Weighted Flow Algorithms (WFA) for stochastic particle coagulation

Stochastic particle-resolved methods are a useful way to compute the time evolution of the multi-dimensional size distribution of atmospheric aerosol particles. An effective approach to improve the efficiency of such models is the use of weighted computational particles. Here we introduce particle weighting functions that are power laws in particle size to the recently-developed particle-resolved model PartMC-MOSAIC and present the mathematical formalism of these Weighted Flow Algorithms (WFA) for particle coagulation and growth. We apply this to an urban plume scenario that simulates a particle population undergoing emission of different particle types, dilution, coagulation and aerosol chemistry along a Lagrangian trajectory. We quantify the performance of the Weighted Flow Algorithm for number and mass-based quantities of relevance for atmospheric sciences applications.     

Smoothed particle hydrodynamics(SPH) for modeling fluid-structure interactions

Fluid-structure interaction(FSI) is a class of mechanics-related problems with mutual dependence between the fluid and structure parts and it is observable nearly everywhere, in natural phenomena to many engineering systems. The primary challenges in developing numerical models with conventional grid-based methods are the inherent nonlinearity and timedependent nature of FSI, together with possible large deformations and moving interfaces. Smoothed particle hydrodynamics(SPH) method is a truly Lagrangian and meshfree particle method that conveniently treats large deformations and naturally captures rapidly moving interfaces and free surfaces. Since its invention, the SPH method has been widely applied to study different problems in engineering and sciences, including FSI problems. This article presents a review of the recent developments in SPH based modeling techniques for solving FSI-related problems. The basic concepts of SPH along with conventional and higher order particle approximation schemes are first introduced. Then, the implementation of FSI in a pure SPH framework and the hybrid approaches of SPH with other grid-based or particle-based methods are discussed. The SPH models of FSI problems with rigid, elastic and flexible structures, with granular materials, and with extremely intensive loadings are demonstrated. Some discussions on several key techniques in SPH including the balance of accuracy, stability and efficiency, the treatment of material interface, the coupling of SPH with other methods, and the particle regularization and adaptive particle resolution are provided as concluding marks.     

An adaptive speech enhancement method for siren noise cancellation

Siren noises usually severely disturb the intelligibility of voice communication inside the cabs of police, paramedic and fire vehicles. It is often desired that such unwanted noise can be removed from the speech signal. In this paper, a new method is proposed to adaptively cancel siren noises and enhance speech signals. Based on the characteristics of siren noises, an anti-speech filter and a time delayer are employed in the single and dual channel noise cancellation systems to reduce the siren noises. Experiment results demonstrate that the effectiveness of the proposed method for canceling the siren noises and the performance of the enhanced speech signal is satisfying.     

An interaction potential based lattice Boltzmann method with adaptive mesh refinement (AMR) for two-phase flow simulation

The lattice Boltzmann method (LBM) for two-phase flow simulation is often hindered by insufficient resolution at the interface. As a result, the LBM simulation of bubbles in bubbling flows is commonly limited to spherical or slightly deformed bubble shapes. In this study, the adaptive mesh refinement method for the LBM is developed to overcome such a problem. The approach for this new method is based on the improved interaction potential model, which is able to maintain grid-independent fluid properties in the two-fluid phases and at the interface. The LBM–AMR algorithm is described, especially concerning the LBM operation on a non-uniform mesh and the improved interaction potential model. Numerical simulations have been performed to validate the method in both single phase and multiphase flows. The 2D and 3D simulations of the buoyant rise of bubbles are conducted under various conditions. The agreement between the simulated bubble shape and velocity with experiments illustrates the capability of the LBM–AMR approach in predicting bubble dynamics even under the large bubble deformation conditions. Further, the LBM–AMR technique is capable of simulating a complex topology change of the interface. Integration of LBM with AMR can significantly improve the accuracy and reduce computation cost. The method developed in this study may appreciably enhance the capability of LBM in the simulation of complex multiphase flows under realistic conditions.     

主动声纳的自适应显示数据缩减算法

本文讨论了主动声纳系统中主动检测结果的数据显示问题.首先介绍传统显示数据缩减的几种算法,在这些基础上,我们提出一种新颖的自适应显示数据缩减算法.此算法利用宽带信号特征和主动检测统计特征,通过分段加权平均并取用子段平均最大值的方法,自适应地缩减待显示数据.仿真结果表明,此算法可以提高数据输出的信息量,减小输出信噪比损失,其性能优于传统的几种显示数据缩减算法,有助于提高人机交互的可靠性,并且算法简单,适于实时实现.     

Dzyaloshinskii-Moriya interactions in systems with fabrication induced strain gradients: An ab-initio study

The Dzyaloshinskii-Moriya (DM) vectors arising from a fabrication induced perpendicular strain gradient in a film of bcc Fe are calculated by a combination of the ab initio density functional electron theory with a micromagnetic model. It is shown that even for large strain gradients the absolute values of the DM vectors are 1-2 orders of magnitude smaller than those calculated recently for a Fe bilayer on W(1 1 0). The breaking of the symmetry of the static and dynamic properties of a magnetic vortex with respect to its polarization is discussed for systems with DM interactions.     

Modified FRITIOF code: negative charged particle production in high energy nucleus–nucleus interactions

Calculations of nucleus–nucleus interaction characteristics at energies 3.2–3.5 GeV per nucleon performed within the framework of a modified FRITIOF model and a cascade-evaporation model (CEM) are presented in comparison with corresponding experimental data. It is shown that the FRITIOF model describes the data better than the CEM does.     

相对论电子束在等离子体中的能量沉积

 用3维粒子模拟程序LARED-P研究了束-等离子体不稳定性, 不稳定性激发的强电磁场使电子束在非常短的距离内沉积能量。对于10 MeV的单能电子束,束电子数目占总电子数目5％的情况下,最终约损失14％的束能量。推导了等离子体的色散关系,得出了增长率。     

An adaptive scanning scheme for effective whole field stress separation in digital photoelasticity

In photoelasticity, the method of obtaining the individual values of principal stresses/normal stresses separately is referred to as stress separation. Shear difference method is one of the widely used techniques for stress separation in digital photoelasticity. Normally a simple raster scanning approach is used in shear difference method in which stress separation is carried out for all the lines within the model domain by either row-wise horizontally or column-wise vertically starting from the boundary pixels. This requires the presence of a free boundary to start the integration scheme for every row of interest, which is not always possible in most of the practical problems. In order to overcome this, in this paper, an adaptive scanning scheme is proposed so that stress separation can be carried out even if there is only one free boundary pixel available in the model. The new scanning scheme is validated using the theoretically generated data for the problem of a ring subjected to internal pressure. Later, the applicability of this scheme is demonstrated by using two other example problems.     

An adaptive regression method for infrared blind-pixel compensation

Blind pixel compensation is an ill-posed inverse problem of infrared imaging systems and image restoration. The performance of a blind pixel compensation algorithm depends on the accuracy of estimation for the underlying true infrared images. We propose an adaptive regression method (ARM) for blind pixel compensation that integrates the multi-scale framework with a regression model. A blind-pixel is restored by exploiting the intra-scale properties through the nonparametric regressive estimation and the inter-scale characteristics via parametric regression for continuous learning. Combining the respective strengths of a parametric model and a nonparametric model, ARM establishes a set of multi-scale blind-pixel compensation method to correct the non-uniformity based on key frame extraction. Therefore, it is essentially different from the traditional frameworks for blind pixel compensation which are based on filtering and interpolation. Experimental results on some challenging cases of blind compensation show that the proposed algorithm outperforms existing methods by a significant margin in both isolated blind restoration and clustered blind restoration.     

An adaptive SPH method for strong shocks

We propose an alternative SPH scheme to usual SPH Godunov-type methods for simulating supersonic compressible flows with sharp discontinuities. The method relies on an adaptive density kernel estimation (ADKE) algorithm, which allows the width of the kernel interpolant to vary locally in space and time so that the minimum necessary smoothing is applied in regions of low density. We have performed a von Neumann stability analysis of the SPH equations for an ideal gas and derived the corresponding dispersion relation in terms of the local width of the kernel. Solution of the dispersion relation in the short wavelength limit shows that stability is achieved for a wide range of the ADKE parameters. Application of the method to high Mach number shocks confirms the predictions of the linear analysis. Examples of the resolving power of the method are given for a set of difficult problems, involving the collision of two strong shocks, the strong shock-tube test, and the interaction of two blast waves.     

多模式离子推力器栅极系统三维粒子模拟仿真

栅极系统是离子推力器推力产生的主要部件,推力器的性能和寿命都与栅极系统密切相关.对于具有多种工作模态的离子推力器,基于电流电压入口的仿真可以有效评估推力器的工作状况.采用三维粒子模拟方法对两栅极系统等离子体输运过程进行了仿真,获得了不同模式下的推力器性能参数,对比NSTAR的在轨测试参数,验证了模型的正确性;分析了工作模式变化对栅极区域电场分布和束流状态的影响以及离子推力器多模式设计需求.分析结果表明:远离栅极系统的外凸型屏栅鞘层和内凹型零等势面、低鞍点电势值和平缓的下游电势分布,有利于提高栅极系统离子通过率,抑制电子返流,减小Pits-and-Grooves腐蚀,是离子推力器工作模式的设计方向;提高束流电压会导致发散角损失增大,但可扩展栅极工作电流范围,在束流强度较大的模式下,使束流具有较好的聚焦状态,有利于减小Barrel腐蚀.研究结果为多模式离子推力器工作模式设计提供了参考.     

An adaptive moving mesh method for two-dimensional ideal magnetohydrodynamics

This paper presents an adaptive moving mesh algorithm for two-dimensional (2D) ideal magnetohydrodynamics (MHD) that utilizes a staggered constrained transport technique to keep the magnetic field divergence-free. The algorithm consists of two independent parts: MHD evolution and mesh-redistribution. The first part is a high-resolution, divergence-free, shock-capturing scheme on a fixed quadrangular mesh, while the second part is an iterative procedure. In each iteration, mesh points are first redistributed, and then a conservative-interpolation formula is used to calculate the remapped cell-averages of the mass, momentum, and total energy on the resulting new mesh; the magnetic potential is remapped to the new mesh in a non-conservative way and is reconstructed to give a divergence-free magnetic field on the new mesh. Several numerical examples are given to demonstrate that the proposed method can achieve high numerical accuracy, track and resolve strong shock waves in ideal MHD problems, and preserve divergence-free property of the magnetic field. Numerical examples include the smooth Alfvén wave problem, 2D and 2.5D shock tube problems, two rotor problems, the stringent blast problem, and the cloud–shock interaction problem.     

水下网络突发分组前导码检测的自适应阈值算法

本文针对水声信道的特点,提出了一种适于水下传感器网络突发通信分组前导码检测的自适应阈值算法。该算法根据估计的信号瞬时功率自适应地调整阈值,以取代固定阈值算法中的预设常数阈值。进行了理论分析和仿真,证明该算法具有恒虚警的特点,检测概率比固定阈值算法有明显的提高。针对检测到相关函数旁瓣的情况,加入了滑动窗口。该算法经过了湖试验证,性能良好,具有实用价值。     

An optimisation design of adaptive illumination for a multi-reflective 3D scene

An illumination optimisation technique applied to multi-reflective 3-D machine vision based on a projector-camera system is introduced, in which the projector plays a key role to compensate for surface reflectance at each pixel to be inversely proportional to the brightness of the pixel under ambient light. The adaptive illumination technology was achieved by iterations emphasising different illumination intensities according to different surface orientations and requiring an accurate correspondence between the projector pixels and the camera pixels. In order to establish the most effective correspondence to prepare for subsequent adaptive illumination, 4 kinds of grating patterns, including sinusoidal, rectangular, triangular, and dual-frequency sinusoidal grating patterns, were projected and compared. The iterations were halted when an optimally lit scene was obtained; the further experiments under weak and strong light searched for the best method of illumination optimisation and confirmed the reliability of the adaptive illumination. The proposed optimisation design could run in real time and became a viable solution for industry.     

一种基于信号功率的水声通信线性调频同步信号自适应阈值检测算法

由于线性调频信号具有一定的抗多普勒效应能力,在水下突发通信中作为同步序列得到了广泛的应用。在对线性调频信号进行同步的过程中发现,当采用固定判决门限检测方法时,往往会造成检测概率的减小或者虚警概率的增大,针对这一问题,本文提出了一种根据信号功率,并在滑动窗口中进行比较判决,自动调整相关峰检测门限值的自适应算法。通过仿真和湖试验证,该方法性能良好,具有一定的实用价值。     

An adaptive nonlocal means filter based on fuzzy domain for laser speckle reduction

A new speckle reduction method, i.e., an adaptive nonlocal means based on fuzzy domain (FANLM), is proposed for polarization imaging despeckling. With this method, speckle is removed by an adaptive nonlocal means in Laplacian pyramid domain. The proposed FANLM method calculates the similarity measure in the fuzzy domain, which can make the similarity measure more precise and accurate. In view of the similarity neighborhood selection problem, an adaptive selection algorithm is proposed. The performance of the proposed FANLM method has been compared with the classic methods. Experimental results show that the visual quality and evaluation indexes outperform the other methods. The researches have important reference values in removing speckle noise.     

An SU(2) model for weak interactions

It is shown that it is possible to obtain neutral current weak interaction in agreement with experiments in a pure SU(2) model. The main experimental consequences are the relation mW₃? √2 mW± and the prediction that mW± ? 53 GeV and mW₃ ? 75GeV.     

Method for independent control of particle size and distance in rhodium epitaxy on TiO2(110)-(1×2) surface An STM study

A method for independent control of the particle size and distance is presented for rhodium epitaxy on TiO₂(110)-(1×2) surface. The real space imaging of the surface morphology was performed by scanning tunneling microscopy. The amount of the deposited rhodium was checked by Auger electron spectrometry. The method consists of two steps: (i) evaporation of 0.001–0.050 ML equivalent of rhodium at room temperature with a post-annealing at 1100 K (“seeding”); (ii) post-deposition of rhodium for growing of the Rh nanoparticles formed in step (i) (“growing”). The mechanism of this procedure is based on the large difference of the surface diffusion coefficient between Rh adatoms and Rh nanocrystallites larger than 1–2 nm. In the first step the average distance between the metal particles is controlled in the range 5–200 nm, the second step determines the particles size (2–50 nm). This work demonstrates that the diffusion processes of metal nanoparticles of different sizes and the growing modes of the crystallites can be studied in detail by application of seeded surfaces.     

An image-shifting technique based on grey-scale classification for particle image velocimetry

The image-shifting techniques are used to overcome the directional ambiguity of particle image displacement in the measurement of particle image velocimetry (PIV). This paper proposes an image-shifting technique based on grey-scale classification for PIV. By calculating the unified grey-scale statistical frequency of each interrogated unit, the directional ambiguity is resolved without any special requirement of the camera, and the particle image displacement is calculated synchronously. This image-shifting technique can be realized by controlling the difference in the light intensity of two lasers. Using this new technique, a PIV system was developed and used to measure the diesel spray flow. The displacement vector map of fuel particle in the spray flow was obtained, and the structure of the spray flow was investigated. The application confirmed that the image-shifting technique is viable and effective.