mean environmental conditions of the difference in warming on the cold and warm side of SST discontinuities is explored. The time dependence is important for the impact on remote sensing of SST, and it is found to be short enough that substantial masking of SST gradients can occur during the first six hours of the diurnal heating cycle, but the effect would continue to grow if calm and solar heating persist for several subsequent days. An integrated effect of this uneven net heating is seen in the seasonal masking of subsurface temperature gradients in the Gulf of Mexico and Florida Straits. 相似文献
249.
用解剖镜镜检的方法鉴定和分析了2001年2、5、8月,2002年5、8、11月以及2003年2月在胶州湾7个航次中浅海Ⅲ型网垂直拖网样品中的砂壳纤毛虫。结果表明,各个航次优势种(丰度)为:2001年2月,Tintinnopsis brasiliensis1500-23900个/m3;2001年5月,Codonellopsis mobilis80600-624000个/m3,Leprotintinnus neriticus920-13842个/m3,Fav-ella campanula184-2728个/m3,T.kofoidi0-368个/m3;2001年8月,Stenosemella sp.0-982个/m3,T.radix98-19114个/m3,L.neriticus0-5932个/m3,E.panamensis0-1318个/m3,T.gracilis0-317个/m3,L.nordqvisti O-158个/m3;2002年5月,C.mobilis22400-1567500个/m3,L.neriticus900-96300个/m3,C.parvicollis0-75600个/m3;2002年8月,E.panamensis0-2604个/m3,C.mobilis0-992个/m3,T.gracilis0-256个/m3,L.nordqvisti0-102个/m3;2002年11月,C.ostenfeldi0-13003个/m3,L.neriticus0-713个/m3,T.schotti0-316个/m3;2003年1月,砂壳纤毛虫丰度低干50个/m3。下列5种是新记录种:C.ostenfeldi,T.schotti,T.kofoidi,L.neriticus,Stenosemella sp.。 相似文献
250.
A.P. Martin 《Progress in Oceanography》2003,57(2):125-174
Explanations for the patchy distribution of marine phytoplankton are critically reviewed with the focus on the role played by lateral advection and mixing. Generating mechanisms for what is increasingly seen as a ubiquitous feature of the oceans have been sought in biology, behaviour, population dynamics, physics and various combinations thereof. As the mesoscale and sub-mesoscale (1-500 km) contain the “weather” of the ocean—eddies and fronts capable of strong stirring and mixing of any tracers advected by them—the physical circulation in this regime may have a strong role in the generation of patchiness. As a consequence, the focus here is on the ways the turbulent currents at these scales can interact with other processes to produce the complex spatial structures in phytoplankton distributions seen in countless satellite images and cruise data. The mechanisms reviewed include patch formation (KiSS theory), filamentation, Turing and related instabilities, diffusion waves, shear effects and turbulent stirring. Parallel to this is a discussion of observations and of the techniques that have been used to analyse them, particularly spectral analysis. It will be seen that many of the limitations in extant theories stem from a paucity of data and ambiguity in its interpretation. A synthesis of recent developments in sampling, instrumentation, image analysis and turbulence theory is used to suggest alternatives to conventional approaches, to highlight important work in related fields and to motivate new lines of research. 相似文献