Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry   总被引：2，自引：0，他引：2  

Daniel R. Parsons  Ian J. Walker  Giles F. S. Wiggs 《Geomorphology》2004,59(1-4):149

A Computational Fluid Dynamics (CFD) model (PHOENICS™ 3.5) previously validated for wind tunnel measurements is used to simulate the streamwise and vertical velocity flow fields over idealized transverse dunes of varying height (h) and stoss slope basal length (L). The model accurately reproduced patterns of: flow deceleration at the dune toe; stoss flow acceleration; vertical lift in the crest region; lee-side flow separation, re-attachment and reversal; and flow recovery distance. Results indicate that the flow field over transverse dunes is particularly sensitive to changes in dune height, with an increase in height resulting in flow deceleration at the toe, streamwise acceleration and vertical lift at the crest, and an increase in the extent of, and strength of reversed flows within, the lee-side separation cell. In general, the length of the separation zone varied from 3 to 15 h from the crest and increased over taller, steeper dunes. Similarly, the flow recovery distance ranged from 45 to >75 h and was more sensitive to changes in dune height. For the range of dune shapes investigated in this study, the differing effects of height and stoss slope length raise questions regarding the applicability of dune aspect ratio as a parameter for explaining airflow over transverse dunes. Evidence is also provided to support existing research on: streamline curvature and the maintenance of sand transport in the toe region; vertical lift in the crest region and its effect on grainfall delivery; relations between the turbulent shear layer and downward forcing of flow re-attachment; and extended flow recovery distances beyond the separation cell. Field validation is required to test these findings in natural settings. Future applications of the model will characterize turbulence and shear stress fields, examine the effects of more complex isolated dune forms and investigate flow over multiple dunes.  相似文献   

北太平洋副热带逆流区涡旋动能谱的季节性变化及其机制     

李宏杰  徐永生 《海洋与湖沼》2017,48(5):932-943

本文利用了23年的卫星高度计数据和WOA13气候态月平均温盐资料,考察了北太平洋副热带逆流(STCC)区涡旋动能谱及其涡旋尺度季节变化的动力过程。为了揭示其动力机制,本文采用了斜压2.5层模式并结合动能串级的理论进行分析。结果表明,在STCC区由于海洋层结及地转流的垂向剪切发生了季节性变化,从而产生的斜压不稳定是导致涡旋动能谱季节变化的原因。涡旋动能最大的时间发生在5—6月份,滞后于斜压最不稳定发生的时间(3月份)约2—3个月左右,这是由于斜压不稳定产生的扰动需要一定时间才能发展成振幅足够大的涡旋。斜压不稳定提供的能量使得涡旋相互作用加强,产生了动能逆向串级,动能谱向更大尺度转移。涡旋能量尺度在3月份仅为280km,而在9月份达到最高值335km左右。另一方面,我们发现STCC区动能谱斜率及动能谱通量也有季节变化,在涡旋动能最大的5—6月份,当尺度小于罗斯贝变形尺度时,谱斜率达到1k–3,而动能谱通量达到最大值。对STCC区涡动能谱及涡旋尺度季节变化的研究,对深入认识中尺度涡旋的产生及其演变机制有着重要的意义。  相似文献   

地球磁极倒转的星地碰撞成因   总被引：4，自引：2，他引：2  

刘广润  张宏泰 《地球科学》2005,30(3):371-376

地球磁场多次发生南北(正负)磁极位置的变换, 即极性倒转, 这已为大家所公认; 但造成这种极性倒转的原因, 则是迄今未能很好解答的一个难题.基于地球磁场的发电机效应理论和星地碰撞的动力效应研究, 探讨了外星撞击地球造成地磁场极性倒转的可能性.研究表明, 当外星沿与地球自转的正逆不同方向撞击地球时引起的地球转速快慢变化, 可导致地球内部核、幔圈层之间的转速相对快慢关系(相对运动方向)发生改变, 从而受其控制的液核涡旋方向及相应的地磁场方向也会随之改变, 于是就形成地磁极性倒转.这是一个新的思路, 它给地磁极性倒转提出了一个简明的动力机制解释.   相似文献   

Investigation of the Solar Differential Rotation of Compact Magnetic Elements for 1966 – 1986     

D. R. Japaridze  M. S. Gigolashvili  V. J. Kukhianidze 《Solar physics》2009,255(2):203-210

The differential rotation of compact magnetic elements during activity cycles 20 and 21 (1966 – 1986) is studied by using solar synoptic charts. For each hemisphere the compact magnetic elements with the polarity of the circumpolar magnetic field have larger rotation rates than the elements with the opposite polarity. This difference in rotation rates is present during the whole cycle except during the polarity reversal of the circumpolar field.  相似文献   

Secondary flow deflection in the lee of transverse dunes with implications for dune morphodynamics and migration   总被引：3，自引：0，他引：3  

Ian J. Walker  Dan H. Shugar 《地球表面变化过程与地形》2013,38(14):1642-1654

Measurements of lee‐side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee‐side separation vortex over closely spaced transverse dune ridges. A differential deflection mechanism is presented that explains the three‐dimensional pattern of lee‐side airflow structure for a variety of incident flow angles. These flow patterns produce reversed, along‐dune, and deflected surface flow vectors in the lee that are inferred to result in net ‘lateral diversion’ of sand transport over one dune wavelength for incident angles as small as 10° from crest‐transverse (i.e. 80° from the crest line). This lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees. Reversed and multi‐directional flow occurs for incident angles between 90° and 50°. These results document the three‐dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

非感应起电参数化方案的对比性研究   总被引：1，自引：1，他引：0  

侯团结  雷恒池  牛生杰 《气候与环境研究》2009,14(2):143-152

为进一步研究不同的非感应起电机制对模拟云内电场的可适用性,把3种不同的非感应起电参数化方案引入三维强风暴动力和电耦合数值模式,以2005年6月10日发生在长春的一次雹暴过程为例,模拟分析第一次闪电发生前云内电场的发展情况。结果表明,电荷的产生始于过冷水含量中心,并随着过冷水含量中心的向上发展而上移;在-20～-30 ℃层存在一电荷的累积区,雷达回波跃增出现后的10 min左右,电场迅速增长,且在4～8 min内由不足50 kV·m-1增长到110 kV·m-1以上。采用方案一形成的主电荷区为偶极性结构,而采用方案二和方案三形成的为比较常见的三极性结构。对比其他地区的观测资料及模拟结果,表明相同的云体宏微观条件下,不同方案中极性反转温度的不同对云内中低层电荷的分布存在较大的影响。考虑低层正电荷区的发展及冰相粒子与过冷水含量的配置关系,取极性反转温度为-10～-20 ℃,方案二的模拟结果与实际较一致。  相似文献   

An improved displacement-based adaptive pushover procedure based on factor modal combination rule     

Reza Abbasnia  A. Tajik Davoudi  M. M. Maddah 《地震工程与工程振动(英文版)》2014,13(2):223-241

An accurate estimation of the applied load pattern is an essential component in each pushover procedure. Recently, a number of adaptive pushover methods have been proposed in which the effects of the higher modes as well as the progressive changes in the dynamic characteristics of structures are taken into account to compute the applied load pattern. The basic shortcoming of these advanced pushover methods is related to employing the quadratic modal combination rule, whereby the sign reversals of the modal load vectors are suppressed. In this study, an improved displacement-based adaptive pushover method is developed in which the applied load pattern is computed using the factor modal combination rule(FMC). In the proposed procedure, multiple load patterns, depending on the number of the modes considered, are determined in order to take into account the sign reversals of different modal load vectors. The accuracy of the proposed method is verifi ed for seven moment resisting frame buildings of 3, 9 and 20 stories with regularity or vertically geometric and mass irregularities subjected to 60 earthquake ground motion records. The results showed that the proposed methodology is capable of reproducing the peak dynamic responses with very good accuracy.  相似文献   

Optical and SuperDARN radar observations of duskside shock aurora over Zhongshan Station     

LIU Jianjun  HU Hongqiao  HAN Desheng  LIU Yonghua  ZHANG Qinghe  Akira S Yukimatu 《极地研究(英文版)》2013,24(1)

  相似文献   

基于旋转运动硬化理论的修正剑桥模型的改进     

孔亮  惠治鑫  王燕昌 《岩土力学》2007,28(Z1):124

修正剑桥模型是最早建立和得到广泛承认的经典土体弹塑性模型之一,但不能模拟应力路径转折时土体的应力-应变特性以及应力引起的各向异性。将旋转运动硬化理论引入到剑桥模型中,给出了椭圆屈服面的旋转运动硬化机制,在不增加任何模型参数的情况下,把等向硬化的修正剑桥模型扩展为旋转运动硬化模型。扩展的新模型既保留了单调加载时的等向硬化的特性,又能反映应力路径转折时土体的本构特性与应力诱发的各向异性,初步验证了模型的有效性。  相似文献   

华西秋雨起止与秋冬季节大气环流转换   总被引：1，自引：0，他引：1  

袁旭  刘宣飞 《气象学报》2013,71(5):913-924

根据1961—2010年平均的逐候NCEP/NCAR再分析资料、1979—2008年平均的逐候CMAP降水资料以及1961—2010年逐候平均的中国553个台站降水资料,讨论了华西秋雨起止日期与秋冬季大气环流转换特征的关系。结果表明,华西地区降水年变化表现为明显的夏、秋双峰特征,8月4—8日(第44候)为双峰间的低谷,10月8—12日(第57候)以后降水降至年平均以下。由此,将华西秋雨建立和结束日期分别确定为8月9—13日(第45候)和10月8—12日(第57候)。华西秋雨的建立对应于东亚夏季风开始向冬季风转变,其标志性环流调整特征是江南地区的西南风转为东南风。东亚经向海平面气压梯度在8月9—13日(第45候)由南高北低转为南低北高,造成850 hPa江南地区的西南风转为东南风,该东南风与来自孟加拉湾的热带西南季风交汇于华西地区,形成风向和水汽的辐合,使得华西地区的降水在夏峰之后再次增强,华西秋雨由此建立。华西秋雨的结束则对应于孟加拉湾热带西南季风结束和东亚冬季风完全建立,其标志性环流调整特征是孟加拉湾地区的西南风转为东北风。随着东亚纬向海平面气压梯度由北向南依次发生东高西低向东低西高的转变,东亚冬季风也逐步向南推进,9月8—12日(第51候)东北冬季风到达江南地区,10月8—12日(第57候)进一步推进到南海地区,此时来自孟加拉湾的热带西南季风消失,造成华西地区完全受大陆冷高压控制,东亚季风经圈环流也转为冬季型哈得来环流,东亚冬季风完全建立,华西秋雨也随之结束。因此,华西秋雨起止可能与东亚夏季风、南亚夏季风向冬季风的转变时间不同步有关,东亚季风与南亚季风的共同作用使得华西秋雨成为亚洲夏季风在中国大陆上的最后一个雨季。  相似文献