A0 mode interaction with a plate free edge: theory and experiments at very low frequency by thickness product

When a plane acoustic wave reaches a medium with an impedance infinite or null, it experiences a phase shift of zero or pi and its amplitude on the edge is maximum or vanishes. The case of a flexion wave (A0 Lamb wave) at a free end is also simple; its amplitude is multiplied by a factor 2 square root 2 and the phase shift is pi/2. The evanescent wave at the origin of these phenomena, perfectly described by the classical flexural plate theory, is identified as the imaginary A1 mode of the exact Rayleigh-Lamb theory. The experiences confirm the theoretical predictions.     

On phase ordering behind the propagating front of a second-order transition

In a real system the heating is nonuniform, and a second-order phase transition to a broken-symmetry phase occurs by propagation of a temperature front. Two parameters, the cooling rate τ _Q and the transition front velocity ν _T determine the nucleation of topological defects. Depending on the relation of these parameters, two regimes are found: in the regime of fast propagation defects are created according to the Zurek scenario for the homogeneous case, while in the slow-propagation regime vortex formation is suppressed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 96–101 (10 January 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Implant assisted-magnetic drug targeting: Comparison of in vitro experiments with theory

Implant assisted-magnetic drug targeting (IA-MDT) was studied both in vitro and theoretically, with extensive comparisons made between model and experiment. Magnetic drug carrier particles (MDCPs) comprised of magnetite encased in a polymer were collected magnetically using a ferromagnetic, coiled, wire stent as the implant and a NdFeB permanent magnet for the applied magnetic field. A 2-D mathematical model with no adjustable parameters was developed and compared to the 3-D experimental results. The effects of the fluid velocity, stent and MDCP properties, and magnetic field strength on the performance of the system were evaluated in terms of the capture efficiency (CE) of the MDCPs. In nearly all cases, the parametric trends predicted by the model were in good agreement with the experimental results: the CE always increased with decreasing velocity, increasing magnetic field strength, increasing MDCP size or magnetite content, or increasing wire size. The only exception was when experiments showed an increase in the CE with an increase in the number of loops in the wire, while the model showed no dependence. The discrepancies between experiment and theory were attributed to phenomena not accounted for by the model, such as 3-D to 2-D geometric and magnetic field orientation differences, and interparticle interactions between the MDCPs that lead to magnetic agglomeration and shearing force effects. Overall, this work showed the effectiveness of a stent-based IA-MDT system through both in vitro experimentation and corroborated theory, with the designs of the ferromagnetic wire and the MDCPs both being paramount to the CE.     

Numerical simulation of experiments on turbulent natural convection of heat generating liquid in cylindrical pool

Available experimental data on heat transfer of a melt with volumetric heat generation are analyzed in order to use them for validating the computer codes that describe a core catcher. The problem for CFD simulation of the experiments on heat transfer by laminar and turbulent natural convection is described. Information that can be obtained from experiments for verifying the models of convective heat transfer in a melt is analyzed. The effect of variable viscosity on the integral heat flux is discussed. Calculation results are represented and compared with experimental data on temperature distribution and integral heat transfer. The calculations are in good agreement with the experiment. The results are numerically extrapolated to the range of Rayleigh numbers up to 7 · 10¹⁶. It is concluded that the CFD calculations with the κ-ɛ turbulence model can be used in problems concerned with analysis of melt convection in a core catcher.     

Electron diamagnetic effect on axial force in an expanding plasma: experiments and theory

The axial force imparted from a magnetically expanding current-free plasma is directly measured for three different experimental configurations and compared with a two-dimensional fluid theory. The force component solely resulting from the expanding field is directly measured and identified as an axial force produced by the azimuthal current due to an electron diamagnetic drift and the radial component of the magnetic field. The experimentally measured forces are well described by the theory.     

Simulation of COPO II Lo experiments on natural convection of heat generating liquid at high Rayleigh numbers

The methodology is described and the results are presented concerning numerical modeling of COPO II Lo experiments on heat transfer in liquid with internal heat generation at very high internal Rayleigh numbers typical for natural convection in a core melt that can appear during progress of severe accident at a nuclear power plant (NPP). The work is keeping in the course of development of CFD-based tool for quantitative analysis of heat transfer in a stratified molten pool of different configurations possible in severe accident scenarios with melt retention in the reactor vessel or in the VVER core catcher. Such CFD methodology would be used for testing of simplified correlation models for simulation of the core melt interaction with NPP structures in system code SOCRAT. During verification the available experimental data on the core melt thermohydraulics were analyzed, and it was concluded that they are insufficient to measures of CFD quality. The data uncertainties, along with the complexities of convective flow, uncertainties of the reactor core melt conditions, limitations of experimental possibilities and of turbulence modeling, actually constrain the multivariate CFD simulations of natural convection at very high Rayleigh numbers. RANS turbulence models only can be efficiently applied here, and they are to be checked for such purposes. In a series of numerical modeling of COPO II Lo experiments and some others, availability of a k-? realizable model with included buoyancy effects was estimated, and the optimal set of CFD options was formed for minimizing numerical artifacts. It was demonstrated that in the investigated range of Rayleigh numbers the k-? model works qualitatively correctly, but is inclined to systematical deformation of the melt boundary heat transfer distribution. This allows one to use this model for qualitative multivariate CFD estimations but requires improvement of the model or finding of its efficient and more exact equivalent.     

Nonlinear dynamics of a convection loop: A quantitative comparison of experiment with theory

The flow in a loop of fluid, heated at constant flux over the bottom half and cooled at constant temperature over the top half, is shown to be qualitatively and quantitatively described by the nonlinear dynamics of the Lorenz model over a wide range of parameters encompassing three different chaotic flow regimes. The boundaries of the five flow regimes are determined experimentally and compared with a scaling relationship which relates the physical parameters of the loop and working fluid to the parameters of the Lorenz model. The characteristics of the three chaotic flow regimes are also discussed.     

热成像系统前置栅网结构的衍射效应分析

针对热像仪前置栅网后产生的伪图像,以经典光学理论为基础,利用光的波动性质分析了栅网的前置对热成像影响的衍射作用机理,并且对非单色光目标情况也做了研究分析,进而建立了基于该成像机理下的数学模型.结果表明,当代入实际状态时的各项参数并用MATLAB对该模型进行图像仿真后,所得仿真图像与实际当中的伪图像相比较,取得了很好的相似性,达到了模型对这种现象进行描述的预期目的,并提出了需要进一步研究的问题和关键技术. 关键词： 热成像 衍射 傅里叶变换 栅网结构     

Inducing desorption of organic molecules with a scanning tunneling microscope: theory and experiments

A scanning-tunneling microscope has been used to induce efficient local desorption of benzene from Si(100) at low currents (<100 pA), sample biases (approximately -2.4 V) and temperatures (22 K). A theoretical model based upon first principles electronic structure calculations and quantum mechanical wave packet dynamics describes this process as occurring via transient ionization of a pi state of the adsorbed molecule. This model accounts for the unexpected efficiency and sharp threshold of the yield.     

The theory of electron-scattering coincidence experiments

A general expression for the electron-scattering coincidence cross section for the reaction A₁(e, e′ X) A₂ with a nuclear target is derived in the one-photon exchange approximation. The result is exact to lowest order in α, the fine-structure constant. It is expressed in terms of four kinematic factors involving the electron scattering variables in the laboratory frame, and four combinations of transition matrix elements of the nuclear current operator expressed in the center-of-momentum (COM) frame. The nuclear matrix elements are decomposed into transition amplitudes of definite angular momentum using a helicity analysis. General expressions for the angular distribution of particle X in the COM frame are then derived. The analysis is independent of the detailed structure of the nucleus and particle X and depends only on general symmetry considerations and the existence of a local electromagnetic current operator for the hadronic target. A unitary transformation from the helicity basis for the final particle X and A₂ to an LS coupling basis is relevant if X is massive and a finite number of total angular momenta J are involved in the reaction. Tables of angular correlation coefficients are given for the case where the initial nucleus A₁ has J₁^π = 0⁺. They constitute one of the most useful results of this paper.Connection is made in the “static limit,” and with the assumption that the reaction proceeds through a finite number of Breit-Wigner resonances with a corresponding factorization of the electroproduction transition matrix elements, to the familiar electromagnetic transition multipole moments involving excitation of a nuclear state J^π. The relation to previous work by de Forest and by Drechsel and Überall is discussed.Analytic expressions for the coincidence cross sections are given for spin-zero systems and some very simple, basic models of nuclear “giant resonance” excitations. It is hoped that they will be useful in obtaining insight into the coincidence cross section and in planning future experiments.Finally, a reanalysis of the recent Stanford data of Calarco et al. on ${}^{12}\text{C}\text{(e, e′ p}{}_0\text{)}{}^{11}\text{B}\text{(}\text{3}\text{2}{}^{\mathrm{?}}\text{)}$ in the vicinity of the giant dipole resonance in ¹²C is carried out using a very simple nuclear model but retaining all the terms in the coincidence cross section, some of which were previously neglected.     

The effect of HF field on diffusion and convection in a plasma column

A cylindrical plasma model with both axial and azimuthal steady magnetic field is used. Nonlinear excitation of time averaged electric currents by the HF field is analyzed. Ponderomotive forces of the HF field also lead to translational and rotational plasma motions. Radial plasma diffusion can be modified by the presence of the HF field at sufficiently low frequencies. The analysis can be applied to problems of anomalous transport.     

Single-walled carbon nanotubes formation with a continuous CO2-laser: experiments and theory

It has been proved [1] that the use of a CO₂-laser system operating in continuous wave mode (cw) can be efficiently used for the production of carbon single-walled nanotubes (SWNTs). In this article we first describe in detail the variable experimental conditions (different ambient gases, static gas pressure, and gas flow) for SWNT formation and summarize the results of the characterization studies of the synthesized materials. Second, we analyze the influence of the different experimental conditions on the SWNTs formation process. We show that the heat transport, kinetic, and diffusion processes allow us to explain seemingly different formation conditions in a qualitative and semi-quantitative agreement with the experimental results. The presented self-consistent scenario for nanotube formation in a gas phase allowed us to propose future experiments on testing the mechanism of nanotube formation.     

Acoustic radiation pressure on a rigid cylinder: an analytical theory and experiments

Simple and general analytical expressions are derived for the acoustic radiation force on a long rigid cylinder with a small diameter, whose axis is perpendicular to the wave propagation. Results are expressed in terms of the time-averaged densities of kinetic and potential energies of the incident sound field. For the case of a standing-wave field, which was used in these experiments, the theoretical results agree well with the experimental observations.     

边界耦合的Marangoni对流边界层问题的近似解析解

研究了由温度梯度引起的Marangoni对流边界层问题.由于动量方程和能量方程的边界条件耦合,利用相似变换将偏微分方程组转化为常微分方程非线性边界值问题.通过巧妙引入摄动小参数对速度和温度边界层方程同时渐近展开求解,得到了问题的近似解析解,并对相应的动量、能量传递特性进行了讨论. 关键词： Marangoni对流 近似解析解 渐近展开     

The solid-liquid interface free-energy of Pb: comparison of theory and experiments

The solid-liquid interface free-energy γ(sl) is a key parameter controlling nucleation and growth during solidification and other phenomena. Different experimental techniques are available for its evaluation but results are often widely scattered and strongly depend on the technique used. One of the best examples in this sense is the case of elemental Pb, with values for γ(sl) differing by as much as 150% between nucleation rate experiments and contact angle data. Even using simple many-body potentials, theoretical calculations of γ(sl) can exceed this level of accuracy and thus be employed to assess the present experimental data. To this purpose, atomistic calculations are performed in conjunction with two different many-body potentials for Pb. These show good agreement with nucleation rate and depression of melting point experiments and support the case for a reassessment of the measurements reported from contact angle data. Possible sources of errors that might have affected these experiments are critically discussed, showing how the magnitude of the anisotropy in the interfacial free energies can be important in closing the gap between the different sets of experimental data.