对山字型构造初应力场解析解的受力模型和边界面力的探讨

本文对李四光先生提出的山字型构造受力模型的选择、面力和解析解的关系等进行了探讨,并研究了另一种受力模型的面力和解析解的变化规律。通过对这两种受力模型合理性的分析,明确指出较为合理的受力模型。

露采高边坡模型初始地应力场快速生成方法研究

为防止露采高边坡建模中初始地应力场畸变,采用子模型法和等效应力梯度边界面力法,用等效应力梯度边界面力替代简化部分水平构造应力,施加侧边界条件。以黄山石灰石矿山高边坡为工程实例,建立取至负地形的高边坡整体计算模型和简化子模型,计算对比整体大模型与简化子模型初始地应力场模拟结果。结果表明:整体大模型与简化子模型水平方向和竖直方向应力误差较小,证明此数值模型简化法可行;以工程边界作为简化子模型侧边界,节点和单元简化率约52%,可快速模拟初始地应力场,提高计算效率。研究结果为高边坡整体大模型初始地应力快速生成提供新思路。

Scattering wave field around a cavity with circular cross-section embedded in saturated soil using boundary element method

Based on Biot’s theory and considering the properties of a cavity,the boundary integral equations for the numerical simulation of wave scattering around a cavity with a circular cross-section embedded in saturated soil are obtained using integral transform methods.The Cauchy type singularity of the boundary integral equation is discussed.The effectiveness of the properties of soil mass and incident field on the dynamic stress concentration and pore pressure concentration around a cavity is analyzed.Our results are in good agreement with the existing solution.The numerical results of this work show that the dynamic stress concentration and pore pressure concentration are influenced by the degree of fluid–solid coupling as well as the pore compressibility and water permeability of saturated soil.With increased degree of fluid–solid coupling,the dynamic stress concentration improves from 1.87 to 3.42 and the scattering becomes more significant.With decreased index of soil mass compressibility,the dynamic stress concentration increases and its maximum reaches 3.67.The dynamic stress concentration increases from 1.64 to 3.49 and pore pressure concentration improves from 0.18 to 0.46 with decreased water permeability of saturated soil.

Stress field near an interface edge of linear hardening materials

The elastic-plastic singular stress field near an interface edge of bounded linear hardening material is substantially as same as that of bonded elastic materials whose Young' s modulus and Poisson ratio are substituted by equivalent values, respectively. Further investigation by the elasto-plastic boundary element method (BEM) on the stress field near the interface edge showed that the stress field there can be divided into three regions: the domain region of the elastic-plastic singular stress field, the transitional region and the elastic region. The domain region of the elastic-plastic singular stress becomes larger with the increasing of the linear hardening coefficient. When the linear hardening coefficient decreases to a certain value, the effective stress in most of the yield zone equals approximately the yield stress. The stress distribution in the elastic region under small-scale yielding condition was also investigated.

Modeling of strength and deformation of overconsolidated clays based on bounding surface plasticity

An elastoplastic constitutive model for overconsolidated clays is established in the framework of the critical state theory and bounding surface plasticity theory. The bounding surface is defined as the maximum yield surface in the loading history. A yielding ratio, i.e., an internal variant, is defined as the size ratio of the current yield surface to the corresponding bounding surface. The yielding ratio instead of the overconsolidation ratio(OCR) is used to evaluate the strength and stress-strain behaviors of overconsolidated clays in the shearing process. The bounding stress ratio incorporating the effect of the yielding ratio is used to characterize the potential failure strength of the overconsolidated clays. The dilation stress ratio taking into account the effect of the yielding ratio is applied to describe the dilatancy behaviors of the overconsolidated clays. Comparisons between model predictions and test data show that the proposed model could well capture the strength and stress-strain behaviors of normally consolidated and overconsolidated clays.

A Study on the Transport Process in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells

Gas diffusion layer(GDL) plays a great important role in proton exchange membrane fuel cell(PEMFC).Water transport mechanism in GDL is still not clear.In the present study,an ex-situ transparent setup is built to visualize the transport phenomena and to measure the threshold pressure of water in GDL at different temperatures.It is found that the relationship between the breakthrough pressure and the temperature is nearly linear(i.e.the pressure decreases linearly with the increase of temperature).To avoid the problems faced by the continuum models,the pore network model is developed to simulate the liquid water transport through the carbon paper.A uniform pressure boundary condition is used in simulation and the results are similar to the ones obtained in the experiment.The reason is that the contact angle and surface tension coefficient of water in GDLs change accordingly with the change of temperature.

Experimental Study of Turbulent Boundary Layers on Groove/Smooth Flat Surfaces

This paper presents an experimental investigation of the turbulent boundary layers on both groove and smooth flat surfaces. The flow structures were shown in a water tunnel using the hydrogen-bubble flow visualization technique. The measurement results indicate that: (1) the grooves can effectively reduce accumulation of low-speed fluids, decrease the number of the low-speed streaks and depress oscillation of the streaks in the sublayer; (2) the grooves can restrain forming of the horseshoe vortices in the buffer region; (3) the grooves bate oscillation and kinking of the quasi-streamwise vortices and restrain production of the hairpin vortices and the ring vortices, reducing both frequency and intensity of the turbulence bursting; (4) the grooves directly affect the flow structures in the sublayer of the boundary layer and then modulate the flow field up to the buffer region and the logarithmic region by restraining development and interaction of the vortices.

Relationship of atmospheric boundary layer depth with thermodynamic processes at the land surface in arid regions of China

The atmospheric boundary layer （ABL） is an important physical characteristic of the Earth＇s atmosphere. Compared with the typical ABL, the ABL in arid regions has distinct features and is formed by particular mechanisms. In this paper, the depth of the diurnal and nocturnal ABLs and their related thermodynamic features of land surface processes, including net radiation, the ground-air temperature difference and sensible heat flux, under typical summer and winter conditions are discussed on the basis of comprehensive observations of the ABL and thermodynamic processes at the land surface carried out in the extreme arid zone of Dunhuang. The relationships of the ABL depth in the development and maintenance stages with these thermodynamic features are also investigated. The results show that the depth of the ABL is closely correlated with the thermodynamic features in both development and maintenance stages and more energy is consumed in the development stage. Further analysis indicates that wind velocity also affects ABL development, especially the development of a stable boundary layer in winter. Taken together, the analysis results indicate that extremely strong thermodynamic processes at the land surface are the main driving factor for the formation of a deep ABL in an arid region.

Numerical simulation of gurney flaps lift-enhancement on a low reynolds number airfoil

Two-dimensional steady Reynolds-averaged Navier-Stokes （RANS） equations with transition shear stress transport （SST） model were solved to investigate the effects of Gumey flaps on the aerodynamic performance of a low Reynolds number airfoil. This airfoil was designed for flight vehicles operating at 20 km altitude with freestream velocity of 25 rn/s. The chord length （C） of this airfoil is 5 m and the corresponding Reynolds number is 7.76× 10^5. Gurney flaps with the heights ranging from 0.25%C to 3%C were investigated. It has been shown that Gurney flaps can enhance not only the prestall lift but also lift-to-drag ratio in a certain range of angles of attack. Specially, at cruise angle of attack （3°）, Gurney flap with the height of 0.5%C can increase lift-to-drag ratio and lift coefficient by 1.6% and 12.8%, respectively. Furthermore, the mechanisms of Gumey flaps to improve the aerodynamic performance were illustrated by analyzing the surface pressure distribution, streamlines and trailing-edge flow structure for this low Reynolds number airfoil. Specially, distinguished from some other numerical researches, the flow details such as the laminar separation bubble and transition phenomena for low Reynolds number airfoil with Gumey flaps were investigated and it was found that Gurney flaps can delay the transition onset position at small angles of attack （≤2°）. However, with the increase of angles of attack, Gurney flaps will promote the boundary layer transition.

Implementation of a Roughness Element to Trip Transition in Large-eddy Simulation

In aerodynamics, the laminar or turbulent regime of a boundary layer has a strong influence on friction or heat transfer. In practical applications, it is sometimes necessary to trip the transition to turbulent, and a common way is by use of a roughness element （e.g. a step） on the wall. The present paper is concerned with the numerical im- plementation of such a trip in large-eddy simulations. The study is carried out on a flat-plate boundary layer con- figuration, with Reynolds number Rex=l.3x 106. First, this work brings the opportunity to introduce a practical methodology to assess convergence in large-eddy simulations. Second, concerning the trip implementation, a volume source term is proposed and is shown to yield a smoother and faster transition than a grid step. Moreover, it is easier to implement and more adaptable. Finally, two subgrid-scale models are tested： the WALE model of Nic0ud and Ducros （Flow Turbul. Combust., vol. 62, 1999） and the shear-improved Smagorinsky model of Ldv^que et al. （J. Fluid Mech., vol. 570, 2007）. Both models allow transition, but the former appears to yield a faster transition and a better prediction of friction in the turbulent regime.

Experimental Investigation of Dimensionless Velocity and Shearing Stress in Boundary Layer Flow on Continuous Moving Surface in Power Law Fluids

An analysis is carried out to study the steady flow characteristics from a continuous flat surface moving in a parallel free stream of non-Newtonian power law fluid. The constitutive equations of the fluid are transformed into dimensionless ones. The velocity field is measured by Particle Image Velocimetry. Experimental results are obtained for the distribution of velocity. The influence of wall velocity ratio parameter on boundary layer flow field is observed in the experiment. Dimensionless velocity distribution and shearing stress distribution are obtained by post-processing experimental results. The effects of various physical parameters like velocity ratio parameter and similarity variable on various momentum transfer characteristics are discussed in detail and shown graphically. It is indicated that dimensionless velocity increases with velocity ratio parameter and similarity variable, and that dimensionless shearing stress decreases with velocity ratio parameter and similarity variable.

Impact of Lower Boundary Condition of Richards＇ Equation on Water, Energy, and Soil Carbon Based on Coupling Land Surface and Biogeochemical Models

Soil moisture has a significant influence on water, energy, and carbon biogeochemical cycles. A numerical method for solving Richards＇ equation is usually used for simulating soil moisture. Selection of a lower boundary condition for Richards＇ equation will further affect the simulation results for soil moisture, water cycle, energy balance, and carbon biogeochemical processes. In this study, the soil water movement dynamic sub-model of a hydrologically based land surface model, the variable infiltration capacity （VIC） model, was modified using the finite difference method （FDM） to solve a mixed form of Richards＇ equation. In addition, the VIC model was coupled with a terrestrial biogeochemical model, the Carnegie Ames Stanford Approach model of carbon, nitrogen, and phosphorus （CASACNP model）. The no-flux boundary （NB） and free-drainage boundary （FB） were selected to investigate their impacts on simulations of the water, energy, and soil carbon cycles based on the coupling model. The NB and FB had different influences on the water, energy, and soil carbon simulations. The water and energy simulations were more sensitive, while the soil carbon simulation was less sensitive to FB than to NB. Free-drainage boundary could result in lower soil moisture, evaporation, runoff, and heterotrophic respiration and higher surface soil temperature, sensible heat flux, and soil carbon content. The impact of the lower boundary condition on simulation would be greater with an increase in soil permeability. In the silt loam soil case, evaporation, runoff, and soil respiration of FB were nearly 169, 13%, and 1% smaller, respectively, compared to those of NB.

The effects of slip condition and fluid flow through a channel multiple stenoses on micropolar of non-uniform cross-section

In this paper, steady incompressible micropolar fluid flow through a non-uniform channel with multiple stenoses is considered. Assuming the stenoses to be mild and using the slip boundary condition, the equations governing the flow of the proposed model are solved, and closed-form expressions for the flow characteristics （resistance to flow and wall shear stress） are derived. The effects of different parameters on these flow characteristics are analyzed. It is observed that both the resistance to the flow and the wall shear stress increase with the heights of the stenoses and the slip parameter; but decrease with the Darcy number, b^rthermore, the effects of the wall exponent parameter, the cross-viscosity coefficient and the micropolar parameter on the flow characteristics are discussed.