模拟退火算法在线热源反问题数值求解中的应用

提出采用模拟退火算法(simulated annealing,SA)来数值求解线热源反问题.探讨了如何设计算法使之适合反问题求解,并给出了算法求解的伪代码;通过线源正问题的模拟数据,使用设计的SA算法进行反问题求解,以此来验证算法求解的准确性和可靠性,并对一组实测数据进行了计算.结果表明,该算法不但可以实现两个参数同时、快速反演,而且具有求解精度高,对初始条件依赖少,编制容易等优点.     

最小平方卷积反演的一种快速迭代算法

本文介绍一种最小平方卷积反演快速迭代算法，其特点是：（１）迭代过程由ＦＦＴ实现；（２）收敛因子收敛易于选取，收敛性好；（３）收敛性与迭代初值无关。     

求解二维Helmholtz外问题的一种快速算法

本基于虚拟边界积分法，通过将虚拟积分曲线选为多(单)条圆形曲线，并在这些圆形积分曲线上将未知源强密度函数用Fourier级数展开，同时借助快速数值Fourier逆变换(IFFT)计算程序，提出了一种求解二维Helmholtz外问题的快速算等。该方法由于不需要将分布在虚拟边界上的未知函数进行单元分散，不仅克服了边界元法或虚拟边界元法中由于单元形函数是由低阶多项式函数构成导致其结果只适用于较低频率范围的不足，而且具有很高的计算精度和效率。中给出的数值算例表明了这种快速算法的计算效率是虚拟边界元法的20-80倍。     

一种基于反问题算法的管道壁面热流密度测量技术

为了解决目前传统测量方法中无法准确计量热流密度的问题,在已知管道内沿层流的几处不同点的测温数据的基础上,建立了热流密度计量的反问题数学模型,采用共轭梯度法,给出了简明的热流测量步骤。数值实验表明,在存在较大温度测量误差的情况下,这种方法仍具有较高的计量精度。     

一种高效的匹配场反演混合优化算法

将下山单纯形算法引入差异进化算法,提高了差异进化算法对目标函数梯度信息的利用,改善了差异进化算法的优化效率,由于下山单纯形算法与差异进化算法都是并行算法,混合算法同时具备了并行高效的特点.     

一种用于MPEG-4形状编码的快速运动估计算法

提出了一种新的形状快速运动估计算法,用于加快MPEG-4视频对象的形状编码过程。该算法充分利用了邻近形状块的运动向量之间的相关性、边界形状块的匹配特性。与MPEG-4验证模型中的形状运动估计算法以及现有的改进算法相比,该算法大大提高了形状运动估计的速度,而形状的编码效率并没有降低。     

一种利用运动信息提取目标物图像的算法

提出了一种利用运动信息提取图像中的研究对象的算法。当要提取的目标物是运动物体，并且背景的绝大多数均为静止时，该算法适用。其基本原理是：将不包含目标物，全为静态背景信息的参考图像与在同一背景下拍摄的包含目标物的图像进行比较，把相同的成分视为背景消去，得到目标物图像；同时为了消除各种因素所产生的影响，在比较时引入适当的容差，对比较结果做去噪处理，并通过检验区域连通性，消去与目标域不连通的区域。试验表明，应用该算法无需人机交互，提取精度较高，抗干扰能力强，有较好的实用性。     

一种利用运动信息提取目标物图像的算法

提出了一种利用运动信息提取图像中的研究对象的算法。当要提取的目标物是运动物体 ,并且背景的绝大多数均为静止时 ,该算法适用。其基本原理是 :将不包含目标物 ,全为静态背景信息的参考图像与在同一背景下拍摄的包含目标物的图像进行比较 ,把相同的成分视为背景消去 ,得到目标物图像 ;同时为了消除各种因素所产生的影响 ,在比较时引入适当的容差 ,对比较结果做去噪处理 ,并通过检验区域连通性 ,消去与目标域不连通的区域。试验表明 ,应用该算法无需人机交互 ,提取精度较高 ,抗干扰能力强 ,有较好的实用性     

基于半逆法的一种快速单幅图像去雾算法

针对目前去雾算法实时性较差,对天空等区域的处理不理想以及去雾后的图像视觉效果较差等问题,提出一种新的基于半逆法的快速单幅图像去雾算法.首先从大气散射模型出发,利用改进的半逆算法得到大气整体光照值;其次,基于大气散射光特性,以图像边缘信息为合成条件融合图像的边缘信息和场景深度信息,准确估测大气光幂;然后,根据大气散射模型得到初步复原无雾图像;最后,经过色调调整和细节增强处理,得到一幅真实感强烈的无雾图像.对于深度发生突变或者远景像素点,消除了光晕效应.与其他算法相比,本算法能很好保持色彩和细节信息,具有较好的实时性和鲁棒性.     

制冷工质R22热力性质的一种快速算法

制冷工质热物性的计算是数值模拟技术非常重要的一个环节，开展这一工作的研究且有重要的价值。本文在结合Martin-Hou方程的基础上提出了制冷工质热力性质的一种快速算法，经实例验证，该算法具有精度高、适应范围广、稳定性好等特点，可供制冷工质热物性算法设计的初学者作参考。     

A POD-selective inverse distance weighting method for fast parametrized shape morphing

Efficient shape morphing techniques play a crucial role in the approximation of partial differential equations defined in parametrized domains, such as for fluid-structure interaction or shape optimization problems. In this paper, we focus on inverse distance weighting (IDW) interpolation techniques, where a reference domain is morphed into a deformed one via the displacement of a set of control points. We aim at reducing the computational burden characterizing a standard IDW approach without significantly compromising the accuracy. To this aim, first we propose an improvement of IDW based on a geometric criterion that automatically selects a subset of the original set of control points. Then, we combine this new approach with a dimensionality reduction technique based on a proper orthogonal decomposition of the set of admissible displacements. This choice further reduces computational costs. We verify the performances of the new IDW techniques on several tests by investigating the trade-off reached in terms of accuracy and efficiency.     

A shape optimization approach for simulating contact of elastic membranes with rigid obstacles

The obstacle problem consists in computing equilibrium shapes of elastic membranes in contact with rigid obstacles. In addition to the displacement u of the membrane, the interface Γ on the membrane demarcating the region in contact with the obstacle is also an unknown and plays the role of a free boundary. Numerical methods that simulate obstacle problems as variational inequalities share the unifying feature of first computing membrane displacements and then deducing the location of the free boundary a posteriori. We present a shape optimization-based approach here that inverts this paradigm by considering the free boundary to be the primary unknown and compute it as the minimizer of a certain shape functional using a gradient descent algorithm. In a nutshell, we compute Γ then u, and not u then Γ. Our approach proffers clear algorithmic advantages. Unilateral contact constraints on displacements, which render traditional approaches into expensive quadratic programs, appear only as Dirichlet boundary conditions along the free boundary. Displacements of the membrane need to be approximated only over the noncoincidence set, thereby rendering smaller discrete problems to be resolved. The issue of suboptimal convergence of finite element solutions stemming from the reduced regularity of displacements across the free boundary is naturally circumvented. Most importantly perhaps, our numerical experiments reveal that the free boundary can be approximated to within distances that are two orders of magnitude smaller than the mesh size used for spatial discretization. The success of the proposed algorithm relies on a confluence of factors- choosing a suitable shape functional, representing free boundary iterates with smooth implicit functions, an ansatz for the velocity of the free boundary that helps realize a gradient descent scheme and triangulating evolving domains with universal meshes. We discuss these aspects in detail and present numerous examples examining the performance of the algorithm.     

Application of the genetic algorithm for microwave imaging of a layered dielectric object via the regular shape expansion technique

A novel method combining the genetic algorithm (GA) and regular shape expansion technique is reported for electromagnetic imaging of a multilayer dielectric object of arbitrary shape. By measuring the scattered field, the shape, location, size, and permittivity of each layer of the object are retrieved quite successfully. The forward problem is solved based on the equivalent source current and the method of moments (MoM), while the inverse problem is reformulated as an optimization problem. The optimization problem is solved by the proposed method. Numerical simulation shows that good image reconstruction can be obtained for various multilayer dielectric objects as long as the noise level is ≤−20 dB. © 1999 John Wiley & Sons, Inc. Int J Imaging Syst Technol 10, 347–354, 1999     

A wideband fast multipole boundary element method for three dimensional acoustic shape sensitivity analysis based on direct differentiation method

This paper presents a wideband fast multipole boundary element approach for three dimensional acoustic shape sensitivity analysis. The Burton-Miller method is adopted to tackle the fictitious eigenfrequency problem associated with the conventional boundary integral equation method in solving exterior acoustic wave problems. The sensitivity boundary integral equations are obtained by the direct differentiation method, and the concept of material derivative is used in the derivation. The iterative solver generalized minimal residual method (GMRES) and the wideband fast multipole method are employed to improve the overall computational efficiency. Several numerical examples are given to demonstrate the accuracy and efficiency of the present method.     

Multifrequency near-field acoustic tomography and holography of 3D subbottom inhomogeneities

A method of coherent multifrequency acoustic tomography and holography of spatially localized subbottom inhomogeneities in shallow seas is proposed. This method is based on solving of the near-field inverse scattering problem that makes it possible to realize a subwavelength resolution. It involves the analysis of measurement data obtained by the 2D transversal scanning with the source-receiver system along the sea bottom, over the area of sounded inhomogeneities. The solution begins with the Born approximation, where the original 3D integral equation for the scattered field is reduced to the 1D Fredholm equation of the first kind relative to the depth profile of the lateral spectrum of inhomogeneities. When solving this integral equation for each pair of spectral components, the generalized discrepancy method is in use. Then, corrections to the Born approximation can be obtained in the proposed iterative procedure. For distributed inhomogeneities, the inverse Fourier transform of the retrieved spectrum gives their 3D distribution that can be visualized as tomography images. For solid targets, this spectrum is used to obtain their shape (i.e. to solve the problem of computer holography). Corresponding results of the numerical simulation are presented.     

A three dimensional He-recoil MWPC for fast polarized neutrons

The basic properties of a high pressure He-MWPC are investigated with respect to its application as a neutron polarimeter for energies between 10 and 60 MeV.     

基于近场声全息振速重构的3种滤波方法比较

针对近场声全息反向重构时边缘误差造成的检测精度低的问题,开展基于振速反向重构的近场声全息滤波方法对比研究。以一个高为11.5 cm、半径为4.2 cm的圆柱形发射换能器为研究对象,通过数值仿真分析和实验测量对比二维Harris滤波窗函数、改进后的二维Harris滤波窗函数和WZ滤波窗函数对边缘误差的抑制效果和反向重构声场幅值的误差大小。结果表明,3种滤波窗函数都可以在较短的反向重构距离范围内有效降低边缘误差,利用Harris滤波窗函数进行滤波在反向重构声场的幅值方面误差最小,但边缘误差抑制效果最差;WZ滤波窗函数对反向重构距离的适用性最好,在更大的反向重构距离时,其对边缘误差的抑制效果更好,反向重构误差更小。     

Inverse acoustic scattering by solid obstacles: topological sensitivity and its preliminary application

A to pological sensitivity approach is developed for the imaging of penetrable solid obstacles embedded in a fluid background medium by way of inverse acoustic scattering. To this end, an asymptotic form for the scattered field caused by a nucleating spherical solid obstacle in the reference fluid medium is derived within the boundary integral equation framework, where the required limiting behaviour of the acceleration field inside the perturbation is established based on solutions of two simplified fluid–solid interaction problems. With this result, the equivalence of acoustic scattering by fluid and solid scatterers in terms of asymptotic behaviour is observed and numerically validated. The direct and adjoint-field topological sensitivity expressions for transient and time-harmonic excitations are obtained accordingly. The utility of the proposed method as a tool for preliminary obstacle reconstruction and bulk modulus characterization is illustrated through numerical examples and an exploratory experimental study. On the basis of adjusted topological sensitivity formulas for fluid reference domains containing pre-existing solid inhomogeneities, an iterative 3D obstacle reconstruction approach is established and its performance with synthetic data is demonstrated.     

A three-dimensional image reconstruction algorithm for electrical impedance tomography using planar electrode arrays

We present a three-dimensional non-iterative reconstruction algorithm developed for conductivity imaging with real data collected on a planar rectangular array of electrodes. Such an electrode configuration as well as the proposed imaging technique is intended to be used for breast cancer detection. The algorithm is based on linearizing the conductivity about a constant value and allows real-time reconstructions. The performance of the algorithm was tested on numerically simulated data and we successfully detected small inclusions with conductivities three or four times the background lying beneath the data collection surface. The results were fairly stable with respect to the noise level in the data and displayed very good spatial resolution in the plane of electrodes.