圆柱体出水运动的自由面效应及水冢现象分析

以圆柱体为例,研究自由面附近刚性物体的受力状态,分析垂直运动圆柱体出水时的自由面效应以及水冢现象.建立基于势流理论考虑自由面效应的数学模型,并用边界元方法求解该模型.求解过程中考虑非线性自由面运动学以及动力学边界条件.通过求解控制方程以及边界条件,模拟恒速刚性圆柱体出水时的水冢现象及其穿越水冢过程.通过数值解和实验值的比较验证方法的可靠性.分析结构出水时的压力变化、自由液面变形、圆柱体所受的垂向合外力以及流场速度矢量的变化,并展开讨论.结果表明:考虑自由面效应的非线性方法能够更准确地预报自由面附近刚性运动物体的受力特性.     

The effect of surface contact conditions on grain boundary interdiffusion in a semi-infinite bicrystal

The Kirkendall effect is conditioned by active diffusion as well as by active sources and sinks for vacancies. In the case of grain boundaries under the condition of negligible bulk diffusion, the Kirkendall effect is highly localized and responsible for the formation of an extra material wedge in the grain boundary, which may lead to high stress concentrations. The Kirkendall effect in grain boundaries of a binary system is described by a set of partial differential equations for the mole fraction of one of the diffusing components and for the stress component normal to the grain boundary completed with the respective initial and boundary conditions. The contact conditions of the grain boundary with the surface layer acting as source of one of the diffusing components can be considered as equilibrium ones ensuring the continuity of generalized chemical potentials of both diffusing components. Thus, the boundary conditions are determined by the difference in chemistry (i.e. how the thermodynamic parameters depend on chemical composition) of the grain boundaries and of the surface layer. The simulations based on the present model indicate a drastic influence of the chemistry on the grain boundary interdiffusion and Kirkendall effect.     

The albedo problem with Fresnel reflection

The classic albedo problem for a half-space is generalised to include the effect of refraction at the boundary by inclusion of the Fresnel boundary conditions. The problem is solved using the Wiener-Hopf technique with both specular and diffuse reflection. The non-singular Fredholm integral equations that arise for the surface angular distribution are solved numerically and the solutions are illustrated by a number of results in graphical and tabular form. The significant effect of refraction on the albedo and the associated angular distributions is highlighted.     

On propagation of anti-plane shear waves in piezoelectric plates with surface effect

Based on the surface piezoelectricity model, the anti-plane or horizontally polarized shear (SH) waves propagating in an infinite piezoelectric plate of nano-thickness are investigated to show the surface effect on wave characteristics. The influence on the overall properties of piezoelectric structures resulting from the surface effect is treated as a spring force exerting on the boundary of the bulk. The frequency equations of anti-symmetric and symmetric waves are presented analytically for the electrically short-circuited case. Numerical results show that the wave properties are size-dependent, and the surface effect becomes very pronounced at a high frequency.     

Critical conditions for dislocation nucleation at surface steps

Dislocation nucleation at a surface step is analyzed based on a general variational boundary integral formulation of the Peierls–Nabarro dislocation model. By modelling the surface step as part of the surface of a three-dimensional crack, the free surface effect is taken into account by transferring the half space problem into an equivalent one in the infinite medium. The profiles of embryonic dislocations, corresponding to the relative displacements between the two adjacent atomic layers along the slip planes, are then rigorously solved through the variational boundary integral method. The critical conditions for dislocation nucleation are determined by solving the stress-dependent activation energies required to activate the embryonic dislocations from their stable to unstable saddle-point configurations. In particular, the effect of the step geometry, such as the height of the step, the dip angle of the slip plane and the inclined angle of the step surface on dislocation nucleation, is quantitatively ascertained. The results show that the atomic-scale surface step can rapidly reduce the critical stress required for dislocation nucleation from the surface by nearly an order of magnitude. The decrease in critical stress as a function of the height of the step is more significant for slip planes with smaller dip angles and surface steps with smaller inclined angles.     

Simulation of surface tension in 2D and 3D with smoothed particle hydrodynamics method

The methods for simulating surface tension with smoothed particle hydrodynamics (SPH) method in two dimensions and three dimensions are developed. In 2D surface tension model, the SPH particle on the boundary in 2D is detected dynamically according to the algorithm developed by Dilts [G.A. Dilts, Moving least-squares particle hydrodynamics II: conservation and boundaries, International Journal for Numerical Methods in Engineering 48 (2000) 1503–1524]. The boundary curve in 2D is reconstructed locally with Lagrangian interpolation polynomial. In 3D surface tension model, the SPH particle on the boundary in 3D is detected dynamically according to the algorithm developed by Haque and Dilts [A. Haque, G.A. Dilts, Three-dimensional boundary detection for particle methods, Journal of Computational Physics 226 (2007) 1710–1730]. The boundary surface in 3D is reconstructed locally with moving least squares (MLS) method. By transforming the coordinate system, it is guaranteed that the interface function is one-valued in the local coordinate system. The normal vector and curvature of the boundary surface are calculated according to the reconstructed boundary surface and then surface tension force can be calculated. Surface tension force acts only on the boundary particle. Density correction is applied to the boundary particle in order to remove the boundary inconsistency. The surface tension models in 2D and 3D have been applied to benchmark tests for surface tension. The ability of the current method applying to the simulation of surface tension in 2D and 3D is proved.     

Elastic fields around a nanosized elliptic hole in decagonal quasicrystals

Based on the variational principle, a continuum theory of surface elasticity and new boundary conditions for qua- sicrystals is proposed. The effect of the residual surface stress on a decagonal quasicrystal that is weakened by a nanoscale elliptical hole is considered. The explicit expressions for the hoop stress along the edge of the hole are obtained using the Stroh formalism. The results show that the residual surface stress and the shape of the hole have a significant effect on the elastic state around the hole.     

The rôle of surface shear stress fluctuations in the generation of boundary layer noise

The influence of unsteady wall shear stress on boundary layer noise and wall pressure fluctuations is discussed. It is argued that in the acoustic analogy theory of boundary layer noise the surface shear stress “dipole” characterizes acoustic propagation and not generation. Analytical results are presented in support of this view which, in addition, indicate that the effect of the surface dipole is to dininish rather than enhance boundary layer radiation at low Mach numbers.     

纳米通道粗糙内壁对流体流动行为的影响

纳米流动系统具有高效、经济等优势,在众多领域具有广泛的应用前景.因该类系统具有极高的表面积体积比,致使界面滑移效应对流动具有显著影响.本文采用分子动力学方法以两无限大平行非对称壁面组成的Poiseuille流动为对象,分析了壁面粗糙度与润湿性变化对通道内流体流动的影响.对于不同结构类型的壁面,需要通过水动力位置来确定固液界面位置,准确计算固液界面位置有助于更好地分析界面滑移效应.研究结果表明,上下壁面不对称会引起通道内流场参数分布的不对称,壁面粗糙度及润湿性的变化会影响近壁面附近流体原子的流动特性,由于壁面凹槽的存在,粗糙壁面附近的数密度分布低于光滑壁面一侧.壁面粗糙度及润湿性的变化会影响固液界面位置,肋高变化及壁面润湿性对通道中速度分布影响较大,界面滑移速度及滑移长度随肋高和润湿性的增大而减小;肋间距变化对通道内流体流动影响较小,界面滑移速度和滑移长度基本保持恒定.     

Transport and free carrier electromagnetic phenomena in semiconductor boundary layer: II. Reflectivity with non-normal incidence and surface plasmons

A classical analysis is given of the reflectivity at oblique incidence on semiconductor surfaces. The theory includes the effect of a space charge boundary layer and yields formulae from which one can obtain the dispersion relation for surface plasmons, for which the effects of the boundary layer are particularly important. An application to experimental data on n-InSb gives satisfactory agreement while yielding a plausible estimate of the surface excess ΔN. The electroreflectance is also studied, including numerical evaluations. It is suggested that measurements at almost grazing incidence could yield valuable information on the surface scattering of carriers.     

Advanced Study of Unsteady Heat and Chemical Reaction with Ramped Wall and Slip Effect on a Viscous Fluid

This paper investigate the effect of slip boundary condition, thermal radiation, heat source, Dufour number,chemical reaction and viscous dissipation on heat and mass transfer of unsteady free convective MHD flow of a viscous fluid past through a vertical plate embedded in a porous media. Numerical results are obtained for solving the nonlinear governing momentum, energy and concentration equations with slip boundary condition, ramped wall temperature and ramped wall concentration on the surface of the vertical plate. The influence of emerging parameters on velocity,temperature and concentration fields are shown graphically.     

Effect of the Earth-surface relief on the impurity propagation process in atmosphere

The propagation of impurities polluting the Earth atmosphere is analyzed theoretically with allowance for their diffusion, the wind transport, and the existence of a boundary layer caused by the Earth surface roughness. It is shown that as opposed to the plane surface, the effect of the boundary layer on the total impurity content, which is measured by an IR radiometer, should be taken into account even for relatively small radii of the field of vision of the device. For large radii of the field of vision and for small heights of the boundary layer, the results we obtained previously for the plane surface remain valid.     

Elastic boundary conditions in the presence of the flexoelectric effect

Boundary conditions for the equations of the theory of elasticity modified by the presence of the flexoelectric effect have been obtained. It has been shown that these conditions do not generally coincide with the boundary conditions of the standard theory of elasticity. In particular, stresses that are proportional to the product of the polarization and surface curvature appear on the surface. Due to the boundary conditions, the uniform polarization deforms the crystal, although this polarization does not enter into the three-dimensional equations of equilibrium.     

Vacuum energy in the presence of dielectric and conducting surfaces

The polarization of the electromagnetic vacuum is examined in the neighborhood of dielectric and conducting surfaces and the energy associated with this polarization is shown to depend on a cutoff related to the microstructure of the boundary. The appearance of the cutoff permits the vacuum energy to be expressed in terms of a surface tension and certain higher shape tensions. For the case of a dielectric boundary the surface tension reproduces the Schmitt-Lucas formula which accounts reasonably for the observed surface tensions of many materials. The curvature tension is also calculated and it seems the effect of this energy may well be accessible to experimental verification. For the case of perfectly conducting surfaces the first four shape tensions are calculated. It is shown that previous calculations of the vacuum energy due to perfectly conducting surfaces are in error and these errors are corrected.     

On the impact of non-sphericity and small-scale surface roughness on the optical properties of hematite aerosols

We perform a comparative modelling study to investigate how different morphological features influence the optical properties of hematite aerosols. We consider high-order Chebyshev particles as a proxy for aerosol with a small-scale surface roughness, and spheroids as a model for nonspherical aerosols with a smooth boundary surface. The modelling results are compared to those obtained for homogeneous spherical particles. It is found that for hematite particles with an absorption efficiency of order unity the difference in optical properties between spheres and spheroids disappears. For optically softer particles, such as ice particles at far-infrared wavelengths, this effect can be observed for absorption efficiencies lower than unity. The convergence of the optical properties of spheres and spheroids is caused by absorption and quenching of internal resonances inside the particles, which depend both on the imaginary part of the refractive index and on the size parameter, and to some extent on the real part of the refractive index. By contrast, small-scale surface roughness becomes the dominant morphological feature for large particles. This effect is likely to depend on the amplitude of the surface roughness, the relative significance of internal resonances, and possibly on the real part of the refractive index. The extinction cross section is rather insensitive to surface roughness, while the single-scattering albedo, asymmetry parameter, and the Mueller matrix are strongly influenced. Small-scale surface roughness reduces the backscattering cross section by up to a factor of 2-3 as compared to size-equivalent particles with a smooth boundary surface. This can have important implications for the interpretation of lidar backscattering observations.     

Photostimulated diffusion of atoms adsorbed on a solid surface

A new light-induced effect—surface photodiffusion—is found in the processes of transfer on a solid surface. The essence of the effect is that the energy of electronic excitation of an adsorbed atom is transformed into that of its motion over the surface. It is shown that, under nonuniform illumination of the surface, the adsorbed atoms are moved away from the illuminated region, with the atomic concentration at the boundary between the illuminated and dark regions exceeding the equilibrium value. The dark and photoinduced diffusion coefficients are measured, and the value of the diffusion barrier height is estimated.     

Acoustic boundary layer influence on scale model simulation of sound propagation: Theory and numerical examples

Scale model simulation of sound propagation above a solid surface will give a systematic and calculable error in the predicted sound field, because the acoustic boundary layer above the surface has an apparent admittance which is not invariant under scaling. The typical error is approximately 5 dB depending on the geometrical configuration, scale and frequency. The effect of the acoustic boundary layer admittance is negligible for sound propagation above an acoustically soft surface (e.g., grassland). One may moreover note, with reference to scale model simulation of concert hall acoustics, that the absorption coefficient of a solid surface increases with frequency.     

高精度格式WCNS-E-5计算物面热流

采用加权紧致非线性格式WCNS-E-5,四阶精度的二阶导数差分近似以及四阶精度的边界格式构造了高精度算法,对高超声速粘性流动的物面热流进行数值研究.首先考察了壁面网格雷诺数对驻点热流的影响,然后开展了边界格式对热流计算结果影响的研究,最后对大攻角钝锥绕流进行了数值模拟.研究结果表明:WCNS-E-5能降低边界层内网格分辨率,全场高精度的WCNS-E-5计算得到的流场图像清晰、真实、分辨率高,热流值准确、可靠.     

Effects of surface and interface energies on the bending behavior of nanoscale multilayered beams

A modified continuum model of the nanoscale multilayered beams is established by incorporating surface and interface energies. Through the principle of minimum potential energy, the governing equations and boundary conditions are obtained. The closed-form solutions are presented and the overall Young's modulus of the beam is studied. The surface and interface energies are found to have a major influence on the bending behavior and the overall Young's modulus of the beam. The effect of surface and interface energies on the overall Young's modulus depends on the boundary condition of the beam, the values of the surface/interface elasticity constants and the initial surface/interface energy of the system. The results can be used to guide the determinations of the surface/interface elasticity properties and the initial surface/interface energies of the nanoscale multilayered materials through nanoscale beam bending experiments.     

Analysis of oscillations of concentration in bounded binary mixtures taking into account surface effects

We consider a binary mixture bordering a substrate and vacuum and forming a film. The distribution of one of the mixture components is described by the Cahn-Hillard equation with asymmetric boundary conditions taking into account (wetting) processes occurring at the boundary and in surface layers of the film and, probably, the effect of a constant external magnetic field. The parameters of the problem characterize the bulk correlation length, pair interaction intensity in the nearest neighbor approximation at the film boundary, and the renormalized magnetic field acting on the spins in the surface layers of the system. The range of parameters generating oscillations of the concentration over the film thickness is determined. Subranges of parameters generating the preset number of oscillations over the film cross section are indicated. The analytic results are qualitatively confirmed in numerical experiments.