排序方式： 共有60条查询结果，搜索用时 31 毫秒
11.
Abstract In a previous paper, Bassom et al. (Proc. R. Soc. Lond. A, 455, 1443–1481, 1999) (BKS) investigated finite amplitude αΩ-dynamo wave trains in a thin turbulent, differentially rotating convective stellar shell; nonlinearity arose from α-quenching. There asymptotic solutions were developed based upon the small aspect ratio ε of the shell. Specifically, as a consequence of a prescribed latitudinally dependent α-effect and zonal shear flow, the wave trains have smooth amplitude modulation but are terminated abruptly across a front at some high latitude θF. Generally, the linear WKB-solution ahead of the front is characterised by the vanishing of the complex group velocity at a nearby point θf; this is essentially the Dee–Langer criterion, which determines both the wave frequency and front location. Recently, Griffiths et al. (Geophys. Astrophys. Fluid Dynam. 94, 85–133, 2001) (GBSK) obtained solutions to the α2Ω-extension of the model by application of the Dee—Langer criterion. Its justification depends on the linear solution in a narrow layer ahead of the front on the short O(θf—θF) length scale; here conventional WKB-theory, used to describe the solution elsewhere, is inadequate because of mode coalescence. This becomes a highly sensitive issue, when considering the transition from the linear solution, which occurs when the dynamo number D takes its critical value D c corresponding to the onset of kinematic dynamo action, to the fully nonlinear solutions, for which the Dee—Langer criterion pertains. In this paper we investigate the nature of the narrow layer for α2Ω-dynamos in the limit of relatively small but finite α-effect Reynolds numbers R α, explicitly ε½ ? R 2 α ? 1. Though there is a multiplicity of solutions, our results show that the space occupied by the corresponding wave train is generally maximised by a solution with θf—θF small; such solutions are preferred as evinced by numerical simulations. This feature justifies the application by GBSK of the Dee—Langer criterion for all D down to the minimum D min that the condition admits. Significantly, the frontal solutions are subcritical in the sense that |D min| ≤ |D c|; equality occurs as the α-effect Reynolds number tends to zero. We demonstrate that the critical linear solution is not connected by any parameter track to the preferred nonlinear solution associated with D min. By implication, a complicated bifurcation sequence is required to make the connection between the linear and nonlinear states. This feature is in stark contrast to the corresponding results for αΩ-dynamos obtained by BKS valid in the limit R 2 α ? ε½, which, though exhibiting a weak subcriticality, showed that the connection follows a clearly identifiable nonbifurcating track. 相似文献
12.
维生素D(VD)是一类具有抗佝偻病作用的类固醇激素,其中维生素D3(VD3)是VD在动物体内的主要存在形式.作为生命活动必不可少的一类物质,VD发挥着广泛且重要的生理学作用.VD在动物体内的代谢过程及相关的代谢酶具有较高的进化保守性,同时也受到多种因素的调控影响.由于鱼类在生活环境方面与陆生脊椎动物存在显著差异,VD在... 相似文献
13.
P.G. Campos J.O. Grimalt L. Berdie J.O. Lopez-Quintero L.E. Navarrete-Reyes 《Organic Geochemistry》1996,25(8):475-488
The isoprenoid, hopanoid and steroid compositions of 15 oils from the most productive oil fields in Cuba were studied to determine source-rock depositional environments and organic matter sources. The oils, which are from the northern geological province of Cuba and can be defined by the position with respect to the overthrust belt, can be grouped into two families: those from the Remedios (1) and those from the Placetas (2) tectonostratigraphic units.Remedios oils contain 17α(H)-diahopane, high relative amounts of 18α(H)-22,29,30-trisnorneohopane and diasteranes, which is indicative of generation from clay-rich source rocks. The crude oils of the Placetas Unit exhibit a sterane/hopane composition consistent with a carbonate origin. Nevertheless, these polycyclic hydrocarbons exhibit significant changes in composition, indicating that several organic matter sources, e.g. a carbonate/evaporitic origin of Varadero and Varadero Sur oils, have contributed to the oils from this Unit. The Remedios oils are more mature [evaluated from the and sterane indices] than the Placetas oils.A wide range of biodegradation levels are encountered in these oils (from 0 to 7–8 using the scale derived by Volkman et al. (1983). The high relative abundance of 25-norhopanes is a distinctive feature of Remedios oils. The presence of these compounds in lightly biodegraded or nondegraded oils corresponds to a mixing of paleobiodegraded oil with more recently sourced nondegraded oil in the reservoir. The most biodegraded oil, Cantel, exhibits and sterane ratios, and hopane ratios that have been altered by microbial attack. 相似文献
14.
The goal of this study is to explain the origin of 234U–238U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with 3He/4He ratios, to gain insight regarding the evolution of groundwater in the region. (234U/238U) activity ratios, or (234U/238U)act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (234U/238U)act, 1.14 ± 0.01, was measured in Ca–HCO3-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little 234U–238U isotopic fractionation. The (234U/238U)act increases to 6.07 ± 0.14 in Na–HCO3–Cl-type groundwater. Preferential migration of 234U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (234U/238U)act and 3He/4He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (234U/238U)act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (14C ages of 6.6 kyrs), with (234U/238U)act of ≥6.07 and large amounts of radiogenic 4He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (234U/238U)act and the addition of excess 4He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced 4He and 234U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic 4He excesses measured in groundwater. 相似文献
15.
The helicity, electromotive force and α-effect produced in a homogeneous, rapidly rotating, electrically conducting fluid by an isolated source of buoyancy at small Elsasser number are calculated, visualized and analyzed. Due to physical symmetries of the system, the integrals of helicity and electromotive force over all space are zero. However, each has a significant non-zero value when integrated over the cross section of the Taylor column. The local α-effect is found to be significantly anisotropic; it is strongest when the applied magnetic field is toroidal and the resulting EMF is parallel to the applied field. 相似文献
16.
Alexander Ruzmaikin Anvar Shukurov Dmitry Sokoloff Sergey Starchenko 《地球物理与天体物理流体动力学》2013,107(1-3):125-139
Abstract We propose a method of derivation of global asymptotic solutions of the hydromagnetic dynamo problem at large magnetic Reynolds number. The procedure reduces to matching the local asymptotic forms for the magnetic field generated near individual extrema of generation strength. The basis of the proposed method, named here the Maximally-Efficient-Generation Approach (MEGA), is the assertion that properties of global asymptotic solutions of the kinematic dynamo are determined by the distribution of the generation strength near its leading extrema and by the number and distribution of the extrema. The general method is illustrated by the global asymptotic solution of the α2-dynamo problem in a slab. The nature of oscillatory solutions revealed earlier in numerical simulations and the reasons for the dominance of even magnetic modes in slab geometry are clarified. Applicability of the asymptotic solutions at moderate values of the asymptotic parameter is also discussed. We confirm this applicability using comparisons with complementary asymptotic expansions and numerical simulations. In particular, this justifies application of the MEGA solutions to estimation of the generation threshold. 相似文献
17.
利用常规观测、FY-2E卫星及EC-Interim 0.5°×0.5°再分析资料对2010-2014年夏季天山及其两侧地区α中尺度对流系统(MαCS)的时空分布特点进行分析,并对典型个例的云图特征和环境条件进行了深入的探讨。结果表明:(1)6月为MαCS出现的高发期且椭圆形MαCS占多数。MαCS形成和发展期主要集中在午后和后半夜,消亡于前半夜,三个时期最易发生时间依次滞后大约2 h,圆形和椭圆形MαCS日频次分别呈单峰和多峰型变化分布。MαCS生命史主要为3~6 h,其中6月生命史分布较广,7-8月较集中;大部分椭圆形MαCS较圆形MαCS生命史和消亡阶段长,圆形MαCS在形成阶段维持时间较长。(2)MαCS多生成于山边平原或浅山区,并在山区主脉上空形成直至成熟,在河谷和山脉两侧的平原区消亡。MαCS成熟期冷云盖长轴长度集中在500~800 km,云顶面积随MαCS出现频次增加而逐渐减小。圆形MαCS发展期移动缓慢,成熟后移速加快,椭圆形MαCS始终移速较慢。MαCS云团TBBmin呈现单峰型且近似正态分布,圆形较椭圆形MαCS的TBB平均梯度大。(3)天山山区MαCS的形成主要是通过层云中多个独立的β中尺度对流云团合并形成。MαCS易发生在高层急流带的抽吸区以及中层低槽前部的辐合上升区,中低层西南和西北气流携带的充沛水汽在大气不稳定层结、不稳定能量持续聚集的背景下辐合上升,促使MαCS不断发展。 相似文献
18.
19.
In mean-field dynamo theory, the electromotive force term 〈u′ × B′〉 due to small-scale fields connects the small-scale magnetic field with the large-scale field. This term is usually approximated as the α-effect, assumed to be instantaneous in time and local in space. However, the approximation is valid only when the magnetic Reynolds number Rm is much less than unity, and is inappropriate when Rm ? 1, which is the condition satisfied in the Earth's core or solar convection zone. We introduce a function φ qr as a non-local and non-instantaneous generalization of the usual α-effect and examine its behaviour as a function of Rm in the range 1/64 ≤ Rm ≤ 10 for a kinematic dynamo model. We use the flow of G.O. Roberts 1972 (Phil, Trans. Roy. Soc. London Ser. A, 1972, 271, 411–454), which is steady and has non-zero helicities and two-dimensional periodicity. As a result, we identify three regions in Rm space according to the behaviour of the function φ qr : (i) Rm ? 1/4, where the function φ qr is local and instantaneous and can be approximated by the traditional α and β effects, (ii) 1/4 ? Rm ? 4, where the deviation from the traditional α and β effects increases and non-localness and non-instantaneousness increase, and (iii) Rm ? 4, where boundary layers develop fully and non-localness and non-instantaneousness are prominent. We show that the non-local memory effect for Rm ? 4 strongly affects the dynamo action and explains an observed augmentation of the growth rate in the dispersion relation. The results imply that the non-local memory effect of the electromotive force should be important in the geodynamo or the solar dynamo. 相似文献
20.
D. Gérard-varet 《地球物理与天体物理流体动力学》2013,107(3-4):171-184
We address mathematical issues raised by the so-called α?effect of dynamo theory, which is a dynamo mechanism arising in conducting flows with small scale fluctuations. Analytical results on the α?effect concern the linear induction equation, and are usually claimed to hold for the whole magnetohydrodynamics (MHD) system, as long as the amplitude of the perturbations is small. We discuss the justification of that claim, in the case of periodic fluctuations of the fields. We show a nonlinear instability result on the MHD system, that predicts dynamo action for a large class of high frequency periodic flows, up to the fully nonlinear regime. 相似文献