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61.
This paper outlines a procedure for the derivation of the differential equations describing the free response of a heaving and pitching ship from its stationary response to random waves. The coupled heave–pitch motion of a ship in random seas is modelled as a multi-dimensional Markov process. The partial differential equation describing the transition probability density function, known as the Fokker-Planck equation, for this process is derived. The Fokker-Planck equation is used to derive the random decrement equations for the coupled heave–pitch motion. The parameters in these equations are then identified using a neural network approach. The method is validated using numerical simulations and experimental results. The experimental data was obtained using an icebreaker ship model heaving and pitching in random waves. It is shown that the method produces good results when the system is lightly damped. An extension for using this method to identify couple heave–pitch motion in realistic seas is suggested. 相似文献
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Application of Markovian models for non-ergodic and non-stationary earthquake times series for the identification of seismic patterns and future projections* 下载免费PDF全文
Hakan Karaca 《地震科学(英文版)》2020,33(2):98-106
The current earthquake forecast algorithms are not free of shortcomings due to inherent limitations. Especially, the requirement of stationarity in the evaluation of earthquake time series as a prerequisite, significantly limits the use of forecast algorithms to areas where stationary data is not available. Another shortcoming of forecast algorithms is the ergodicity assumption, which states that certain characteristics of seismicity are spatially invariant. In this study, a new earthquake forecast approach is introduced for the locations where stationary data are not available. For this purpose, the spatial activity rate density for each spatial unit is evaluated as a parameter of a Markov chain. The temporal pattern is identified by setting the states at certain spatial activity rate densities. By using the transition patterns between the states, 1- and 5-year forecasts were computed. The method is suggested as an alternative and complementary to the existing methods by proposing a solution to the issues of ergodicity and stationarity assumptions at the same time. 相似文献
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针对UBGM(1,1)-Markov模型中存在2个邻近值可能被归属到不同状态,导致预测值产生偏差的问题,结合模糊分类理论,构建基于模糊分类的无偏灰色-马尔科夫模型(unbiased gray-Markov model based on fuzzy classification, FC-UBGM(1,1)-Markov)。首先对UBGM(1,1)模型进行残差修正,然后将修正后拟合值的相对残差序列作为Markov链进行区间划分,再结合模糊分类的隶属度函数,计算相对残差的模糊向量,根据隶属度确定其所属的状态。实际算例表明,该模型比传统UBGM(1,1)-Markov模型的预测效果更好。 相似文献
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This study analyses the temporal clustering, spatial clustering, and statistics of the 2012–2013 Torreperogil-Sabiote (southern Spain) seismic swarm. During the swarm, more than 2200 events were located, mostly at depths of 2–5 km, with magnitude event up to mbLg 3.9 (Mw 3.7). On the basis of daily activity rate, three main temporal phases are identified and analysed. The analysis combines different seismological relationships to improve our understanding of the physical processes related to the swarm's occurrence. Each temporal phase is characterized by its cumulative seismic moment. Using several different approaches, we estimate a catalog completeness magnitude of mc≅ 1.5. The maximum likelihood b-value estimates for each swarm phase are 1.11 ± 0.09, 1.04 ± 0.04, and 0.90 ± 0.04, respectively. To test the hypothesis that a b-value decrease is a precursor to a large event, we study temporal variations in b-value using overlapping moving windows. A relationship can be inferred between change in b-value and the regime style of the rupture. b-values are indicators of the stress regime, and influence the size of ruptures. The fractal dimension D2 is used to perform spatial analysis. Cumulative gamma and beta functions are used to analyse the behaviour of inter-event distances during the earthquake sequence. 相似文献
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Weidong Li Chuanrong Zhang 《International journal of geographical information science》2013,27(6):821-839
In Markov chain random field (MCRF) simulation of categorical spatial variables with multiple classes, joint modeling of a large number of experimental auto and cross-transiograms is needed. This can be tedious when mathematical models are used to fit the complex features of experimental transiograms. Linear interpolation can be used to perform the joint modeling quickly regardless of the number and the complexity of experimental transiograms. In this paper, we demonstrated the mathematical validity of linear interpolation as a joint transiogram-modeling method, explored its applicability and limitations, and tested its effect on simulated results by case studies with comparison to the joint model-fitting method. Simulations of a five-class variable showed little difference in patterns for interpolated and fitted transiogram models when samples were sufficient and experimental transiograms were in regular shapes; however, they neither showed large difference between these two kinds of transiogram models when samples were relatively sparse, which might indicate that MCRFs were not much sensitive to the difference in the detail of the two kinds of transiogram models as long as their change trends were identical. If available, expert knowledge might play an important role in transiogram modeling when experimental transiograms could not reflect the real spatial variation of the categorical variable under study. An extra finding was that class enclosure feature (i.e., a class always appears within another class) was captured by the asymmetrical property of transiograms and further generated in simulated patterns, whereas this might not be achieved in conventional geostatistics. We conclude that (i) when samples are sufficient and experimental transiograms are reliable, linear interpolation is satisfactory and more efficient than model fitting; (ii) when samples are relatively sparse, choosing a suitable lag tolerance is necessary to obtain reliable experimental transiograms for linear interpolation; (iii) when samples are very sparse (or few) and experimental transiograms are erratic, coarse model fitting based on expert knowledge is recommended as a better choice whereas both linear interpolation and precise model fitting do not make sense anymore. 相似文献
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Wei Huang Xintao Liu Yifang Ban 《International journal of geographical information science》2013,27(9):1569-1587
Human mobility patterns can provide valuable information in understanding the impact of human behavioral regularities in urban systems, usually with a specific focus on traffic prediction, public health or urban planning. While existing studies on human movement have placed huge emphasis on spatial location to predict where people go next, the time dimension component is usually being treated with oversimplification or even being neglected. Time dimension is crucial to understanding and detecting human activity changes, which play a negative role in prediction and thus may affect the predictive accuracy. This study aims to predict human movement from a spatio-temporal perspective by taking into account the impact of activity changes. We analyze and define changes of human activity and propose an algorithm to detect such changes, based on which a Markov chain model is used to predict human movement. The Microsoft GeoLife dataset is used to test our methodology, and the data of two selected users is used to evaluate the performance of the prediction. We compare the predictive accuracy (R2) derived from the data with and without implementing the activity change detection. The results show that the R2 is improved from 0.295 to 0.762 for the user with obvious activity changes and from 0.965 to 0.971 for the user without obvious activity changes. The method proposed by this study improves the accuracy in analyzing and predicting human movement and lays the foundation for related urban studies. 相似文献
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