Fine particle deposition to initially starved,stationary, planar beds     

NATHAN T. HAMM  W. BRIAN DADE  CARL E. RENSHAW 《Sedimentology》2009,56(7):1976-1991

Reported here are results from new flume experiments examining deposition and entrainment of inert, silt‐sized particles (with spherical diameters in the range from 20 to 60 μm) to and from planar, impermeable and initially starved beds underlying channel flows. Bed surfaces comprised smooth or fixed sand‐size granular roughness and provided hydraulically smooth to transitionally rough boundaries. Results of these experiments were analysed with a simple model that describes the evolution of vertically averaged concentration of suspended sediment and accommodates the simultaneous delivery to and entrainment of grains from the bed. The rate of particle arrival to a bed diminishes linearly, and the rate of particle entrainment increases by the 5/2 power, as the value of the dimensionless Saffman parameter S = u_*³/g’ν approaches a threshold value of order unity, where u is the conventional friction velocity of the turbulent channel flow, g’ is the acceleration due to gravity adjusted for the submerged buoyancy of individual particles and ν is the kinematic viscosity of the transporting fluid. This transport behaviour is consistent with the notion that non‐cohesive, silt‐sized particles can neither reach nor remain on an impermeable bed under flow conditions where mean lift imposed on stationary particles in the viscous sublayer equals or exceeds the submerged weight of individual particles. Within the size range of particles used in these experiments, particle size and the characteristic size of granular roughness, up to that of medium sand, did not affect rates of dimensionless arrival or entrainment to a significant degree. Instead, a new but consistent picture of fine‐particle transport is emerging. Silt‐sized material, at least, is subject to potentially significant interaction with the bed during intermittent suspension transport at intermediate flow speeds greater than the value required for initiation of transport (ca 20 cm sec^?1) but less than the value (ca 50 cm sec^?1) required by the Saffman criterion ensuring transport in fully passive suspension or, equivalently, ‘wash‐load’.  相似文献   

上海磁悬浮列车工程基岩水准标设计与施工     

顾为栋  龚士良 《上海国土资源》2001,(Z1):43-47

磁悬浮列车是当今世界上最为现代化的陆上交通工具,其对工程沿线地面沉降控制要求严格,对沉降监测的测量基准点的稳定性也有严格要求.上海磁悬浮列车工程专门设计施工了两座基岩水准标,作为沉降监测的测量基准.该基岩标较以往监测标志有重大改进,并兼有高程控制与平面位置控制的作用.本文介绍其设计原理、结构特点与施工技术.  相似文献   

Removal of sediments from the Alleppey nearshore region during post-mud bank formation     

K. K. Ramachandran  M. Samsuddin 《Journal of Earth System Science》1991,100(2):195-203

Formation of mud banks along the southwest coast of India is marked by an unusual calmness of the nearshore area with a large amount of suspensates in the water column and mud deposition. After cessation of the monsoon, the removal of mud from the mud bank region is evident. Sediment dynamics worked out from the textural characteristics suggest resuspension of sediments by waves and subsequent transportation of the suspended load by longshore/coastal currents as the main factors responsible for the removal of sediments from the mud bank region during the post-monsoon season.  相似文献   

Formation of high-grade ignimbrites Part II. A pyroclastic suspension current model with implications also for low-grade ignimbrites   总被引：2，自引：2，他引：0  

Armin Freundt 《Bulletin of Volcanology》1999,60(7):545-567

 Analogue experiments in part I led to the conclusion that pyroclastic flows depositing very high-grade ignimbrite move as dilute suspension currents. In the thermo–fluid–dynamical model developed, the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation. Initial conditions of the currents are derived from column-collapse modeling for magmas with an initial H₂O content of 1–3 wt.% erupting through circular vents and caldera ring-fissures. The flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content, temperature, bottom slope and coarser initial grain size. Progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance. The ash plumes reach stratospheric heights and distribute 30–80% of the erupted material as widespread co-ignimbrite ash. Pyroclastic suspension currents with initial mass fluxes of 10⁷-10¹² kg/s can spread for tens of kilometers with only limited cooling, although they move as supercritical, strongly entraining currents for the eruption conditions considered here. With increasing eruption mass flux, cooling during passage through the fountain diminishes while cooling during flow transport increases. The net effect is that eruption temperature exerts the prime control on emplacement temperature. Pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is T_o>1.3^.T_mw, the minimum welding temperature. High eruption rates, a large fraction of fine ash, and a ring-fissure vent favor the formation of extensive high-grade ignimbrite. For very hot eruptions producing sticky, partially molten pyroclasts, analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation, which reduces runout. As a result, very high-grade ignimbrites cannot spread more than a few tens of kilometers from their source. In cooler pyroclastic currents, particles do not aggregate, and the sedimentation process may involve re-entrainment of particles, which potentially leads to more extensive cooling and longer runout; such effects, however, are only significant when net erosion of substrate occurs. Model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios. The model also provides an alternative explanation of the observed decrease in H/Lratios with ignimbrite mass. Received: 10 May 1998 / Accepted: 21 October 1998  相似文献   

水悬浮体系中蒙脱石和凹凸棒石的互相作用及其意义   总被引：7，自引：0，他引：7  

陈天虎  徐惠芳  鲁安怀  岳书仓  彭书传  汪家权  徐晓春 《矿物学报》2003,23(4):313-318

由于晶体结构的制约，蒙脱石和凹凸棒石具有不同的形态和物理化学性质，在水悬浮体系中必定存在蒙脱石和凹凸棒石的互相作用，但这种互相作用的形式和科学意义未曾被人认知。通过蒙脱石和凹凸棒石分散、悬浮液混合、固液分离处理，经XRD、TEM、SSA分析证实，水悬浮体系蒙脱石和凹凸棒石存在胶体互相作用，并且以面-面形式结合。研究结果的理论和实际意义表现在以下三个方面：①为合理解释凹凸棒石蒙脱石混合粘土吸附现象和物理化学性质提供新的视角；②凹凸棒石可以作为蒙脱石粘土的改性剂，在蒙脱石粘土中添加少量的凹凸棒石即可达到提高比表面积的效果，可以利用蒙脱石和凹凸棒石的互相作用来制备类似柱撑粘土的纳米孔结构材料；③凹凸棒石粘土样品湿法处理时，水悬浮体系中不同矿物胶体颗粒互相作用会影响X射线粉末衍射分析结果，对此现象在粘土矿物定量研究中应引起重视。  相似文献   

钢管混凝土中承拱桥吊杆横梁受力性能分析     

曹晓岩  孙明哲  付金科 《世界地震工程》2004,20(2):73-76

通过足尺寸吊杆横梁试件试验，验证了拱肋为钢管混凝土三角形桁架结构吊杆横梁的设计承载力，并对吊杆横梁的受力及变形进行了分析，提出了吊杆横梁施工的改进技术方案，保证了工程设计和施工质量。  相似文献   

Measurements of powder snow avalanche- nature -     

K. Nishimura  F. Sandersen  K. Kristensen  K. Lied 《Surveys in Geophysics》1995,16(5-6):649-660

Snow cloud growth rates of powder snow avalanche were obtained with analysing the pictures recorded in Ryggfonn, Norway. Although results showed wide scattering, as far as the data obtained in runout zone are concerned, they roughly agreed with the water tank experiments by Beghin and Olagne (1991).Air movement in snow avalanche cloud was measured with an ultra-sonic anemometer in Kurobe Canyon, Japan. It showed rising current existed near the front and downward at the trail. Comparing with drifting snow threshold and particle suspension criterion, entrainment, suspension and deposition of snow particles in the snow cloud were discussed.  相似文献   

悬浮液进样-氢化物发生原子荧光光谱法测定土壤中痕量砷锑硒   总被引：5，自引：2，他引：5  

温晓华  邵超英张琢 何中发 《岩矿测试》2007,26(6):460-464

在原子荧光光谱法传统样品前处理的基础上，提出了悬浮液进样技术。用琼脂作为悬浮剂，水溶液作为校正曲线，对土壤标准样品和实际样品进行测定，结果令人满意，并且省去了冗长的样品化学前处理过程。检出限分别为砷0．33μg／L、锑0．13μg／L、硒0．06μg／L；相对标准偏差分别为：砷3．92％～6．32％、锑1．76％～3．11％、硒3．08％～5．54％；回收率分别为砷98．7％～105．7％、锑99．5％-103．3％和硒94．6％-107．2％。国家一级标准物质的测定结果与标准值相符，实际样品的测定值与传统消化法相比较，结果基本一致。  相似文献   

四渡河大跨悬索桥空间地震响应分析   总被引：2，自引：0，他引：2  

张卉  王志清  彭元诚  龙晓鸿 《地震工程与工程振动》2007,27(3):70-76

非一致激励输入对大跨结构响应有着较大影响。本文以四渡河悬索桥为背景,首先根据该桥动力特性对规范反应谱进行调整,基于修正反应谱并考虑相位非平稳性合成地震动位移;然后编制了调索程序来确定有限元模型的初始平衡构型;采用时程法,分别分析了在纵桥向、横桥向以及竖向的非一致激励(行波输入、多点输入)下的悬索桥地震响应。计算结果表明,纵向非一致激励输入增大了悬索桥的地震响应,横向、竖向非一致激励输入的影响有增有减。  相似文献   

Validation of WEPS for soil and PM10 loss from agricultural fields within the Columbia Plateau of the United States     

G. Feng  B. Sharratt 《地球表面变化过程与地形》2007,32(5):743-753

Wind erosion from agricultural fields contributes to poor air quality within the Columbia Plateau of the United States. Erosion from fields managed in a conventional winter wheat–summer fallow rotation was monitored during the fallow period near Washtucna, WA, in 2003 and 2004. Loss of soil and PM10 (particulates ≤10 µm in diameter) was measured during six high wind events (sustained wind speed at 3 m height >6·4 m s^?1). Soil loss associated with suspension, saltation and creep as well as PM10 emission was used to validate the Wind Erosion Prediction System (WEPS) erosion submodel. Input parameters for WEPS simulations were measured before each high wind event. The erosion submodel produced no erosion for half of the observed events and over‐predicted total soil loss by 200–700 kg ha^?1 for the remaining events. The model appears to over‐predict total soil loss as a result of overestimating creep, saltation and suspension. The model both over‐predicted and under‐predicted PM10 loss. High values for the index of agreement (d > 0·5) suggest that the performance of the model is acceptable for the conditions of this study. While the performance of the model is acceptable, improvements can be made in modeling efficiency by better specifying the static threshold friction velocity or coefficients that govern emissions, abrasion and breakage of silt loams on the Columbia Plateau. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献