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1.
Xiaoxian Zhang A. Glyn Bengough John W. Crawford Iain M. Young 《Journal of Hydrology》2002,260(1-4):75-87
The non-linear solvers in numerical solutions of water flow in variably saturated soils are prone to convergence difficulties. Many aspects can give rise to such difficulties and in this paper we address the gravity term and the prescribed-flux boundary in the Picard iteration. The problem of the gravity term in the Picard iteration is iteration-to-iteration oscillation as the gravity term is treated, by analogy with the time-step advance technique, ‘explicitly’ in the iteration. The proposed method for the gravity term is an improvement of the ‘implicit’ approach of Zhang and Ewen [Water Resour. Res. 36 (2000) 2777] by extending it to heterogeneous soil and approximating the inter-nodal hydraulic conductivity in the diffusive term and the gravity term with the same scheme. The prescribed-flux boundary in traditional methods also gives rise to iteration-to-iteration oscillation because there is no feedback to the flux in the solution at the new iteration. To reduce such oscillation, a new method is proposed to provide such a feedback to the flux. Comparison with traditional Picard and Newton iteration methods for a wide range of problems show that a combination of these two proposed methods greatly improves the stability and consequently the computational efficiency, making the use of small time step and/or under-relaxation solely for convergence unnecessary. 相似文献
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A modified Rankine source panel method is presented for solving a linearized free-surface flow problem with respect to the double-body potential. The method of solution is based on the distribution of Rankine sources on the hull as well as its image and on the free surface. An iterative algorithm is used for determining the free surface and wave resistance using upstream finite difference operator. A verification of numerical modeling is made using the Wigley hull and the validity of the computer program is examined by comparing the details of wave profiles and wave making resistance with Series 60 model. 相似文献
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将起伏曲面B上的位场向下延拓至曲面最低点的平面A的插值-迭代法步骤是：1）将曲面B上的场值放置在水平面A上具有相同水平坐标的点上,作为A上的初值;2）用若干水平面切割B,从A的初值,用快速傅里叶变换法(FFT)向上延拓出这些平面的场值,用插值的方法从这些平面的场值计算曲面B的场值;3）根据B上的实测值与计算值的差值,对A上的值进行加权改正;4）重复步骤2）和3）,直到B上的差值小到可以忽略.这种插值-迭代法具有高的计算速度,比通常的FFT法延拓得更深,可以超过10倍点距.文中给出计算实例. 相似文献
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有限区域流函数和速度势的计算及其在台风中应用 总被引：1，自引：1，他引：0
在Endlich提出求解旋转风和辐散风基础上,本文给出了直接利用风场迭代计算流函数和速度势的方法,该方法不但能求解流函数和速度势,还可将原始风场直接分解为旋转风和辐散风。此方法在有限区域中,能大大降低边界条件对计算结果的影响,不仅能计算准确的涡度和散度,还能很好的重建原始风场。采用此方法,对"0808"号台风"凤凰"进行了诊断分析,得到了台风不同时期的流函数和速度势、旋转风和辐散风。结果发现在台风的不同时期,流函数与高度场相似,可用其低值中心来分析台风移动路径和强度的变化,速度势存在高值区,其中心与高度场低值中心不吻合;旋转风的涡旋中心也可用来反映台风中心,台风南侧辐散风较大,故可用来反映南方水汽供应对台风发展、成熟、衰退的影响。 相似文献
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An analytical approach to small amplitude solutions of the extended nearly circular Sitnikov problem
J. Hagel 《Celestial Mechanics and Dynamical Astronomy》2009,103(3):251-266
The model of extended Sitnikov Problem contains two equally heavy bodies of mass m moving on two symmetrical orbits w.r.t the centre of gravity. A third body of equal mass m moves along a line z perpendicular to the primaries plane, intersecting it at the centre of gravity. For sufficiently small distance from the
primaries plane the third body describes an oscillatory motion around it. The motion of the three bodies is described by a
coupled system of second order differential equations for the radial distance of the primaries r and the third mass oscillation z. This problem which is dealt with for zero initial eccentricity of the primaries motion, is generally non integrable and
therefore represents an interesting dynamical system for advanced perturbative methods. In the present paper we use an original
method of rewriting the coupled system of equations as a function iteration in such a way as to decouple the two equations
at any iteration step. The decoupled equations are then solved by classical perturbation methods. A prove of local convergence
of the function iteration method is given and the iterations are carried out to order 1 in r and to order 2 in z. For small values of the initial oscillation amplitude of the third mass we obtain results in very good agreement to numerically
obtained solutions. 相似文献
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处理转换计算在重磁资料解释中发挥着重要的作用,但一些计算如向下延拓、化极等有时是很不稳定的,在频率域中则表现为其转换因子具有明显的放大作用,所以其FFT理论计算结果是不稳定的.因此,很多研究工作都是围绕增加计算的稳定性、提高计算效果进行的,其中迭代法是近来在研究中受到普遍重视的方法技术,并取得了较好的成果.但也存在对迭代法研究还不够深入,对其存在的缺点认识不够充分、客观等问题,例如,迭代法进行延拓及化极等计算时,对一些具体应用虽能在一定程度上获得较好的计算结果,但却存在计算结果并不会随着迭代次数的增加而得到持续改善的问题,对于原本不稳定的计算,迭代法在迭代次数比较大时,所得的结果依然是不稳定的.为此,本文在对迭代法进行分析研究的基础上,进一步推导了迭代法的通式,并分析了对迭代法收敛性影响的各种因素.分析结果表明:迭代法收敛到FFT理论直接计算结果的决定因素是计算过程中如何选择原始数据到目标数据的映射函数;在选择了合适的映射函数的情况下,迭代次数不仅仅是决定计算成本,而是决定结果好坏的关键因素;增加迭代次数虽然能够使计算收敛到FFT直接计算理论结果,但如果该理论结果本身就是不稳定的,则迭代法计算如果收敛,也是收敛到一个不稳定的结果.所以针对位场处理转换中一些不稳定计算采用迭代法,并没有从根本上解决计算的不稳定性问题. 相似文献
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应用NOAA/AVHRR数据测算局地水稻种植面积方法研究 总被引：29，自引：3，他引：29
本文采用在模糊监督分类中增加迭代过程的方法－－模糊监督分类一迭代法,在分解混合像元的基础上,利用ＡＶＨＲＲ数据求算水稻种植面积。根据稻田与旱地存在温度差异的特点,在分析ＡＶＨＲＲ数据统计特征的基础上,增加了第３和４两通道参加模糊监督分类,从而增加了分类橡元值矢量维数,增强了对水稻的鉴别能力。反采用的迭代法是收敛具有效的,经本方法输出的水稻种植面积百分含量图与实际水稻分布十分吻合,测算出的面积值与Ｔ 相似文献
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MIHT(Modified Iterated Hough Transform)方法用于影像配准时,具有以直线段作为配准基元、线段之间不需同名关系等优点,同时亦存在着收敛速度慢、计算费时等不足。针对MIHT算法的缺点,提出将选权迭代最小二乘法引入到计算过程中,并且文中给出改进算法的具体实施流程。实验证明,与原算法相比较,改进算法的收敛速度和可靠性都有了明显的提高。 相似文献
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数字高程模型是重要的地理空间数据,可以提供丰富的地形信息.为了获得时效性好、分辨率高的数字高程模型,本文以南京市为实验区域,利用德国宇航局TerraSAR-X卫星同一轨道上的两幅StripMap模式SAR影像对,主从影像时间间隔只有66天,空间分辨率为3 m.采用InSAR技术,通过迭代的方法来提高获取DEM的分辨率,并结合JAXA/EORC提供的AW3D30数字表面模型与迭代后的DEM进行融合来解决阴影等问题,最终获取了南京地区的高精度数字高程模型.将实验结果分别与90 m分辨率的SRTM、30 m分辨率的ASTER GDEM、30 m分辨率的AW3D30进行相互对比分析.结果表明,相比三种常用的DEM而言,本文获取的DEM能够更精确的获取地面的细节信息,特别是对于分布稀疏的大型单体建筑物,能够很好的恢复其三维信息,但是对于建筑物分布较为密集的区域,由于传感器视线受阻,不能观测建筑物的全貌,阴影分布较多,导致此类区域的DEM结果不理想,还需进一步深入研究,提高精度及可靠性. 相似文献