基于多维匹配场处理技术的扩展杂波抑制算法

高频天波超视距雷达通过电离层的折射作用 ,具有同时超视距监测大区域范围的能力。但是由于高频HF段的电波传输环境复杂 ,存在相位路径失真和多模式传输等多种不利因素 ,会导致接收到的杂波展宽 ,从而淹没于杂波附近的目标回波。主要讨论了位于相邻探射区域反射的一致杂波的展宽现象 ,在针对该现象的原始匹配场处理方法的基础上 ,提出了多路信号形成多维交叉相关的方法。通过实验仿真说明 ,改进后的方法能够有效地改善扩展杂波的抑制效果     

WCDMA的小区搜索及其实现算法

对WCDMA三步小区搜索各步的实现算法进行了研究，充分考虑了频差对初始小区搜索的影响，给出了各步优化的实现算法。     

Detection Algorithm for LFM Echo of Underwater Moving Targets Based on Discrete Fractional Fourier Transform

The mismatch between echo and replica caused by underwater moving target（UMT）＇s radial velocity degrades the detection performance of the matched filter（MF） for the linear frequency modulation（LFM） signal. By using the focusing property of fractional Fourier transform（FRFT） to that signal, a detection algorithm for UMT＇s LFM echo based on the discrete fractional Fourier transform（DFRFT） is proposed. This algorithm is less affected by the target＇s radial velocity compared with the other MF detection algorithm utilizing zero radial velocity replica（ZRVR）, and the mathematical relation between the output peak positions of these two algorithms exists in the case of existence of target echo. The algorithm can also estimate the target distance by using this relation. The simulation and experiment show that this algorithm＇s detection performance is better than or equivalent to that of the other MF algorithm utilizing ZRVR for the LFM echo of UMT with unknown radial velocity under reverberation noise background.     

Creep crack growth in internally pressurised tubes

This work was conducted as a small part of BRITE-EURAM Project BE 7463. In order to relate results from laboratory sized specimens to those on actual tubular components, test pieces from straight and cold bent carbon manganese steel tubing were tested under internal pressurisation at 360°C. Project work had previously indicated significant residual stress relaxation in cold bent material at 360°C, hence this was also investigated, it being anticipated that such stresses would play a role in the crack growth. Increased time at 360°C before pressurisation increased failure times on both bends and straight tubing. The latter was found also to contain high residual stresses. Such effects must be taken into consideration both during testing and when applying results to plant situations. Preliminary data analysis indicates that crack growth in tubular materials is faster than crack growth rates in both compact tension and three point bend specimens for equivalent K or C* values. Therefore both geometry and size effects have a significant influence on creep crack growth behaviour.     

Nonlinear instability in flagellar dynamics: a novel modulation mechanism in sperm migration?

Throughout biology, cells and organisms use flagella and cilia to propel fluid and achieve motility. The beating of these organelles, and the corresponding ability to sense, respond to and modulate this beat is central to many processes in health and disease. While the mechanics of flagellum–fluid interaction has been the subject of extensive mathematical studies, these models have been restricted to being geometrically linear or weakly nonlinear, despite the high curvatures observed physiologically. We study the effect of geometrical nonlinearity, focusing on the spermatozoon flagellum. For a wide range of physiologically relevant parameters, the nonlinear model predicts that flagellar compression by the internal forces initiates an effective buckling behaviour, leading to a symmetry-breaking bifurcation that causes profound and complicated changes in the waveform and swimming trajectory, as well as the breakdown of the linear theory. The emergent waveform also induces curved swimming in an otherwise symmetric system, with the swimming trajectory being sensitive to head shape—no signalling or asymmetric forces are required. We conclude that nonlinear models are essential in understanding the flagellar waveform in migratory human sperm; these models will also be invaluable in understanding motile flagella and cilia in other systems.     

Optimizing perfectly matched layers in discrete contexts

Perfectly matched layers (PMLs) are widely used for the numerical simulation of wave‐like problems defined on large or infinite spatial domains. However, for both time‐dependent and time‐harmonic cases, their performance critically depends on the so‐called absorption function. This paper deals with the choice of this function when classical numerical methods are used (based on finite differences, finite volumes, continuous finite elements and discontinuous finite elements). After reviewing the properties of the PMLs at the continuous level, we analyze how they are altered by the different spatial discretizations. In the light of these results, different shapes of absorption function are optimized and compared by means of both one‐dimensional and two‐dimensional representative time‐dependent cases. This study highlights the advantages of the so‐called shifted hyperbolic function, which is efficient in all cases and does not require the tuning of a free parameter, by contrast with the widely used polynomial functions. Copyright © 2014 John Wiley & Sons, Ltd.     

An efficient finite element time‐domain formulation for the elastic second‐order wave equation: A non‐split complex frequency shifted convolutional PML

The perfectly matched layer (PML) technique has demonstrated very high efficiency as absorbing boundary condition for the elastic wave equation recast as a first‐order system in velocity and stress in attenuating non‐grazing bulk and surface waves. This paper develops a novel convolutional PML formulation based on the second‐order wave equation with displacements as the only unknowns to annihilate spurious reflections from near‐grazing waves. The derived variational form allows for the use of e.g. finite element and the spectral element methods as spatial discretization schemes. A recursive convolution update scheme of second‐order accuracy is employed such that highly stable, effective time integration with the Newmark‐beta (implicit and explicit with mass lumping) method is achieved. The implementation requires minor modifications of existing displacement‐based finite element software, and the stability and efficiency of the proposed formulation is verified by relevant two‐dimensional benchmarks that accommodate bulk and surface waves. Copyright © 2011 John Wiley & Sons, Ltd.     

水下小孔径阵列远程目标检测方法研究

匹配滤波器频域自适应线谱增强方法是一种基于递归算法的非线性滤波技术,它大大提高了匹配滤波器的检测性能.针对当前该技术使用窄带信号作为发射信号存在可利用的带宽有限,不能充分发挥自适应线谱增强器性能的问题,文章提出将该技术与宽带信号相结合来检测远程目标.仿真显示,该方法在低信噪比条件下获得了较高的信噪比增益和检测概率.海试...     

Asymptotic expansions of the Biot-Savart law for a slender vortex with core variation

The Method of Matched Asymptotic Expansion of Singular Integrals (MAESI) is used to expand the Biot-Savart law in terms of different parameters. This method is first used to find, in terms of the small distance r to a line vortex, the first orders of the known expansion of the potential flow induced by this line vortex. This method is also used to easily compare two equations of motion of a slender vortex filament: the one obtained in an ad-hoc way by a cut-off line-integral technique and the other derived from the Navier-Stokes equations by Callegari and Ting. Finally, this method is used to give the inner expansion of the flow induced by a slender vortex in terms of its slenderness . This is the first inner expansion up to order one in terms of of the Biot-Savart law for a slender vortex. An application of this inner expansion is then given to find the induced velocity of a family of non-circular vortex rings with axisymmetric axial-core variation. In order to understand the time-evolution of these initial conditions to the Navier-Stokes equations, a short time scale is introduced. A quasi-hyperbolic system that describes the leading-order dynamics of the axisymmetric axial core variation on a curved slender vortex filament is finally extracted from the Navier-Stokes equations.     

一种用阈值提取子空间的多步匹配场反演方法

在分析已有的匹配场反演方法的基础上,构造了一种用阈值提取子空间的多步匹配场反演方法。它根据一定反演环境下参数的不同敏感性将参数划分为子集（子空间）,并依次在各敏感子空间内反演。反演时用一定的阈值将目标函数优于阈值的参数区域提取出,最后在提取出的已相对缩减的区域和最后一个子空间（通常是不敏感参数子空间）内联合反演全部参数,求得最优值。这样既可减少反演参数空间又能可靠地保证精确度,避免了已有的子空间方法反演结果受非反演参数失配影响的问题。仿真研究结果表明,本算法比已有的两类算法性能上有明显提高。