Force-coupling method for flows with ellipsoidal particles

The force-coupling method, previously developed for spherical particles suspended in a liquid flow, is extended to ellipsoidal particles. In the limit of Stokes flow, there is an exact correspondence with known analytical results for isolated particles. More generally, the method is shown to provide good approximate results for the particle motion and the flow field both in viscous Stokes flow and at finite Reynolds number. This is demonstrated through comparison between fully resolved direct numerical simulations and results from the numerical implementation of the force-coupling method with a spectral/hp element scheme. The motion of settling ellipsoidal particles and neutrally buoyant particles in a Poiseuille flow are discussed.     

From h to p efficiently: Implementing finite and spectral/hp element methods to achieve optimal performance for low- and high-order discretisations

The spectral/hp element method can be considered as bridging the gap between the – traditionally low-order – finite element method on one side and spectral methods on the other side. Consequently, a major challenge which arises in implementing the spectral/hp element methods is to design algorithms that perform efficiently for both low- and high-order spectral/hp discretisations, as well as discretisations in the intermediate regime. In this paper, we explain how the judicious use of different implementation strategies can be employed to achieve high efficiency across a wide range of polynomial orders. Furthermore, based upon this efficient implementation, we analyse which spectral/hp discretisation (which specific combination of mesh-size h and polynomial order P) minimises the computational cost to solve an elliptic problem up to a predefined level of accuracy. We investigate this question for a set of both smooth and non-smooth problems.     

基于PCA-BP神经网络的光谱消光法颗粒粒径反演算法研究

光谱消光法广泛应用于颗粒粒径测量领域,在利用光谱消光法对颗粒粒径进行反演的过程中,由于颗粒的消光系数存在理论复杂、计算繁琐、收敛速度慢以及求解不稳定等问题,很大程度上影响了整个反演过程的快速性和准确性。且在众多波长的消光数据中,存在较多重复冗余的信息,也很大程度上增加了反演算法的时间。针对光谱消光法粒径反演算法计算繁琐、反演效率低的问题,提出了基于主成分分析（PCA）和BP神经网络的光谱消光颗粒粒径分析方法。基于Mie散射理论对不同粒径、不同波长下的光谱消光值进行了仿真计算,通过对光谱消光数据集的主成分分析及各个波长综合载荷系数的计算,实现了最优特征波长的选取,利用降维后的光谱消光数据训练了PCA-BP神经网络模型,并利用该网络模型计算了粒径颗粒分布。通过仿真计算,比较了PCA-BP神经网络模型与传统的BP神经网络模型的预测精度,并分析了波长数目对两种神经网络模型预测结果的影响。针对训练得到的PCA-BP神经网络模型开展光谱消光法粒径参数反演算法的验证实验,搭建了光谱消光法颗粒粒径参数测量实验系统,测量了粒径范围在0.5～9.7 μm内的6种不同粒径参数的聚苯乙烯标准颗粒。仿真和实验结果表明：基于主成分分析方法可确定各个波长向量之间的相关性,利用综合载荷系数选取最优特征波长对应的消光值对整体的光谱数据具有较好的代表性,可实现光谱数据的降维。相比传统的BP神经网络模型,基于PCA-BP神经网络模型的颗粒粒径分布的分析方法预测精度更高,对于较分散颗粒系的分布参数的预测有更加明显的优势。而且,被选取的波长数较少时,PCA-BP神经网络模型依然有较高的预测精度。利用训练好的PCA-BP神经网络模型对颗粒粒径参数进行实验验证,预测结果可瞬时输出,颗粒粒径分布误差在5%以内,验证了该算法的可行性。     

A Wave Automaton for Wave Propagation in Inhomogeneous Anisotropic Media

This paper presents an extension built on a hexagonal grid of the wave automaton, which was introduced in past few years for describing wave propagation in inhomogeneous media. This new method is capable of computing wave propagation in 2D anisotropic media without the need for introducing interpolating schemes. After a comparison of isotropic single scattering with analytical results using Mie theory, the method is used to compute the field scattered by one anisotropic particle for various orientations of its principal axes. Scattering by a collection of anisotropic particles is also presented.     

Electric field and force on a conducting sphere in contact with a dielectric solid

This paper presents the analysis of electric field and force on a conducting sphere lying on a dielectric solid under a uniform field. To achieve high accuracy, we have applied the analytical method of successively placing three infinite sequences of point and dipole charges (zero- or first-order multipoles). The electric field is highest at the contact point, called the triple junction, where the conductor, the dielectric solid, and the surrounding medium (gas or vacuum) meet together. Both the contact-point field and the force increase with the permittivity ratio of the solid to that of the surrounding medium. The resulting force always attracts the sphere to the solid, in contrast to the repulsive force in the case of a conducting sphere lying on a plane conductor under an external field. We have given very simple formulae for approximating the contact-point field and the force which agree with the precise values within a difference of 3% for permittivity ratios up to 32 and 64, respectively.     

Spectral shift of partially coherent flattened multi-Gaussian beams with a hole passing through ABCD optical systems

Based on flattened multi-Gaussian beam model and partially coherent theory, the expression for cross-spectral density of partially coherent flattened multi-Gaussian beam with a hole was given. The analytical expression for on-axis spectrum of partially coherent flattened multi-Gaussian beam with a hole passing through a paraxial ABCD optical system was derived. The spectral shifts of the partially coherent flattened multi-Gaussian beam with a hole propagating in free space and passing through the lens have been analyzed. The effects of the beam profile, spatial coherence parameter and the system parameters on the relative spectral shift have been discussed. Our results show that the inner radius of flattened multi-Gaussian beam with a hole determines its relative spectral shift in near field and its outer radius determines that in far field. For the flattened multi-Gaussian beam with a hole passing through a lens, the on-axis relative spectral shift changes sharply near the focal plane with the increase in Fresnel number and the spatial coherence parameter.     

煤粒度对煤质近红外定量分析模型的影响

为了减少因煤样粒度而产生的光谱采集误差,研究0.2,1,3和13mm粒度等级下的煤质近红外分析模型。采用PCA方法提取特征信息,建立基于GA-BP和GA-Elman神经网络算法的定量分析模型。实验结果表明,经数据归一化与多元散射校正预处理后,0.2mm粒度等级的光谱与煤炭标准之间的相关性最强,模型的学习精度最高;经平滑处理后1mm粒度等级的分析结果最佳。平滑法对特征谱峰不明显的光谱的预处理效果较差,多元散射校正方法的适用性最强。在0.2mm粒度等级下原光谱的信息准确度最高,1和3mm其次,13mm最差。煤样粒度越大,光谱的不稳定因素越多,从而导致分析模型的负面影响增加。     

基于墨量限制样本的打印机光谱特性化模型修正方法

打印机色彩特性化是打印色彩管理技术的关键。打印机特性化模型修正，是指针对因打印介质及墨盒更换等原因而造成的特性化模型精度下降问题，借助特定修正样本实现原始模型精度校正的过程。为进一步提高此类修正方法的建模精度与效率，提出了一种基于墨量限制样本的打印机光谱特性化修正方法，该方法充分利用新介质墨量限制过程所制备的墨量限制样本，在无需额外修正样本的条件下，实现了原始特性化模型的有效修正。研究以三类不同类型打印介质为例，以现有典型特性化模型修正方法为对照，通过实验对本文方法的有效性进行了验证。结果显示，该方法相较现有方法展现出更为优异的修正效果，在无需额外修正样本的条件下，其光谱修正精度可提升15%～20%，色度修正精度可提升10%～20%，实现了修正精度与效率的同步优化。     

光谱成像仪快速光谱定标方法研究

光谱定标是确定光谱仪器各通道中心波长的过程,为了获取光谱辐亮度,通常需要对光谱仪器进行辐射定标,将光谱仪器输出的数值,映射为物理量——辐亮度。不同的光谱仪器的光谱响应不同,因此还需要在光谱定标过程中确定各个通道的光谱响应。光谱成像仪可以看成是多个光谱仪组成的,需要对所有点的中心波长和光谱响应进行定标。自第一台成像光谱仪诞生以来,其定标方法逐渐固定,通常需要采用光谱分辨率较光谱成像仪更高的单色仪输出准单色光进行光谱定标,其准单色光的光谱带宽远小于光谱成像仪的光谱响应带宽,可以将准单色光抽象为脉冲函数。根据脉冲函数的特性,改变准单色光的波长,扫描光谱成像仪的响应波长范围,是对光谱响应函数进行间隔采样的过程,通过光谱定标数据可以直接得到光谱成像仪的中心波长和光谱响应函数。随着技术的发展,探测器的灵敏度越来越高,光谱成像仪的分辨率也越来越高,为了完成光谱定标,对光谱定标需要的准单色光提出了更高的要求。然而准单色光的带宽越窄,其能量越低,获取满足信噪比要求的数据需要更长的时间,使定标的效率降低。从光谱定标的目的出发,结合准单色光和光谱成像仪光谱响应近似高斯函数的特点,通过理论分析,提出一种利用宽带定标光进行光谱定标的方法,可以有效减少光谱定标的步骤,提高定标的效率,适用于光谱成像仪的快速定标。该方法用于某星载高光谱成像仪的光谱定标,待标定光谱成像仪采用棱镜分光,具有色散非线性的特点,光谱分辨率在2~18 nm之间变化,同时存在较大的谱线弯曲,导致每个像元的中心波长都不同,需要对每个像元进行光谱定标。为了避免分视场定标导致的相邻视场中心波长不连续现象,将单色仪发出的准单色光的光斑照亮整个狭缝,狭缝和单色仪之间放置柱透镜和毛玻璃,其中柱透镜用于汇聚垂直于狭缝方向的光线,提高能量利用率;毛玻璃用于匀化光照,毛玻璃的存在极大地减弱了进入光谱成像仪的能量,结合提出的方法,增加定标光的带宽,提高能量,最终完成了该光谱成像仪的快速定标,利用汞灯的特征光谱验证该成像光谱仪的光谱定标精度为0.23 nm。     

Analysis of Trapping Force and Torque for Fiber-optical Trap by the Finite-differential Time-domain (FDTD) Method

In this paper,the FDTD method is used to calculate the light field distribution in an uncoated fiber probe with a small dielectric particle.By integrating Maxwell′s stress tensor in the FDTD calculation,the time-averaged force exerted on the dielectric particles with different sizes and at different positions is obtained.The calculation results are also verified experimentally.The force near the axis in the vicinity of the tip is about 10-14 N for the incident 10 mW laser of 980 nm.Furthermore,the torque exerting on an ellipsoidal dielectric particle by focusing light field with two fiber probes is calculated and the dependence of this torque on both the declination angle of the ellipsoidal particle and the incident laser power ratio of the two probes is obtained.The detailed intensity map within the particle is also given.