The energy of electric current sheets     

Low  B. C.  Hu  Y. Q. 《Solar physics》1983,84(1-2):83-98

Solar Physics - The free energy associated with current sheets formed by displacing magnetic dipoles in a highly conducting medium is discussed. Specific models are illustrated, based on the idea...  相似文献   

THE INTERLINKAGE OF LARGE-SCALE SYSTEMS BETWEEN HIGH AND LOW LATITUDES RELATED TO PROPAGATION OF THE PLANETARY WAVE AROUND EAST ASIA IN SUMMER   总被引：1，自引：0，他引：1  

王亚非  藤吉康志 《Acta Meteorologica Sinica》2002,(3)

This study has reviewed recent studies about impact of the Rossby wave propagation on theweather of East Asia.Then,we have focused on the northward progression of the subtropical highaffected by the propagations and their interaction.The following results are obtained:(1)The twokinds of meridional Rossby wave propagations often affect the East Asian weather in summer,which propagate in two conflicting directions(northwest-southeastward and southwest-northeast-ward).This is considered as the interaction of the large-scale systems between high and lowlatitudes with wave propagation,especially to the circulation systems around East Asia.(2)Thenorthward progression of the subtropical high is largely restrained,aecompanied by the southwardshift of the convective activity in middle latitudes,under the strong and persistent Rossby wavepropagation from high to low latitudes.And the opposite role could be found in the Rossby wavepropagation from low to high latitudes.  相似文献   

An approach to the study of minerals using infrared photothermal beam deflection spectroscopy     

M. J. D. Low  J. M. D. Tascon 《Physics and Chemistry of Minerals》1985,12(1):19-22

Infrared (IR) spectra of minerals can be recorded by photothermal beam deflection spectroscopy with minimal sample preparation and even with no sample preparation at all. A modulated IR beam is focussed onto the sample, radiation selectively absorbed by the solid warms the solid and the air over it and brings about a refractive index gradient in the air, so that a laser beam grazing the solid's surface is deflected. Measurements of the deflections result in the IR photothermal absorption spectrum. Sample preparation steps can be avoided entirely, i.e., selected areas on the surface of a large specimed can be examined directly, provided that the laser probe beam can pass over the sample area. A minimal sample preparation, consisting of lightly grinding the sample by hand, increases sensitivity and avoids spectral reflection effects. Examples of application of the technique to carbonate and lead minerals are shown.  相似文献   

The energy of electric current sheets     

Y. Q. Hu  B. C. Low 《Solar physics》1982,81(1):107-119

This paper treats two problems on the formation of electric current sheets in the highly electrically conducting solar atmosphere. The first problem concerns a vertical current sheet formed by decreasing the distance between a pair of parallel magnetic line-dipoles lying on the photosphere. The solution to this problem was given previously by Priest and Raadu. With an interest in the flare phenomenon, they derived a formula for the energy stored through the presence of the current sheet. We show that this formula is incorrect. Firstly, there is an error of sign in the derivation of Priest and Raadu, so that, when corrected, the formula gives a negative value for the stored energy. Secondly, the formula is shown to refer to an energy quite different from the free energy associated with the current sheet. To calculate for the current free energy, it is important to account for the frozen-in condition in the highly conducting photosphere.The second problem of the paper concerns the current sheet formed by increasing the distance between the pair of line-dipoles. A different field configuration results, with a curved current sheet lying transverse to the vertical. An analysis of the energy properties is given, to compare with the properties of the Priest-Raadu model.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

河川枯水径流与极值形成机理研究   总被引：4，自引：0，他引：4  

李秀云  傅肃性  宋现锋 《中国沙漠》1999

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Dust concentration and chondrule formation     

Alexander Hubbard  Mordecai-Mark Mac Low  Denton S. Ebel 《Meteoritics & planetary science》2018,53(7):1507-1515

Meteoritical and astrophysical models of planet formation make contradictory predictions for dust concentration factors in chondrule-forming regions of the solar nebula. Meteoritical and cosmochemical models strongly suggest that chondrules, a key component of the meteoritical record, formed in regions with solids-to-gas mass ratios orders above the solar nebula average. However, models of dust grain dynamics in protoplanetary disks struggle to surpass concentration factors of a few except during very short-lived stages in a dust grain's life. Worse, those models do not predict significant concentration factors for dust grains the size of chondrule precursors. We briefly develop the difficulty in concentrating dust particles in the context of nebular chondrule formation and show that the disagreement is sufficiently stark that cosmochemists should explore ideas that might revise the concentration factor requirements downward.  相似文献   

Efficient system reliability analysis illustrated for a retaining wall and a soil slope     

B.K. Low  J. Zhang  Wilson H. Tang 《Computers and Geotechnics》2011

Although first-order reliability method is a common procedure for estimating failure probability, the formulas derived for bimodal bounds of system failure probability have not been widely used as expected in present reliability analyses. The reluctance for applying these formulas in practice may be partly due to the impression that the procedures to implement the system reliability theory are tedious. Among the methods for system reliability analysis, the approach suggested in Ditlevsen 1979 is considered here because it is a natural extension of the first-order reliability method commonly used for failure probability estimation corresponding to a single failure mode, and it can often provide reasonably narrow failure probability bounds. To facilitate wider practical application, this paper provides a short program code in the ubiquitous Excel spreadsheet platform for efficiently calculating the bounds for system failure probability. The procedure is illustrated for a semi-gravity retaining wall with two failure modes, a soil slope with two and eight failure modes, and a loaded beam with three failure modes. In addition, simple equations are provided to relate the correlated but unrotated equivalent standard normals of the Low and Tang 2007 FORM procedure with the uncorrelated but rotated equivalent standard normals of the classical FORM procedure. Also demonstrated are the need for investigating different permutations of failure modes in order to get the narrowest bounds for system failure probability, and the use of SORM reliability index for system reliability bounds in a case where the curvature of the limit state surface cannot be neglected.  相似文献   

Probabilistic analysis of underground rock excavations using response surface method and SORM   总被引：6，自引：0，他引：6  

Qing Lü  Bak Kong Low 《Computers and Geotechnics》2011

Probabilistic analysis of underground rock excavations is performed using response surface method and SORM, in which the quadratic polynomial with cross terms is used to approximate the implicit limit state surface at the design point. The response surface is found using an iterative algorithm and the probability of failure is evaluated using the first-order and the second-order reliability method (FORM/SORM). Independent standard normal variables in U-space are chosen as basic random variables and transformed into correlated non-normal variables in the original space of random variables for constructing the response surface. The proposed method is first illustrated for a circular tunnel with analytical solutions considering Mohr–Coulomb (M–C) and Hoek–Brown (H–B) yield criteria separately. The failure probability with respect to the plastic zone criterion and the tunnel convergence criterion are estimated from FORM/SORM and compared to those obtained from Monte Carlo Simulations. The results show that the support pressure has great influence on the failure probability of the two failure modes. For the M–C model, the hypothesis of uncorrelated friction angle and cohesion will generate higher non-performance probability in comparison to the case of negatively correlated shear strength parameters. Reliability analyses involving non-normal distributions are also investigated. Finally, an example of a horseshoe-shaped highway tunnel is presented to illustrate the feasibility and validity of the proposed method for practical applications where numerical procedures are needed to calculate the performance function values.  相似文献   

Nonlinear bend stiffener analysis using a simple formulation and finite element method     

LOW Ying Min  SHEEHAN John M 《中国海洋工程》2011,25(4):577-590

Flexible marine risers are commonly used in deepwater floating systems.Bend stiffeners are designed to protect flexible risers against excessive bending at the connection with the hull.The structure is usually analyzed as a cantilever beam subjected to an inclined point load.As deflections are large and the bend stiffener material exhibits nonlinear stress-strain characteristics,geometric and material nonlinearities are important considerations.A new approach has been developed to solve this nonlinear problem.Its main advantage is its simplicity;in fact the present method can be easily implemented on a spreadsheet.Finite element analysis using ABAQUS is performed to validate the method.Solid elements are used for the bend stiffener and flexible pipe.To simulate the near inextensibility of flexible risers,a simple and original idea of using truss elements is proposed.Through a set of validation studies,the present method is found to be in a good agreement with the finite element analysis.Further,parametric studies are performed by using both methods to identify the key parameters and phenomena that are most critical in design.The most important finding is that the common practice of neglecting the internal steel sleeve in the bend stiffener analysis is non-conservative and therefore needs to be reassessed.  相似文献   

System Reliability Assessment for a Rock Tunnel with Multiple Failure Modes     

Qing Lü  Chin Loong Chan  Bak Kong Low 《Rock Mechanics and Rock Engineering》2013,46(4):821-833

This paper presents a practical procedure for assessing the system reliability of a rock tunnel. Three failure modes, namely, inadequate support capacity, excessive tunnel convergence, and insufficient rockbolt length, are considered and investigated using a deterministic model of ground-support interaction analysis based on the convergence–confinement method (CCM). The failure probability of each failure mode is evaluated from the first-order reliability method (FORM) and the response surface method (RSM) via an iterative procedure. The system failure probability bounds are estimated using the bimodal bounds approach suggested by Ditlevsen (1979), based on the reliability index and design point inferred from the FORM. The proposed approach is illustrated with an example of a circular rock tunnel. The computed system failure probability bounds compare favorably with those generated from Monte Carlo simulations. The results show that the relative importance of different failure modes to the system reliability of the tunnel mainly depends on the timing of support installation relative to the advancing tunnel face. It is also shown that reliability indices based on the second-order reliability method (SORM) can be used to achieve more accurate bounds on the system failure probability for nonlinear limit state surfaces. The system reliability-based design for shotcrete thickness is also demonstrated.  相似文献