Debye Entropic Force and Modified Newtonian Dynamics

Verlinde has suggested that the gravity has an entropic origin, and agravitational system could be regarded as a thermodynamical system. It is well-known that the equipartition law of energy is invalid at very low temperature. Therefore, entropic force should be modified while the temperature of the holographic screen is very low. It is shown that the modified entropic force is proportional to the square of the acceleration, while the temperature of the holographic screen is much lower than the Debye temperature T_D. The modified entropic force returns to the Newton's law of gravitation while the temperature of the holographic screen is much higher than the Debye temperature. The modified entropic force is connected with modified Newtonian dynamics (MOND). The constant a₀ involved in MOND is linear in the Debye frequency ω_D, which can be regarded as the largest frequency of the bits in screen.We find that there do have a strong connection between MOND and cosmology in the framework of Verlinde's entropic force, if the holographic screen is takento be bound of the Universe. The Debye frequency is linear in the Hubble constant H₀.     

Surface relaxation and thermal expansion for the (001) face of α-Fe and Cu

The surface relaxation and the near-surface enhancement of thermal expansion have been calculated for the (001) face of a bcc crystal, α-Fe, and an fcc crystal, Cu. The calculations make use of the anharmonic perturbation formalism of Dobrzynski and Maradudin; the results for certain equal-time vibrational correlation functions which arise in this formalism are also presented. The crystal potential is described in terms of several kinds of short-range empirical interatomic potentials, such as have been used in studies of defects in bulk; in the near-surface region, the effects of surface redistribution of the electron distribution are modelled by the addition of a simple surface Madelung (SSM) force. The effect of the SSM force is to limit severely the usual outward relaxation driven by short-range interatomic potentials. For Fe(001), the five and one-half percent outward static relaxation driven by the short-range potentials acting alone is changed to a one percent inward static relaxation when the SSM force is incorporated; for Cu(001), the comparable change is from a one percent outward relaxation to a one-half percent outward relaxation. On the other hand, the SSM force makes only a small effect on the surface-enhanced thermal expansion coefficients (STEC) for interplanar spacings. The STEC for the outermost spacing is between 2.5. and 3.0 times of that for the bulk at the Debye temperature for both Fe(001) and Cu(001); for the second interplanar spacing, the STEC is smaller than 1.5 times of that of the bulk at the Debye temperature. The ratios of the near-surface mean-square amplitudes (MSA) to those of the bulk at high temperatures are, for Fe(001), about 1.75 for z-components (normal to surface) and 1.55 for x-components (parallel to surface) in the surface layer; for Cu(001), about 1.95 for z-components and 1.30 for x-components. The interplanar correlation functions, while smaller than the MSA on an absolute scale, do show considerable surface-enhancement, particularly for the zz-compoments. For example, the zz-correlation between an atom in the outermost layer and its nearest neighbor in the next layer is nearly twice the comparable bulk correlation above the Debye temperature for both Fe(001) and Cu(001).     

The selection of layer thicknesses to control acoustic radiation force profiles in layered resonators

Ultrasonic standing waves can be used to generate radiation forces on particles within a fluid. A number of authors have derived detailed representations of these forces but these are most commonly applied using an approximation to the energy distribution based upon an idealized standing wave within a mode based upon rigid boundaries. An electro-acoustic model of the acoustic energy distribution within a standing wave with arbitrary thickness boundaries has been expanded to model the radiation force on an example particle within the acoustic field. This is used to examine the force profile on a particle at resonances other than those predicted with rigid boundaries, and with pressure nodes at different positions. A simple analytical method for predicting modal conditions for combinations of frequencies and layer thickness characteristics is presented, which predicts that resonances can exist that will produce a pressure node at arbitrary positions in the fluid layer of such a system. This can be used to design resonators that will drive particles to positions other than the center of the fluid layer, including the fluid/solid boundary of the layer, with significant potential applications in sensing systems. Further, the model also predicts conditions for multiple subwavelength resonances within the fluid layer of a single resonator, each resonance having different nodal planes for particle concentration.     

Universality in crossover function for convection in fluids with a free surface

We show that in the onset of convection in a thin fluid layer with a free surface, the passage from surface tension driven to buoyancy driven convection with changing thickness of the fluid layer follows a universal curve and can be calculated very accurately using a variational method. We have shown that the balance between surface tension traction to buoyancy force determines the crossover length scale of the fluid which is independent of viscosity or thermal diffusivity. We suggest a scenario near critical point of fluids in which this crossover can be observed.     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

Wave propagation in heterogeneous layers of the Earth

Abstract

This paper represents the analytical and numerical study of the propagation of horizontally polarized shear wave in two inhomogeneous layers lying over an isotropic homogeneous semi-infinite elastic medium. The inhomogeneity of the upper layer is caused by the exponential variation of rigidity and density with the depth, whereas the second layer exhibits inhomogeneity due to quadratic variation in rigidity of the medium. The dispersion relation has been obtained in closed form using Debye asymptotic expansion which is the major highlight of this study. The comparative study and some important peculiarities have been revealed by means of graphical illustrations.     

The geometrical and vibrational properties of the Rh(111) surface

Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.     

Dynamic stability of a sandwich plate with a constraining layer and electrorheological fluid core

The dynamic stability problems of a sandwich plate with a constraining layer and an electrorheological (ER) fluid core subjected to an axial dynamic force are investigated. The rectangular plate is covered in an ER fluid core and a constraining layer to improve the stability of the system. Effects of the natural frequencies, static buckling loads, and loss factors on the dynamic stability behavior of the sandwich plate are studied in the paper. Rheological property of an ER material, such as viscosity, plasticity, and elasticity may be changed when applying an electric field. The modal damper and the natural frequencies for the sandwich plate are calculated for various electric fields. When an electric field is applied, the damping of the system is more effective. In this study, finite element method and the harmonic balance method are used to calculate the instability regions of the sandwich plate. The ER fluid core is found to have a significant effect on the dynamic stability regions.     

Thermodynamic properties of BaRuO3

We have investigated the specific heat, thermal expansion and thermodynamic properties of BaRuO₃ in rhombohedral, hexagonal and cubic phase using the modified rigid ion model (MRIM) by incorporating the effect of lattice distortions. The calculated Debye temperature, specific heat and other thermal properties reproduce well with the available experimental data, implying that MRIM represents properly the nature of the perovskite-type BaRuO₃. Besides, we have reported the thermal expansion (α), bulk modulus (B), cohesive energy (?), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ _D), and Gruneisen parameter (γ).     

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对等离子体气动激励控制边界层进行了数值仿真。将等离子体气动激励对边界层的作用建模成动量和热量。通过由基于表面放电的二维流体体力模型得到的等离子体气动激励的体力分布函数,得到向边界层注入的动量和热量分布,将动量和热量以源项的形式引入N-S方程求解。研究了等离子体气动激励的强度、激励电极的数目、来流速度的大小以及热量项的大小对等离子体气动激励作用效果的影响,仿真结果与实验一致。     

Screened Forces between two Positive Ions in an Electron Bath

The force between two ions imbedded in a hot and dense electron gas is studied using classical Molecular Dynamics with a regularized Coulomb potential. It is found to be well represented by the Debye form but with a screening length that differs from the Debye length except at weak coupling.     

The recoilless fraction of the oxide surface layer formed on small iron particles

Small iron particles with oxide surface layer were studied by Mössbauer Spectroscopy at various temperatures in the range of 5K to 300K. The Debye temperature of the oxide layer and the temperature dependence of f2/f1 were obtained, where f1 and f2 are the recoilless fractions for the inner α-iron core and the oxide layer respectively. Furthermore, the method of estimating the thickness of the oxide layer was improved.     

翼型绕流电磁控制的实验和数值研究

分布在弱电介质溶液中的电磁力(Lorentz力)，可以有效地控制边界层的流动.利用以转动水槽为主的实验系统和基于双时间步Roe格式的数值方法，对翼型绕流的电磁控制进行了实验和数值研究.结果表明，对于一定攻角的翼型，电磁力可以控制其绕流形态.当电磁力方向与流动方向相同时，可以抑制分离，消除涡街，其效果与减小攻角类似.当电磁力的方向与流动方向相反时，可在流场中形成大涡组成的涡街，增强流体的混合能力，其效果与增大攻角类似.     

Buoyancy Force in Liquid Fluidized Beds of Mixed Particles

The literature represents the buoyancy force on particles in a fluidized bed in two different ways: one is based on the density of the fluid alone and the other is based on the density of the suspension comprised of the fluid and solid. To clarify this problem is especially important for mixing/segregation phenomena in fluidized beds of mixed particles. This paper, presents a brief review of the literature and provides experimental evidence to conclude for the correct expression of the buoyancy force in fluidized beds.     

翼型绕流电磁控制的实验和数值研究

分布在弱电介质溶液中的电磁力(Lorentz力),可以有效地控制边界层的流动.利用以转动水槽为主的实验系统和基于双时间步Roe格式的数值方法,对翼型绕流的电磁控制进行了实验和数值研究.结果表明,对于一定攻角的翼型,电磁力可以控制其绕流形态.当电磁力方向与流动方向相同时,可以抑制分离,消除涡街,其效果与减小攻角类似.当电磁力的方向与流动方向相反时,可在流场中形成大涡组成的涡街,增强流体的混合能力,其效果与增大攻角类似. 关键词： 电磁力 翼型绕流 流体控制     

温度对微界面摩擦影响的研究

以微观界面摩擦为研究对象,分析了温度变化对材料摩擦性能的影响.基于Towle剪切强度-温度经验公式和晶格热动力学理论,推导出摩擦力与温度之间的理论计算公式.理论分析表明:当界面温度低于材料的德拜温度时,摩擦力随着温度的增加而降低.理论计算结果与原子力显微镜实验结果对比,发现二者趋势一致,表明本文提出的理论和方法可行. 关键词： 界面摩擦 真实接触面积 温度 摩擦力     

The mechanism of the attracting acoustic radiation force on a polymer-coated gold sphere in plane progressive waves

Acoustic plane progressive waves incident on a sphere immersed in a nonviscous fluid exert a steady force acting along the direction of wave motion. It is shown here that when an elastic gold sphere is coated with a polymer-type (polyethylene) viscoelastic layer, this force becomes a force of attraction in the long wavelength limit. Kinetic, potential and Reynolds stress energy densities are defined and evaluated with and in the absence of absorption in the layer. Without absorption, the mechanical energy density counteracts the Reynolds stress energy density, which causes a repulsive force. However, in the case of absorption, the attractive force is predicted to be a physical consequence of a mutual contribution of both the mechanical and the Reynolds stress energy densities. This condition provides an impetus for further designing acoustic tweezers operating with plane progressive waves as well as fabricating polymer-coated gold particles for specific biophysical and biomedical applications.     

Acoustic enhancement of diffusion in a porous material

Lattice Boltzmann simulations are used to model the enhancement of diffusion which results from Eckart (attenuation driven) acoustic streaming in model porous material. Comparisons are made to Fickian diffusion where no flow is present and the diffusion when a fluid jet is used, which represents a more conventional method of enhancement. We show that streaming can produce a higher diffusion rate for the same average flow velocity and propose that this is the result of the continuation within the material of the driving force that produces the acoustic streaming.     

电磁力滤波与快速反射镜光学补偿在潜航器光轴稳定控制中的应用

搭载在潜航器上的光电桅杆是光电跟瞄的重要装置.当潜航器在水下高速行进时,海水会在物体表面形成脱体边界层和涡街,涡街的生成和脱体会引起阻力和升力的大幅度波动,从而对光轴稳定性产生极大的扰动.本文首先基于电磁场和流体力学的基本控制方程,通过层次结构网格下的有限体积法探讨了电磁流体表面控制对潜航器绕流流场的影响和消涡减振效果;其次,分析并获得了快速反射镜(fast steering mirror,FSM)的结构特性、传递函数和PID控制策略;最后,以潜航器光路模型为研究背景,结合电磁流体的滤波特性和FSM的传递函数,论证了复合控制对潜载光电跟瞄系统稳定性提高的效果.结果表明,壁面流向电磁力能很好地调控潜航器绕流边界层,抑制涡激振动、减少光学系统的输入噪声,在此基础上通过FSM实现二次补偿,可以进一步提高光学系统跟踪的精度.本研究是电磁流体控制在光电领域的探索,也是对传统流体力学实验方法的拓展,因此具有一定的科学意义和实用价值.