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961.
The saltation regime is very important for understanding the sediment transport mechanism. However,there is no consensus on a model for the saltation regime. This study answers several questions raised with respect to the Eulerian-Lagrangian modeling of sediment transport. The first question is why the previous saltation models that use different combinations of hydrodynamic forces yielded acceptable results? The second question is which shear lift model(i.e. a shear lift expression and its coefficient) is more appropriate? Another important question is which hydrodynamic forces have greater contributions to the saltation characteristics of a sediment particle? The last question is what are the contributions of the turbulence fluctuations as well as effects of using two-and three-dimensional(2 D and 3 D) models on the simulation results? In order to fairly answer these questions, a systematic study was done by considering different scenarios. The current study is the first attempt to clearly discuss these issues. A comprehensive 3 D saltation model for non-cohesive sediment was developed that includes all the hydrodynamic forces acting on the particle. The random nature of sediment transport was included using turbulent flow and bed-particle collision models. The eddy interaction model was applied to generate a3 D turbulent flow field. Bed-particle collisions were considered using the concept of a contact zone and a corresponding contact point. The validation of the model was done using the available experimental data for a wide range of sediment size(0.03 to 4.8 cm). For the first question, the results indicated that some of the hydrodynamic effects show opposing trends and some have negligible effects. With these opposing effects it is possible to adjust the coefficients of different models to achieve acceptable agreement with the same experimental data while omitting some aspects of the physics of the process. A suitable model for the shear lift force was developed by linking the lift coefficient to the drag coefficient and the contributions of the hydrodynamic forces and turbulence fluctuations as well as the consequences of using of 2 D and 3 D models were studied. The results indicate that the shear lift force and turbulent flow fluctuations are important factors for the saltation of both sand and gravel, and they cannot be ignored. 相似文献
962.
结合均匀场地模型和分层均匀场地模型,采用数值试验的方法研究了场地地震反应分析中侧向人工边界距离的选取问题。分析了各主要因素对人工边界距离选取的影响,得到了如下结论:单侧人工边界距离与土层深度之比L/h是影响人工边界距离选取的主要参数;随着侧向人工边界距离的增加,有限元模型的计算误差并不一直减小,在局部范围内,会出现计算误差随着边界距离的增加而增大的现象;场地硬度、阻尼比和激振频率等因素不会影响计算误差随边界距离的变化规律,但采用自由场边界时受输入地震动频谱特性的影响较为明显;对水平激振下的一维场地模型来说,采用结合自由度边界时所需边界距离最短,自由场边界次之,采用粘性边界达到收敛时所需边界距离最长;无论哪种边界条件,当边界距离大于某一范围后,都能得到稳定的计算结果;相对于均质场地模型来说,实际的分层均匀场地建模时可适当减小侧向人工边界距离。 相似文献
963.
964.
基于地震波场能量构建的能量互相关成像条件,具有易实现、物理意义明确及背向散射压制效果明显等优势.但是,目前构建的能量互相关成像条件仅适用于二阶弹性波方程,难以直接应用于一阶弹性波方程.为此,本文针对一阶弹性波方程,基于能量守恒定理及能量密度,构建以速度-应力为参数的能量范数以表征弹性波场能量,将速度-应力能量范数拓展为能量内积以提取弹性波场反射能量.震源端与检波端的基矢量正方向保持一致的基础上,构建得到可有效压制背向散射的弹性波能量成像条件.数值模拟结果表明:该成像条件可以得到背向散射压制、振幅有效保持的能量成像结果. 相似文献
965.
Based on the output data from 1997 to 2000 obtained by the MITgcm's (general circulation model) adjoint assimilation method, volume, heat and salt transports through the Luzon Strait are calculated. The results indicate that there are obvious different characteristics between 1997 and 1998~2000 on the transports through the Luzon Strait. During 1997, theLuzon Strait had a mean net westward transport of 3.93×106 m3/s with a maximum transport of 7.34×106 m3/s in October. During 1998~2000, the Luzon Strait possessed an annual mean eastward transport of 0.93×106, 1.80×106 and 1.00×106 m3/s respectively with a maximum eastward transport of 4.10×106/3.31×106 m3/s in July 1998/1999 and 2.06×106 m3/s in April 2000, respectively. Moreover, the transports in 1997 indicated a difference from the other years, i.e.,that the ranges of westward inflows expanded more obviously to north of the Luzon Strait and downwards exceedingthose of the other years. The westward inflows expanded horizontally to the north part of the Luzon Strait until 21°N. 相似文献
966.
Based on the historical evolution of the Hangzhou Bay, by making use of the conclusions made by the previous research workers and the integration of concrete data, five distinct impact indicators of the sediment from the Changjiang Estuary and the East China Sea to the Hangzhou Bay are summarized. Numerical calculation and analysis indicate that the scouring and deposition of seabed in the Hangzhou Bay are subject to the direct impact of the evolution of the Changjiang Estuary, and the growth and decline and the direction of the sandy bar at Nanhuizui give traces to the sediment transport between the Changjiang Estuary and the Hangzhou Bay. The transport of sediment from the Changjiang Estuary to the Hangzhou Bay occurs mainly in winter and spring seasons and the increase of the Changjiang River runoff and the decrease of sediment charge have caused scouring in the northern coast of the Hangzhou Bay and the seabed erosion along the frontal margin of the Changjiang River Delta. 相似文献
967.
Based on the historical evolution of the Hangzhou Bay, by making use of the conclusions made by the previous research workers and the integration of concrete data, five distinct impact indicators of the sediment from the Changjiang Estuary and the East China Sea to the Hangzhou Bay are summarized. Numerical calculation and analysis indicate that the scouring and deposition of seabed in the Hangzhou Bay are subject to the direct impact of the evolution of the Changjiang Estuary, and the growth and decline and the direction of the sandy bar at Nanhuizui give traces to the sediment transport between the Changjiang Estuary and the Hangzhou Bay. The transport of sediment from the Changjiang Estuary to the Hangzhou Bay occurs mainly in winter and spring seasons and the increase of the Changjiang River runoff and the decrease of sediment charge have caused scouring in the northern coast of the Hangzhou Bay and the seabed erosion along the frontal margin of the Changjiang River Delta. 相似文献
968.
波浪作用下渤海湾近岸海域污染物的输移扩散规律 总被引:13,自引:3,他引:10
用物理模型实验和数学模型计算相结合的方法,研究了均匀缓坡岸滩上,规则波及不规则波浪作用下形成的沿岸流及其对岸边排放污染物输移扩散的影响。针对渤海湾的地形和主要波浪方向,研究了渤海湾主要排污口附近单纯波浪以及波浪、潮流共同作用下近岸海域内流动速度分布与相应污染物输移扩散规律。结果表明,在渤海湾近岸海域(一定范围内),污染物输移扩散受到波浪作用的影响,表现为平行岸线方向。波浪作用使得远离污染物排放口的滩涂受污染的影响增大。 相似文献
969.
The aim of this study is to examine the annual regime of channel scour and fill by monitoring bed‐elevation changes in a reach of Squamish River in southwestern British Columbia, Canada. Sonar surveys of 13 river cross‐sections in a sandy gravel‐bed single‐channel study reach were repeated biweekly over a full hydrologic year (1995/6). The survey results show that bedload movement occurs as waves or pulses forming bedwaves that appear to maintain an overall coherence with movement downstream. These bedwaves propagate downstream by a mode here termed pulse scour and pulse fill, a process distinguished from the conventional mode of scour and fill commonly associated with flood events (here termed local scour and local fill). Bedwave celerity was estimated to be about 15·5 m d−1 corresponding to a bedwave residence time in the study reach of almost one hydrologic year. The total amount of local bed‐elevation change ranged between 0·22 m and 2·41 m during the period of study. Analysis of the bed‐elevation and flow data reveals that, because of the bedwave phenomenon, there is no simple relation between the mean bed‐elevation and discharge nor any strong linear correlation among cross‐sectional behaviour. The bed‐elevation data also suggest that complex changes to the bed within a cross‐section are masked when the bed is viewed in one dimension, although no definitive trends in bed behaviour were found in the two‐dimensional analysis. Although a weak seasonal effect is evident in this study, the bed‐elevation regime is dominated by sediment supply‐driven fluctuations in bedload transport occurring at timescales shorter than the seasonal fluctuation in discharge. The study also indicates that bed‐elevation monitoring on Squamish River, and others like it, for purposes of detecting and measuring aggradation/degradation must take into account very considerable and normal channel‐bed variability operating at timescales from hours to months. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
970.
本文对监测地下深部气体组分含量变化,在地震预报中的作用作了论述,对开展天山地震带气体地震前兆研究的条件和SP—2305色谱仪观测条件的选择作了分析,最后,对气体测量在地震预报中的效应进行了探讨。 相似文献