On Deformed Riemannian Extensions Associated with Twin Norden Metrics

The main purpose of this paper is to study the deformed Riemannian extension▽g +VG··in the cotangent bundle, where G is a twin Norden metric on the base manifold.     

HT－7托卡马克中等离子体平衡研究

本文解决了二维轴对称近似下带铁芯的托卡马克中等离子体平衡问题，计算了ＨＴ－７托卡马克中的等离子体平衡位形以及极向场系统的非线性电感和垂直场系数。最后应用Ｋｉｒｃｈｈｏｆｆ方程组和平衡垂直场公式得到了一组等离子体、加热场和垂直场线圈的电流波形的自洽曲线。     

On the reversed flow and oscillating wake in an asymmetrically heated channel

The detailed processes of flow reversal in a buoyancy-induced flow through a one-side-heated vertical channel of finite height were simulated numerically. It is of interest to note that the wake above the heated plate is oscillatory at high Rayleigh number and there exists a minimum in the transient variation of the average Nusselt number. Additionally, the predicted steady average Nusselt number and induced flow rate are correlated by empirical equations.     

邻二氮杂苯-水复合物氢键相互作用的理论研究

用密度泛函理论B3LYP方法和MP2方法分别对邻二氮杂苯与水形成1∶1和1∶2复合物的基态氢键结构与相互作用能进行了理论计算,结果表明复合物之间存在较强的氢键N…H-O相互作用,在复合物中,水的H-O对称伸缩振动频率明显红移.同时,使用含时密度泛函理论方法计算了邻二氮杂苯单体及复合物的第一1(n,π)和1(π,π)激发态的垂直激发能.     

Formulation of a linear three-dimensional hydrodynamic sea model using a Galerkin-eigenfunction method

The three dimensional linear hydrodynamic equations which describe wind induced flow in a sea are solved using the Galerkin method. A basis set of eigenfunctions is used in the calculation. These eigenfunctions are determined numerically using an expansion of B-splines. Using the Galerkin method the problem of wind induced flow in a rectangular basin is examined in detail. A no-slip bottom boundary condition with a vertically varying eddy viscosity distribution is employed in the calculation. With a low (of order 1 cm²/s) value of viscosity at the sea bed there is high current shear in this region. Viscosities of the order of 1 cm²/s) value of viscosity at the sea bed there is high current shear in this region. Viscosities of the order of 1 cm²/s near the sea bed together with high current shear in this region are physically realistic and have been observed in the sea. In order to accurately compute the eigenfunctions associated with large (of order 2000 cm²/s at the sea surface to 1 cm²/s at the sea bed) vertical variation of viscosity, an expansion of the order of thirty-five B-splines has to be used. The spline functions are distributed through the vertical so as to give the maximum resolution in the high shear region near the sea bed. Calculations show that in the case of a no-slip bottom boundary condition, with an associated region of high current shear near the sea bed, the Galerkin method with a basis set of the order of ten eigenfunctions (a Galerkin-eigenfunction method) yields an accurate solution of the hydrodynamic equations. However, solving the same problem using the Galerkin method with a basis set of B-splines, requires an expansion of the order of thirty-five spline functions in order to obtain the same accuracy. Comparisons of current profiles and time series of sea surface elevation computed using a model with a slip bottom boundary condition and a model with a no-slip boundary condition have been made. These comparisions show that consistent solutions are obtained from the two models when a physically relistic coefficient of bottom friction is used in the slip model, and a physically realistic bottom roughness length and thickness of the bottom boundary layer are employed in the no-slip model.     

A solution for the vertical variation of stress,rather than velocity,in a three-dimensional circulation model

A simple technique is presented that allows a numerical solution to be sought for the vertical variation of shear stress as a substitute for the vertical variation of velocity in a three-dimensional hydrodynamic model. In its most general form the direct stress solution (DSS) method depends only upon the validity of an eddy viscosity relation between the shear stress and the vertical gradient of velocity. The rationale for preferring a numerical solution for shear stress to one for velocity is that shear stress tends to vary more slowly over the vertical than velocity, particularly near boundaries. Consequently, a numerical solution can be obtained much more efficiently for shear stress than for velocity. When needed, the velocity profile can be recovered from the stress profile by solving a one-dimensional integral equation over the vertical. For most practical problems this equation can be solved in closed form. Comparisons are presented between the DSS technique, the standard velocity solution technique and analytical solutions for wind-driven circulation in an unstratified, closed, rectangular channel governed by the linear equations of motion. In no case was the computational effort required by the velocity solution competitive with the DSS when a physically realistic boundary layer was included. The DSS technique should be particularly beneficial in numerical models of relatively shallow water bodies in which the bottom and surface boundary layers occupy a significant portion of the water column.     

Performance of optical chaotic communication systems using multimode vertical cavity surface emitting lasers

Influence of side modes on the performance of optical chaotic communication systems is studied theoretically. Three coding techniques, namely chaotic modulation, chaotic masking and chaotic shift keying, are also considered in the investigation. It can be shown that communication system using chaotic shift keying has better immunity to side mode. On the other hand, frequency division multiplexing using multimode vertical cavity surface emitting laser is proposed to double the transmission capacity of the communication system.     

Investigation of Adsorption of Xanthan Gum on Enamel by Tapping Mode Atomic Force Microscopy

By using tapping mode atomic force microscopy (TMAFM), a polymer layer was found on the enamel surface after the exposure to xanthan gum solutions. The layer thickness is closely related to the exposure time and the concentration of xanthan gum solution. The thickness data were evaluated by a Kruskal-Wallis test and Box-Whisker Plot at a 95% confidence level (p＜0. 05), and a statistically significant difference among the thickness data groups was demonstrated. After the exposure to 1000, 400, 100 mg/L xanthan gum solutions, the mean of layer thickness at the adsorption equilibrium is in the ranges of 103.5--122.06,82.4--88.94 and 45. 27--55.55 nm, respectively. This phenomenon suggests that the viscosity modifying a-gents in the beverage might be adsorbed on the enamel surface during consumption, which may form a barrier that can protect the enamel from being attacked by acid and therefore reduce dental erosion.     

Prediction of the interfacial shear-stress in vertical annular flow

Many improvements of the Wallis correlation for the interfacial friction in annular flow have been proposed in the literature. These improvements give in general a better fit to data, however, their physical basis is not always justified. In this work, we present a physical approach to predict the interfacial shear-stress, based on the theory on roughness in single-phase turbulent pipe flows. Using measured interfacial shear-stress data and measured data on roll waves, which provide most of the contribution to the liquid film roughness, we show that the interfacial shear-stress in vertical annular flow is in very close agreement with the theory. We show that the sand-grain roughness of the liquid film is not equal to four times the mean film thickness, as it is assumed in the Wallis correlation. Instead, the sand-grain roughness is proportional to the wave height, and the proportionality constant can be predicted accurately using the roughness density (or solidity). Furthermore, we show that our annular flow, which is in similar conditions to others in the literature, is fully rough. Hence, the bulk Reynolds number should not appear in the prediction of the interfacial friction coefficient, as is often done in the improvements of the Wallis correlation proposed in the literature.     

Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies

Aquatic flow over a submerged vegetation canopy is a ubiquitous example of flow adjacent to a permeable medium. Aquatic canopy flows, however, have two important distinguishing features. Firstly, submerged vegetation typically grows in shallow regions. Consequently, the roughness sublayer, the region where the drag length scale of the canopy is dynamically important, can often encompass the entire flow depth. In such shallow flows, vortices generated by the inflectional velocity profile are the dominant mixing mechanism. Vertical transport across the canopy–water interface occurs over a narrow frequency range centered around f _v (the frequency of vortex passage), with the vortices responsible for more than three-quarters of the interfacial flux. Secondly, submerged canopies are typically flexible, coupling the motion of the fluid and canopy. Importantly, flexible canopies can exhibit a coherent waving (the monami) in response to vortex passage. This waving reduces canopy drag, allowing greater in-canopy velocities and turbulent stresses. As a result, the waving of an experimental canopy reduces the canopy residence time by a factor of four. Finally, the length required for the set-up and full development of mixing-layer-type canopy flow is investigated. This distance, which scales upon the drag length scale, can be of the same order as the length of the canopy. In several flows adjacent to permeable media (such as urban canopies and reef systems), patchiness of the medium is common such that the fully developed condition may not be representative of the flow as a whole.