排序方式: 共有47条查询结果,搜索用时 15 毫秒
21.
In the recent experiment on the HL-2A tokamak, two types of improved confinement regimes have been achieved in different configurations. One is the improved confinement regime in limiter configuration during electron cyclotron resonant heating (ECRH), characterized by a sharp decrease in Hα emission accompanied by an increase in the total radiation of plasma, the line averaged electron density and the stored energy of plasma. The other is high confinement regime (H-mode) in divertor configuration during a combination of ECRH and Neutral beam injection (NBI) heating, characterized with edge localized modes (ELMs) besides the features mentioned above. The ELMs are found to be localized on the plasma edge (r/a ≥ 0.8), causing average losses of particles and stored energy in the ranges of about 1--3% and 3--5% respectively during a single ELM event. So far, the ELMs observed in the HL-2A are type III ELMs with low amplitude and high repetition frequency in a range from 200 Hz to 350 Hz. An investigation of the radiated power density profiles shows that radiative cooling effect plays a significant role in the transition back to the L-mode and the triggering of ELM events. 相似文献
22.
We consider the behavior of charged particles with an anisotropic initial velocity distribution in a magnetic trap with approaching mirrors in connection with the problem of particle acceleration in solar flares. We show that, irrespective of the charge sign, the efficiency of confinement and acceleration increases with increasing anisotropy factor of the initial distribution α = (T⊥/T∥)1/2. For a positive electric potential of the trap plasma relative to the mirrors, the emerging additional effect of ion expulsion form the trap increases with αi. The derived estimate of the electric potential suggests an amplification of the initial perturbation and the development of instability. 相似文献
23.
Jean-Gabriel Bréhéret Jean-Jacques Macaire Amélie Fleury Agathe Fourmont Ingeborg Soulié-Märsche 《Comptes Rendus Geoscience》2003,335(5):479-485
The autochthonous magnesian carbonates and the gypsum depicted in the Holocene deposits from the ‘Marais de Sarliève’ (Limagne, France) result from an alkaline, magnesian and sulphated composition of waters. This is also recorded by the composition of algal micro- and macrophytes. This may be explained by the dissolved fluxes from the Oligocene catchment and from a restriction. During the historical period, the lake evolved towards a lesser restriction. To cite this article: J.-G. Bréhéret et al., C. R. Geoscience 335 (2003). 相似文献
24.
《Sedimentology》2018,65(2):597-619
Fluvial channel‐belt clustering has recently been documented using quantitative metrics for systems dominated by autogenic controls. It has long been recognized that allogenic forcing (tectonic and eustatic controls) can lead to confinement of fluvial systems, resulting in clustering of channel belts. To date, no study has quantitatively documented the differences in channel‐belt clustering, compensational stacking of channel belts and interchannel‐belt connectivity in unconfined and confined systems. This study quantitatively compares world‐class outcrops of an unconfined fluvial system (Palaeocene lower Wasatch Formation) with outcrops of a confined fluvial system (Cretaceous Dakota Sandstone). Two new methods have been developed to quantitatively document channel‐belt clustering and intrachannel‐belt connectivity. These new methods, and other previously developed methods, are used to document an increase in channel‐belt clustering and intrachannel‐belt connectivity downdip in both systems. Additionally, it was found that channel belts within the unconfined system stack more compensationally than those in the confined system. These new methods and empirical relationships can be used for predicting intrachannel‐belt connectivity, and accurately modelling unconfined and confined fluvial systems in the subsurface. 相似文献
25.
26.
Nonlinear static analysis procedures are key tools in evaluating the performance of existing buildings and verifying the design
of seismic retrofits in seismically active regions. In this procedure, nonlinear force-displacement or moment-curvature (M-φ) behavior needs to be defined. In the ATC-40 document, values of M-φ have been proposed to model elements in a nonlinear procedure. However, these values need to be investigated to determine
if they are representative of actual values. In this paper, an attempt has been made to numerically derive M-φ curves to simulate actual performance. Then, these curves are compared with the ATC-40 recommended curves with respect to
various parameters. The study indicated that ATC-40 suggested values are conservative in nature in most situations. 相似文献
27.
This paper presents the engineering geological properties and support design of a planned diversion tunnel at the Boztepe dam site that contains units of basalt and tuffites. Empirical, theoretical and numerical approaches were used and compared in this study focusing on tunnel design safety. Rock masses at the site were characterized using three empirical methods, namely rock mass rating (RMR), rock mass quality (Q) and geological strength index (GSI). The RMR, Q and GSI ratings were determined by using field data and the mechanical properties of intact rock samples were evaluated in the laboratory. Support requirements were proposed accordingly in terms of different rock mass classification systems. The convergence–confinement method was used as the theoretical approach. Support systems were also analyzed using a commercial software based on the finite element method (FEM). The parameters calculated by empirical methods were used as input parameters for the FEM analysis. The results from the two methods were compared with each other. This comparison suggests that a more reliable and safe design could be achieved by using a combination of empirical, analytical and numerical approaches. 相似文献
28.
The results of an experimental study aimed at characterizing the behavior of concrete for high confining pressures (up to 500 MPa) are reported. The main characteristics of the response under deviatoric conditions are quasi‐linearity in the elastic regime, stress‐path dependency, and gradual change from compressibility to dilatancy under increasing deviatoric stress. By performing the cyclic triaxial compression tests with several load–creep–unload and reload cycles, the time influence on the overall behavior was detected. Further, whether the main features of the observed behavior can be described within the framework of elastic/viscoplasticity theory was investigated. For this purpose, Cristescu's (Rock Rheology. Kluwer Academic Publishers: The Netherlands, 1989) approach was used. It was shown that the proposed elastic/viscoplastic model captures the main features of concrete behavior at high pressures. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
29.
This study focuses on identifying concrete behavior under severe triaxial loadings (near field detonation or ballistic impacts). In order to reproduce high stress levels with well‐controlled loading paths, static tests have been carried out on concrete samples by mean of a very high‐capacity triaxial press (stress levels on the order of 1 GPa). It is a longstanding fact that the water/cement ratio (W/C), upon entering the concrete composition, is a major parameter affecting the porosity and strength of the cement matrix of hardened concrete. The objective of this article is to quantify the effect of this ratio on concrete behavior under conditions of high confinement. From the composition of a reference ‘ordinary’ concrete (i.e. W/C=0.6), two other concretes have been produced with W/C ratios equal to 0.4 and 0.8, respectively. This article presents experimental results and their analysis regarding the effect of water/cement ratio (W/C) on concrete behavior under high confinement. It shows that when placed under high confinement, concrete behaves like a granular stacking composed of concrete without any influence from the level of cement matrix strength. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
30.
The Effect of the Stress Path on Squeezing Behavior in Tunneling 总被引:1,自引:0,他引:1
Summary The interplay between support systems and the rock when tunneling under squeezing conditions is normally studied by means
of two-dimensional analyses. The present paper shows that the underlying plane strain assumption involved in a two-dimensional
analysis may lead, under certain conditions, to ground pressure and deformation values that are considerably lower than those
produced by stress analyses that take into account spatial effects in the vicinity of the tunnel face. The differences are
due to the stress path dependency in the elasto-plastic behavior of the ground and, more specifically, to the inability of
the plane strain model to map the actual radial stress history, which involves a complete radial unloading (and, later, a
re-loading) of the tunnel boundary over the unsupported span. This inherent weakness of any plane strain analysis is relevant
from the design standpoint, particularly for heavily squeezing conditions that require a yielding support. For the majority
of tunneling conditions and methods, however, involving as they do the completion of a stiff support within a few meters of
the face, the errors introduced by the plane strain assumption are not important from a practical point of view. 相似文献