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We identified a Kelvin-Helmholtz billow from vertical turbulence velocity and instantaneous heat flux signals obtained from
airfoil shear probes and thermistors mounted on a research submarine. The vertical turbulence velocity indicates that the
horizontal scale of the billow was about 3.5 m. The spectral slope of the vertical turbulence velocity component is close
to −2, revealing the flow is two-dimensional. We show a remarkable agreement between the length scales of the observed billow
and those computed from direct numerical simulations based on similar conditions.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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M. K. Rama Varma Raja G. C. Asnani P. S. Salvekar A. R. Jain D. Narayana Rao S. Venkoba Rao P. Kishore M. Hareesh 《Journal of Earth System Science》1999,108(4):287-295
Contrary to the prevalent belief that tropical region is characterized by convective clouds rather than by layer clouds, we
have suggested that deep convective clouds occur on meso-scale, but layer clouds occur on larger synoptic-scale with a relatively
small region of deep convective clouds. Sustenance of deep convective clouds is inhibited by the presence of inertio-gravity
waves, which have alternating layers of upward and downward motion in the vertical. We have also shown that inertio-gravity
waves generate regions of relatively strong horizontal velocity, vertically separated by layers of relatively weak horizontal
velocity. Layers of strong horizontal velocity are created by inertio-gravity wave system through convergence of vertical
flux of horizontal momentum. We have also suggested that horizontal convergence/divergence of moisture flux is generated by
inertio-gravity waves, giving rise to vertically alternating layers of high/low humidity, and visible or sub-visible clouds.
Layers of high humidity become layers of strong radar reflectivity at frequency of 53 MHz at which MST Radar at Gadanki, near
Tirupati, India, operates. These observations, more than 2,50,000 in number, for vertical grid points, spread over all the
months of the year, have helped us, among other observations, to arrive at these conclusions. Further, the analysis suggests
that the main source of strong MST radar reflectivity is not mechanical turbulence as is commonly believed. 相似文献
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本文用二维MHD数值模拟研究了地球磁层顶同时存在速度剪切和磁场剪切时Kelvin-Helmholtz不稳定性(K-H)和撕裂模不稳定性(TM)的耦合过程。在雷诺数和磁雷诺数确定时,Alfvèn马赫数值(MA)对耦合特性起决定性作用。在本文选取的参数条件下,若MA<0.4,自发TM占主导地位;当0.4≤MA<1.4时,TM受到K-H明显调制;如果MA≥1.4,K-H引起的涡旋运动起控制作用,导致一种新的不稳定性产生。该不稳定性称作涡旋撕裂模不稳定性。其饱和后长时间渐近状态由一个大尺度的流体涡旋和同心磁岛组成。在地球磁层顶通量传输事件(FTEs)中它可能起着重要作用。 相似文献
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本文提出一个模型,通过此模型得到土星千米波辐射源(Saturn's Kilometric Radiation Source)的分布.这个模型就是:磁层顶的开尔文-亥姆霍兹不稳定性(Kelvin Helmholtz Instability)会导致阿尔芬波沿着弯曲的磁力线传播到土星极区附近,并在这个过程中使土星电离层附近(1Rs高度,Rs为土星半径)的粒子加速沉降,发生回旋不稳定性(Cyclotron Maser Instability),进而产生R-X模式辐射的非寻常波(Extraordinary wave).我们认为,极区发生回旋不稳定性的区域就是土星千米波辐射源的区域.本文求出了磁层顶附近开尔文-亥姆霍兹不稳定性发生在土星本地时(Saturn's Local Time)11 LT以前,得出了阿尔芬波沿磁力线传播到土星需要约383 min,求解出向阳面磁力线弯曲了约80°,最后得出土星千米波辐射源分布从黎明段的7~9 LT延伸到傍晚段的18 LT,并且黎明段分布的纬度比傍晚段低. 相似文献
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纹层是组成层理的基本单位,既可以是低能深水沉积环境的产物,也可能形成于高能环境中。在受风暴影响的区域,微小的泥质颗粒能够在流动过程中发生絮凝形成絮粒并沉积下来形成纹层。贵州习水吼滩剖面下奥陶统桐梓组碳酸盐岩纹层发育,纹层中风暴沉积构造丰富,包括风暴侵蚀面和渠模等。基于细致的野外观察和室内沉积学研究,将在风暴作用影响下形成的纹层划分为5个类型:平行厚纹层、单向交错薄纹层、平行薄纹层、波状厚纹层和水平厚纹层。采用EDS元素面扫描方法,发现亮、暗纹层中都含有铝、硅和镁元素且分布都比较均匀,而暗纹层所含黏土矿物稍多,所以暗纹层中3种元素的含量比亮纹层的稍高。利用开尔文—赫姆霍兹不稳定性解释了亮、暗纹层的形成机制,即絮粒与未发生絮凝的微小颗粒在开尔文—赫姆霍兹不稳定性作用下交替沉积,其中以絮粒为主的纹层更容易发生重结晶形成亮纹层。根据各纹层不同的垂向结合序列划分出3种风暴序列,且从Ⅰ型到Ⅲ型风暴作用逐渐减弱。 相似文献
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The Kelvin-Helmholtz instability is believed to be an important means for the transfer of energy, plasma, and momentum from the solar wind into planetary magnetospheres, with in situ measurements reported from Earth, Saturn, and Venus. During the first MESSENGER flyby of Mercury, three periodic rotations were observed in the magnetic field data possibly related to a Kelvin-Helmholtz wave on the dusk side magnetopause. We present an analysis of the event, along with comparisons to previous Kelvin-Helmholtz observations and an investigation of what influence finite ion gyro radius effects, believed to be of importance in the Hermean magnetosphere, may have on the instability. The wave signature does not correspond to that of typical Kelvin-Helmholtz events, and the magnetopause direction does not show any signs of major deviation from the unperturbed case. There is thus no indication of any high amplitude surface waves. On the other hand, the wave period corresponds to that expected for a Kelvin-Helmholtz wave, and as the dusk side is shown to be more stable than the dawn side, we judge the observed waves not to be fully developed Kelvin-Helmholtz waves, but they may be an initial perturbation that could cause Kelvin-Helmholtz waves further down the tail. 相似文献
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Masaki N. Nishino Hiroshi Hasegawa Yoshifumi Saito Iannis Dandouras Henri Rème Alessandro Retinò Elizabeth Lucek 《Planetary and Space Science》2011,59(7):502-509
Kelvin-Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. It is excited on the tail-flanks of the Earth's magnetosphere where the flowing magnetosheath plasma and the stagnant magnetospheric plasma sit adjacent to each other. This instability is thought to induce vortical structures and play an important role in plasma transport there. While KHI vortices have been detected, the earlier observations were performed only on one flank at a time and questions related to dawn-dusk asymmetry were not addressed. Here, we report a case where KHI vortices grow more or less simultaneously and symmetrically on both flanks, despite all the factors that may have broken the symmetry. Yet, energy distributions of ions in and around the vortices show a remarkable dawn-dusk asymmetry. Our results thus suggest that although the initiation and development of the KHI depend primarily on the macroscopic properties of the flow, the observed enhancement of ion energy transport around the dawn side vortices may be linked to microphysical processes including wave-particle interactions. Possible coupling between macro- and micro-scales, if it is at work, suggests a role for KHI not only within the Earth's magnetosphere (e.g., magnetopause and geomagnetic tail) but also in other regions where shear flows of magnetized plasma play important roles. 相似文献
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Centred at the interface between the sea-breeze and the return flow aloft, Kelvin-Helmholtz billows (KHB) are an important
feature of the turbulent structure of some sea-breeze circulations (SBCs). In other SBCs, there are no prominent KHBs observed.
Factors governing the appearance of billows are determined from a database of 139 sea breezes, constructed from two years
of summertime surface observations at a site on the south coast of England. Post-frontal oscillations occur in the surface
data for some SBCs and are interpreted as indicating possible KHBs aloft. The SBCs are formed under a wide range of synoptic
conditions, enabling various measures of possible billow occurrence to be related to properties of the large-scale, ambient
flow. Consistent with laboratory experiments of density currents, KHBs are suppressed for propagation into a head wind and
enhanced with a tail wind. They are also found to be enhanced for stronger ambient wind speeds, while large-scale coast-parallel
flow is effective in suppressing the billows. 相似文献
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