基于CFD的船舶斜浪三自由度运动仿真研究

针对船舶在斜浪中的多自由度耦合运动问题, 建立了三维粘性流耐波性数值波浪水池, 采用边界条件造波法生成高精度的斜向规则波, 通过同时求解RANS方程和刚体运动学方程, 结合网格整体移动方法和滑移网格方法, 实现了船舶斜浪航行的垂荡、纵摇及横摇三自由度耦合运动数值模拟。给出了DTMB5512船模斜浪中的垂荡、纵摇及横摇的频率响应函数, 与线性切片理论计算结果进行比较, 吻合良好。该方法可为船舶斜浪航行的耐波性预报提供一种有效的途径。     

A radar ocean imaging model for small to moderate incidence angles

Based on a first-principles scattering theory applicable to small to moderate incidence angles, a new imaging model for ocean features is proposed. In contrast to the Alpers and Hennings Bragg-based model, the new model incorporates the full ocean wave spectrum, utilizes Hughes' suggested spectral decay rate formula and contains no adjustable parameters. For typical ocean currents, the new model predicts realistic values for L-band cross section modulations and comparable L-band and X-band cross section modulations. These latter results are shown to be due to backscatter from ocean waves that are significantly longer than the Bragg resonant wave. By comparison the Bragg-based imaging model is shown to predict that X-band modulations will be at least one order of magnitude weaker than L-band modulations.     

Signatures of resonant and non-resonant scattering mechanisms on radar images of internal waves

The results of two polarization airborne radar imagery tests of the ocean surface obtained during the JUSREX'92 experiment are presented. It is shown that the traditional composite surface model with small-scale 'Bragg' waves superposed over larger gravity waves can not explain either the contrasts of internal wave surface manifestations in conditions of a stable atmospheric boundary layer at low grazing angles (LGA), or the apparent difference between the images obtained at different polarizations in unstable atmospheric conditions. We attribute this discrepancy to the presence of mesoscale steep waves, which produce non-resonant scattering and make different relative contributions to the total cross sections for the two polarizations. The possibility of distinguishing between surface manifestations of atmospheric and oceanic origin is also discussed.     

Numerical calculation of the runaway electron distribution function and associated synchrotron emission

Synchrotron emission from runaway electrons may be used to diagnose plasma conditions during a tokamak disruption, but solving this inverse problem requires rapid simulation of the electron distribution function and associated synchrotron emission as a function of plasma parameters. Here we detail a framework for this forward calculation, beginning with an efficient numerical method for solving the Fokker–Planck equation in the presence of an electric field of arbitrary strength. The approach is continuum (Eulerian), and we employ a relativistic collision operator, valid for arbitrary energies. Both primary and secondary runaway electron generation are included. For cases in which primary generation dominates, a time-independent formulation of the problem is described, requiring only the solution of a single sparse linear system. In the limit of dominant secondary generation, we present the first numerical verification of an analytic model for the distribution function. The numerical electron distribution function in the presence of both primary and secondary generation is then used for calculating the synchrotron emission spectrum of the runaways. It is found that the average synchrotron spectra emitted from realistic distribution functions are not well approximated by the emission of a single electron at the maximum energy.     

Megasonic agitation for enhanced electrodeposition of copper

In this paper we propose an agitation method based on megasonic acoustic streaming to overcome the limitations in plating rate and uniformity of the metal deposits during the electroplating process. Megasonic agitation at a frequency of 1 MHz allows the reduction of the thickness of the Nernst diffusion layer to less than 600 nm. Two applications that demonstrate the benefits of megasonic acoustic streaming are presented: the formation of uniform ultra-fine pitch flip-chip bumps and the metallisation of high aspect ratio microvias. For the latter application, a multi-physics based numerical simulation is implemented to describe the hydrodynamics introduced by the acoustic waves as they travel inside the deep microvias.     

Diffusion and equilibration in 2D fluid codes

The problems associated with diffusion and rapid equipartition of energy between electrons and ions in two-dimensional fluid codes are investigated numerically. It is shown that a commonly used procedure of averaging the exchange term over the hydrodynamic timescales of interest can be problematic in modelling the equipartition unless very small timesteps are used. A simple model, in which the electron and ion diffusion equations are solved simultaneously, is found to prevent the problems introduced by the average exchange model. The simultaneous solution method increases storage requirements as large scale matrix equations have to be solved. Various algorithms are considered in terms of the increase in storage and execution time required to implement the simultaneous solution method. In particular, a new simple point-iterative algorithm is introduced which is particularly efficient at solving the above problem and offers much scope when applied as a solution method for multi-species diffusion and equilibration problems.     

Simulations of lateral mixing in cross-channel flow

Cross flow phenomena between connected sub-channels are studied by means of numerical simulations based on lattice-Boltzmann discretization. The cross (that is lateral) transfer is largely due to macroscopic instabilities developing at two shear layers. The characteristic size and advection velocity of the instabilities favorably compare with experimental results from the literature on a geometrically similar system. The strength of the cross flow strongly depends on the Reynolds number, with cross flow developing only for Reynolds numbers (based on macroscopic flow quantities) larger than 1360. Mass transfer between the sub-channels has been assessed by adding a passive scalar to the flow and solving its transport equation. As a result of the intimate connection of cross flow and lateral mass transfer, also the mass transfer coefficient is a pronounced function of Re.     

Direct excitation of xenon by ballistic electrons emitted from nanocrystalline‐silicon planar cathode and vacuum‐ultraviolet light emission

Abstract— To verify the possible use of energetic electrons for direct excitation of inert gas molecules, a nanocrystalline‐silicon (nc‐Si) planar ballistic emitter is operated in a high‐pressure xenon gas ambience. Under the pulse drive, vacuum‐ultraviolet (VUV) light emission is detected without any signs of discharge. The transient behavior of the VUV light emission properly corresponds to that of the nc‐Si emitter. In accordance with quantitative analyses of electron‐emission characteristics and the VUV output, the electron‐to‐photon conversion efficiency reaches 81% in the relatively efficient emitter case. The VUV output power is mainly determined from the number of electrons with energies compatible the with internal excitation of xenon. The emission spectrum observed at a pressure of 10 kPa shows peaks at 152 and 172 nm, which are thought to be originated from metastable Xe²* states. In contrast to the case of conventional impact ionization, no near‐infrared (NIR) peaks are seen in the spectrum. These results strongly suggest that the incidence of energetic electrons causes direct excitation of xenon molecules followed by radiative relaxation through intermediate states. The generated VUV light can be easily converted to visible light using a phosphor screen. As a discharge‐free VUV light emission, this phenomenon is potentially applicable to mercury‐free, high‐efficacy, and high‐stability flat‐panel light‐emitting device.     

Plane wave diffraction by circular transmission grating; finite-difference approach

Transmission gratings with circular elemental cross section are analyzed for diffraction of plane electromagnetic waves incident at oblique angles of incidence. The new solution method which is based on the finite-difference scheme is simple in nature and involves no unchecked approximation. Numerical results are obtained for various gratings at arbitrary angles of incidence. These results favorably compare with a number of similar data available in the literature. They further reveal some of the important properties of the transmission gratings. The proposed solution technique is also applicable to problems involving finite conductivity, and, therefore, should be of considerable interest to engineers and physicists.     

基于时滞的H∞滤波器设计及其在网络中的应用

本文讨论了基于时滞的不确定线性系统的H∞滤波器设计及其在网络中的应用问题. 首先考虑时滞的上下界, 并在不忽略Lyapunov泛函导数中的任何一项的前提下, 利用自由权矩阵方法, 提出了一种新的滤波误差系统的H∞性能分析和滤波器设计方法. 而且, 滤波器的系数矩阵可以通过解一组线性矩阵不等式(LMIs)得到. 然后, 将该方法应用于考虑网络影响的具有参数不确定性的线性系统的滤波问题. 与已有文献的方法相比, 结果的保守性大 大降低. 最后给出的数值实例说明了本文所给方法的有效性和优越性.     

Radar backscattering from breaking wind waves: field observation and modelling

In this article, the results of a field study of the Ka-band (37.5 GHz) radar backscattering from breaking wind waves are presented. Radar and a video camera were simultaneously used to measure the radar cross section (RCS) of the whitecap zone and the characteristics of wave-breaking events. A comparison is made between absolute geometrical lengths, areas and orientations of the whitecaps and the RCS of the whitecaps at moderate (45°) and high (70°) incidence angles. A thin vegetable oil film covering the illuminated area was made to minimize background Bragg backscattering from the surface free of breaking-wave features. At high incidence angles, a linear-like dependence between breaking crest length and RCS and no significant azimuthal dependence are found. At moderate angles of incidence, a prominent azimuthal variation of the whitecap's RCS is shown, while a linear-like relationship between the active breaking area and the whitecap's RCS is obtained in presence of the film on the sea surface. The results are compared to the model of quasi-specular reflection from the breaking waves. It is shown that accounting for backscattering from the droplets produced by the breaking crest improves prediction of the whitecap's RCS given by this model at high incidence angles.     

Modeling of ionization composition in argon plasma with fast electrons

Discharge and laser produced plasmas often have non-equilibrium electron distributions containing fast electrons that may have a profound effect on the ionization balance. The influence of high energy electrons on the ionization balance in the collisional-radiative equilibrium model for optically thin plasma is considered forthwith. A nonmaxwellian electron distribution with fast electrons at energies and concentrations is used for calculating collisional rates; this according to the Hartree-Fock-Slater (HFS) quantum-statistical model in the average atom approximation. The calculative approach is based on the distorted wave approximation along with numerical and semiclassical wave functions calculated in the self-consistent HFS potential. The results from ionization equilibrium modeling in argon plasma at temperatures T ∼ 5–1000 eV with fast electron energies ranging from 1 to 10 keV and their different relative concentrations (from 0.1–10%) are analyzed. It is shown that fast electrons increase the impact ionization rates and significantly change the ionization stage as compared with equilibrium plasma.     

Scattering and Doppler analysis for electrically large nonlinear sea surfaces: a field-based semi-deterministic model

This paper presents a semi-deterministic facet model based on the combination of the two-scale theory and the small slope approximation (SSA). Based on the model, normalized radar cross section (NRCS) and the Doppler spectra from electrically large nonlinear ocean scenes are analysed at moderate and large incidence angles. The analytic scattering amplitude of small-scale capillary waves based on the local configuration angles is conducted. Different from the conventional two-scale model (TSM), the model is a field-based rather than RCS-based model, which considerably widens its potential applications. In order to take into account the influence of nonlinear hydrodynamics on echo characteristics, the choppy wave model (CWM) is adopted to simulate nonlinear surfaces. Several examples of bistatic NRCS and Doppler spectra are given with comparison and analysis, which validate the rationality of the model at moderate and large incidence angles. In addition, the running time and memory consumption comparisons can indicate the model is more efficient than the SSA.     

Effect of front guide nozzle shape on the flow characteristics in an augmentation channel of a direct drive turbine for wave power generation

There is an increasing interest in cross flow turbines(also known as Banki turbines) for small and low head applications because of their simple structure as well as low capital and maintenance costs.The present work aims at implementing the direct drive turbine(DDT) of cross flow type for wave power generation.A numerical wave tank was used to simulate the waves and after obtaining the desired wave properties;the augmentation channel plus the front guide nozzle and rear chamber were integrated to the numer...     

涡激振动行波动力学特征的数值模拟

采用数值模拟分析了水中大长径比结构物涡激振动行波动力学特征,以及水动力学阻尼和模型初始张力对于行波动力学特征的影响.数值模拟模型采用改进的尾流振子模型,该模型考虑了涡激振动横向运动和顺流向运动之间的耦合,考虑了张力沿模型长度的变化和流体的耗散.给出了用于评估行波在整个振动波中所占比例的方法,该方法采用行波椭圆来定量表征行波所占的比例,即行波比.数值模拟结果显示,行波比总体上呈现随流速增加的趋势,但在模态阶数变化的临界流速上突然降低.水动力学阻尼显著影响着行波比,阻尼比越大,行波比越大,行波对于整个振动波的贡献就越大.不同张力情况下,行波比突然下降的临界速度不同.初始张力影响模型的固有频率,影响模态阶数发生转变的流速,从而影响了行波比突然下降的临界流速.     

Numerical study of droplet impact and coalescence in a microline patterning process

The droplet impact and coalescence on a substrate, which is applicable to the manufacture of microlines, is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid–gas interface or droplet shape is tracked by a sharp-interface level-set method which is modified to account for the change between advancing and receding contact angles at the liquid–gas–solid interline. The numerical results show that the droplet impact and merging pattern depends significantly on the advancing and receding contact angles. Also, the effect of droplet spacing on the droplet motion is investigated to find the optimal conditions in manufacturing microlines.     

An accelerated algorithm for full band electron–phonon scattering rate computation

Computing scattering rates of electrons and phonons stands at the core of studies of electron transport properties. In the high field regime, the interactions between all electron bands with all phonon bands need to be considered. This full band interaction implies a huge computational burden in calculating scattering rates. In this study, a new accelerated algorithm is presented for this task, which speeds up the computation by two orders of magnitude (100 times) and dramatically simplifies the coding. At the same time, it visually demonstrates the physical process of scattering more clearly.     

A lumped Galerkin method for the numerical solution of the modified equal-width wave equation using quadratic B-splines

The numerical solution of the one-dimensional modified equal width wave (MEW) equation is obtained by using a lumped Galerkin method based on quadratic B-spline finite elements. The motion of a single solitary wave and the interaction of two solitary waves are studied. The numerical results obtained show that the present method is a remarkably successful numerical technique for solving the MEW equation. A linear stability analysis of the scheme is also investigated.