共查询到20条相似文献，搜索用时 31 毫秒
1.
Irene M. Moroz 《地球物理与天体物理流体动力学》2013,107(3-4):313-314
Abstract The model equations describing two-dimensional thermohaline convection of a Boussinesq fluid in a rotating horizontal layer are known to support multiple instabilities, depending on the values of certain control parameters (Arneodo et al., 1985). Most of these multiple instabilities have already been studied for double or triple diffusive convection, where behaviours ranging from simple steady to irregular motions have been found. Here we consider the one remaining bifurcation mentioned by Arneodo et al. (1985): the interaction between a steady and an oscillatory convection roll when the linear spectrum for a single wavenumber comprises one zero and one pair of purely imaginary eigenvalues. The method of centre manifolds and normal forms is used to derive evolution equations for the amplitudes of the convection rolls close to bifurcation and the behaviours associated with the equations is discussed. 相似文献
2.
E. J. DOUZE 《Geophysical Prospecting》1971,19(2):253-264
Optimum filters can be computed using orthogonal coordinates obtained from the eigenvalues and eigenvectors of the autocorrelation matrix. The method is used to obtain unit distance prediction error filters. The output of a unit distance prediction error filter when applied to the input wavelet is an impulse at zero time. The effect on the output of added white noise is easily obtained using the approach through the orthogonal coordinates. The added white noise results in output wavelets which are no longer impulses at zero time. The decrease in time resolution gives a filter that does not increase undesirable high frequency noise as much as filters computed without white noise. Orthogonal coordinates with little signal energy can be omitted from the filter computation resulting in output wavelets resembling those computed using added white noise. 相似文献
3.
A new method is introduced for the identification of modal quantities of self-adjoint distributed-parameter systems. The method uses the temporal and spatial orthogonality properties of distributed-parameter systems to form a pseudo-Rayleigh quotient. The stationary values of the pseudo-Rayleigh quotient can be determined by the solution of an eigenvalue problem, where the eigenvalues and eigenfunctions provide the frequencies and natural modes of the distributed-parameter system. By way of formulation, the method is insensitive to zero mean measurement noise and is applicable to continuous as well as discrete systems. Numerical examples are presented in which the modal quantities of a simply-supported beam and a discrete model of a membrane are identified. The results are compared with another modal identification technique, namely, the Ibrahim time domain method. 相似文献
4.
Min Liang Denis Marcotte Nicolas Benoit 《Stochastic Environmental Research and Risk Assessment (SERRA)》2014,28(7):1733-1741
The estimation of overburden sediment thickness is important in hydrogeology, geotechnics and geophysics. Usually, thickness is known precisely at a few sparse borehole data. To improve precision of estimation, one useful complementary information is the known position of outcrops. One intuitive approach is to code the outcrops as zero thickness data. A problem with this approach is that the outcrops are preferentially observed compared to other thickness information. This introduces a strong bias in the thickness estimation that kriging is not able to remove. We consider a new approach to incorporate point or surface outcrop information based on the use of a non-stationary covariance model in kriging. The non-stationary model is defined so as to restrict the distance of influence of the outcrops. Within this distance of influence, covariance parameters are assumed simple regular functions of the distance to the nearest outcrop. Outside the distance of influence of the outcrops, the thickness covariance is assumed stationary. The distance of influence is obtained thru a cross-validation. Compared to kriging based on a stationary model with or without zero thickness at outcrop locations, the non-stationary model provides more precise estimation, especially at points close to an outcrop. Moreover, the thickness map obtained with the non-stationary covariance model is more realistic since it forces the estimates to zero close to outcrops without the bias incurred when outcrops are simply treated as zero thickness in a stationary model. 相似文献
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6.
Michael R. E. Proctor 《地球物理与天体物理流体动力学》2013,107(1):311-324
Abstract The kinematic dynamo problem is considered for certain steady velocity fields with symmetries that are plausible in a rapidly rotating convective system. By generalizing results proved for the mean field dynamo model by Proctor (1977a), it is shown that for a related “comparison problem” with modified boundary conditions, the eigenvalues are degenerate if there is no axisymmetric mean circulation, with modes of dipole and quadrupole parity excited with equal ease. The comparison problem can be shown to be closely similar to the dynamo problem when there is a region unfavourable to dynamo action surrounding the dynamo region. The near-symmetries found by Roberts (1972) for the mean field model are invoked to suggest that a close correspondence is likely even when this region is absent. It is therefore conjectured that such mean motions may be important in explaining the observed preference for solutions of dipole parity by planetary dynamos. 相似文献
7.
采样数据长度对潮汐因子计算精度的影响 总被引：1，自引：0，他引：1
选取福建省地下流体3个测点的水位资料,以提取M2波幅度为例,根据所选取采样数据长度的不同分别对提取出的M2波幅度进行比较。结果表明,M2波幅度随农历时间的变化而改变,在小潮时几乎接近于零,且不同长度的采样数据的对比证明,以月尺度数据求出的M2波幅度最佳。 相似文献
8.
With the opening of ancient Tibetan structures to visitors worldwide, human load has become the principal live load on these structures. This project studies the properties of the floor structure of an ancient Tibetan building and its behavior under human-induced load effects. Tests were conducted with static and dynamic crowd load, including stepping and jogging by people at a fixed position. The tests show that the floor structure does not behave as a continuous slab. It takes the load from local areas with minimal load transference properties. The acceleration response shows significant peaks when the footstep frequency is close to the natural frequency of the structure, but the human jogging excitation frequency does not have any obvious effect on the structural response. The elastic modulus of the Agatu material is estimated to be close to zero from the measured natural frequency of the slab. The Agatu material is concluded to be a discrete compacted material with insignificant contribution to the structural rigidity of the floor slab. 相似文献
9.
The Erzurum Basin has received more attention in petroleum potential research because of its particularity in geographic and tectonic position. There remains debate on the basement structure of the basin since igneous rocks and faults make the structure and stratigraphy more complicated. We utilize gravity data to understand the structure of the Erzurum Basin. This study describes an edge enhancement technique based on the eigenvalues and determinant obtained from the curvature gravity gradient tensor (CGGT). The main goal of this technique is to delineate structural boundaries in complex geology and tectonic environment using CGGT. The results obtained from theoretical data, with and without Gaussian random noise, have been analyzed in determining the locations of the edges of the vertical-sided prism models. The zero contours of the smallest eigenvalue delineate the spatial location of the edges of the anomalous sources. In addition, 3-D gravity inversion of Bouguer anomalies has been used with purpose to estimate the structure of the substrata to allow modeling of the basement undulation in the Erzurum basin. For this reason, the Parker–Oldenburg algorithm helped to investigate this undulation and to evidence the main linear features. This algorithm reveals presence of basement depths between 3.45 and 9.06 km in the region bounded by NE–SW and E–W trending lineaments. We have also compared the smallest eigenvalue zero contours with the HGM images and Tilt derivative (TDR) of Bouguer anomaly map of the study area. All techniques have agreed closely in detecting the horizontal locations of geological features in the subsurface with good precision. 相似文献
10.
H. Burtscher P. Cohn L. Scherrer H.C. Siegmann G. Faraci A.R. Pennisi V. Privitera R. Cristofolini V. Scribano 《Journal of Volcanology and Geothermal Research》1987,33(4)
Volcanic aerosol emissions have been studied for the first time by in situ photoelectric charging. Explorative studies on Mt Etna reveal large concentrations of particles below 1000 nm with the spectrum peaking in the size range of 100–1000 nm diameter. Although a large fraction of the particles is already charged upon emission, the net electrical charge carried by the aerosol turned out to be close to zero.Particles with high photoelectric yield vary greatly in their relative abundance and seem to occur mainly at active points of the volcano. 相似文献
11.
分析实际桥梁结构的各种损伤状况,采用矩阵摄动理论分析桥梁结构发生损伤时固有频率及振型向量的一阶、二阶摄动结果,并讨论了固有振动特性对结构单元损伤的灵敏性。在理论分析的基础上,编制桥梁结构固有振动特性及其在损伤下的摄动分析程序,并用以分析简支梁、连续梁及拱结构损伤引起的固有振动的摄动结果。分析结果表明:单一单元损伤引起固有频率的摄动结果很小;不同位置处单元损伤与固有频率摄动结果的关系曲线与对应振型曲线具有相似性,但在中间支承处或结构端部有扭转约束处单元损伤影响显著;而单元损伤对振型向量影响极小。 相似文献
12.
Modifications are made to the revised Morgan–Morgan–Finney erosion prediction model to enable the effects of vegetation cover to be expressed through measurable plant parameters. Given the potential role of vegetation in controlling water pollution by trapping clay particles in the landscape, changes are also made to the way the model deals with sediment deposition and to allow the model to incorporate particle‐size selectivity in the processes of erosion, transport and deposition. Vegetation effects are described in relation to percentage canopy cover, percentage ground cover, plant height, effective hydrological depth, density of plant stems and stem diameter. Deposition is modelled through a particle fall number, which takes account of particle settling velocity, flow velocity, flow depth and slope length. The detachment, transport and deposition of soil particles are simulated separately for clay, silt and sand. Average linear sensitivity analysis shows that the revised model behaves rationally. For bare soil conditions soil loss predictions are most sensitive to changes in rainfall and soil parameters, but with a vegetation cover plant parameters become more important than soil parameters. Tests with the model using field measurements under a range of slope, soil and crop covers from Bedfordshire and Cambridgeshire, UK, give good predictions of mean annual soil loss. Regression analysis of predicted against observed values yields an intercept value close to zero and a line slope close to 1·0, with a coefficient of efficiency of 0·81 over a range of values from zero to 38·6 t ha?1. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
13.
Algorithm for recognizing Pc3 geomagnetic pulsations in 1-s data from INTERMAGNET equatorial observatories 总被引：1，自引：0，他引：1
N. R. Zelinskiy N. G. Kleimenova O. V. Kozyreva S. M. Agayan Sh. R. Bogoutdinov A. A. Soloviev 《Izvestiya Physics of the Solid Earth》2014,50(2):240-248
The methods are suggested for analyzing the data of three-component geomagnetic observations in order to automatically recognize time anomalies-pulsations in the geomagnetic field. These methods include preliminary bandpass filtering of the data, calculating the eigenvalues of the covariance matrix of magnetic components in a moving time window, computing the generalized variance of the eigenvalues (generalization is understood as raising to a power that is distinct from the traditional power of 2), averaging the variance, and identifying the time intervals marked by the presence of pulsations by the criterion of the averaged variance of eigenvalues to exceed a certain threshold specified by the fuzzy-logic methods. 相似文献
14.
Closed-form expressions are obtained to calculate the approximate complex eigenvalues and eigenvectors of a system composed of a non-classically damped primary structure and a single degree of freedom oscillator. The expressions are obtained through a systematic second order perturbation analysis of a transformed eigenvalue problem of the combined system. The possibility of tuning between the structure and equipment is considered. The dynamic properties of the combined system are derived in terms of the complex eigenvalues and eigenvectors of the supporting structure and the frequency, mass and damping ratio of the equipment. Examples demonstrating the accuracy of the expressions for the eigenvalues and eigenvectors are presented. These eigenproperties are used for generation of floor response spectra for non-classically damped structures to incorporate the dynamic interaction effects between the structure and equipment. 相似文献
15.
汶川8.0级地震前龙门山断裂带能量场变化 总被引：3，自引：0，他引：3
在大地震的孕育和发生过程中,沿断裂带的应力-应变状态与断裂带周围微震活动有着密切的关系.震前沿构造断裂带附近地震活动的能量释放更能反映地震孕育的动态过程.本文采用自然正交函数展开方法,讨论了2008年汶川8.0级地震前沿龙门山断裂带地震活动能量场的时间变化.结果表明,在汶川8.0级地震前,沿龙门山断裂带能量场的几个主要特征值对应的典型场的时间因子出现非常明显的短期上升变化,可能是该次地震短期异常信息.通过对比分析1976年唐山地震等震例认为,研究沿活动断裂带地震活动能量场的时间变化,可以对地震中长期判断有较大危险性的活动断裂区进行有目的的重点监测和预测. 相似文献
16.
应用自然正交分量(NOC)方法,对1995～2006年中国地区36个地磁台的地磁场观测数据进行了计算,获得了上述数据矩阵的本征值和相应的空间域、时间域本征向量,并就其时空分布与中国地磁模型（CGM）进行了对比分析,探讨了它的物理意义.研究表明,上述观测数据包含了丰富的地磁内、外源场信息,其中地磁场要素F、D、I第一、第二本征值及相应的空间域和时间域的第一、第二本征向量所描述的地磁基本场时间变化与CGM吻合较好. 相似文献
17.
A method, based on the Hilbert–Huang spectral analysis, has been proposed by the authors to identify linear structures in which normal modes exist (i.e., real eigenvalues and eigenvectors). Frequently, all the eigenvalues and eigenvectors of linear structures are complex. In this paper, the method is extended further to identify general linear structures with complex modes using the free vibration response data polluted by noise. Measured response signals are first decomposed into modal responses using the method of Empirical Mode Decomposition with intermittency criteria. Each modal response contains the contribution of a complex conjugate pair of modes with a unique frequency and a damping ratio. Then, each modal response is decomposed in the frequency–time domain to yield instantaneous phase angle and amplitude using the Hilbert transform. Based on a single measurement of the impulse response time history at one appropriate location, the complex eigenvalues of the linear structure can be identified using a simple analysis procedure. When the response time histories are measured at all locations, the proposed methodology is capable of identifying the complex mode shapes as well as the mass, damping and stiffness matrices of the structure. The effectiveness and accuracy of the method presented are illustrated through numerical simulations. It is demonstrated that dynamic characteristics of linear structures with complex modes can be identified effectively using the proposed method. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
18.
The main purpose of this study is to evaluate the potential of simulating the profiles of the mean velocity and turbulence intensities for the steep open channel flows over a smooth boundary using artificial neural networks. In a laboratory flume, turbulent flow conditions were measured using a fibre‐optic laser doppler velocimeter (FLDV). One thousand and sixty‐four data sets were collected for different slopes and aspect ratios at different locations. These data sets were randomly split into two subsets, i.e. training and validation sets. The multi‐layer functional link network (MFLN) was used to construct the simulation model based on the training data. The constructed MFLN models can almost perfectly simulate the velocity profile and turbulence intensity. The values of correlation coefficient (γ) are close to one and the values of root mean square error (RMSE) are close to zero in all conditions. The results demonstrate that the MFLN can precisely simulate the velocity profiles, while the log law and Reynolds stress model (RSM) are less effective when used to simulate the velocity profiles close to the side wall. The simulated longitudinal turbulence intensities yielded by the MFLN were also fairly consistent with the measured data, while the simulated vertical turbulence intensities by the RSM were not consistent with the measured data. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
19.
基于特征结构配置的结构鲁棒控制算法及仿真 总被引：2，自引：0，他引：2
本文提出了结构系统的鲁棒主动控制算法，其目的是求取状态反馈控制律，在闭环系统具有希望特征值前提下，使得闭环系统特征值关于参数摄动具有最小的灵敏度。基于状态反馈特征结构配置参数化方法和矩阵特征值灵敏度分析理论，该问题转化为含有约束条件的优化问题，并给出了求解该鲁棒控制问题的算法。该方法给出了特征值灵敏度函数的参数化表示，且直接基于结构系统矩阵，故便于工程应用。三自由度层间剪切型结构地震作用下的仿真分析，表明所提方法的有效性。 相似文献
20.
Fabric ‘shape’, based on the relative values of three eigenvalues calculated from fabric data, has been used to differentiate sediment facies and infer deformation histories. The eigenvalues are based on samples drawn from parent populations, and as such are subject to statistical variance due to sampling effects. In this paper, the degree of statistical variance in fabric data for two types of subglacial till from Breidamerkurjøkull, Iceland, is investigated using ‘bootstrapping’ techniques, in which empirical ‘confidence regions’ are built up by repeated resampling of the original data. The experiments show that, for each till type, the observed between‐sample variability in the fabrics lies within the boundaries associated with random variations, indicating that the observed range of fabric shapes within each till type is likely to be entirely the product of sampling effects. Differences in fabric shape between till types are generally greater than that associated with random variations, indicating that their fabric shape characteristics, as measured by eigenvalues, are significantly different. Nevertheless, the results suggest that great care should be exercised when using a‐axis fabric data to differentiate sedimentary facies, or to infer subtle differences in physical processes or conditions. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献