Mining the inherent persistence property of the time series of wind power is crucial for forecasting and controlling wind power.Few common methods exist that can fully depict and quantify the persistence property.Base...Mining the inherent persistence property of the time series of wind power is crucial for forecasting and controlling wind power.Few common methods exist that can fully depict and quantify the persistence property.Based on the definition of the active power output state of a wind farm,this paper describes the statistical persistence property of the duration time and state transition.Based on the results of our analysis of significant amounts of wind power field measurements,it is found that the duration time of wind power conforms to an inverse Gaussian distribution.Additionally,the state transition matrix of wind power is discovered to yield a ridge property,the gradient of which is related to the time scale of interest.A systemaic methodology is proposed accordingly,allowing the statistical characteristics of the wind power series to be represented appropriately.展开更多
Using the limit surface slope as a criterion of wave breaking, a simple model for estimating the spatial fraction of breaking surface of sea at an instant, which is regarded as the whitecap coverge in this paper, is a...Using the limit surface slope as a criterion of wave breaking, a simple model for estimating the spatial fraction of breaking surface of sea at an instant, which is regarded as the whitecap coverge in this paper, is analytically derived from the probability density of surface slope based on Gaussian statistics. The resulting fraction is found depending on the fourth moment of wave spectum, m(4), as well as the critical threshold of surface slope. By expressing the fourth moment in terms of the Neumann spectrum, a formula linking the fraction and wind speed for fully developed sea states is obtianed. Another formula relating the fraction to both wind speed and fetch (or duration) is achieved by expressing m, in terms of the Krylov spectrum and applying the empirical relationships used in the SMB ocean wave predicting technique. A comparison between these results and the field data of whitecap coverage collected by Monahan and O'Muircheartuigh shows an encouraging agreement.展开更多
Background Hypertension （HTN） is a very prevalent public health problem and as the population ages,cognitive impairment （CI） is also going to be a public health burden.However,the relationship between hypertension...Background Hypertension （HTN） is a very prevalent public health problem and as the population ages,cognitive impairment （CI） is also going to be a public health burden.However,the relationship between hypertension duration and cognitive function declination worldwide is still unknown.The aim of the study was to investigate the effects of HTN duration on CI in Chinese population.Methods At baseline,1 386 HTN patients and 293 normotensive （NT） people were enrolled.The HTN patients were further divided into four subgroups （duration of HTN 〈6 years,6-10 years,11-20 years,and 〉20 years） according to the HTN duration and were screened for cognitive function with neuropsychological tests including mini-mental-state-examination （MMSE） and clock-drawing-test （CDT） in comparison with the NT group.Results More HTN patients had CI （45.3%） than NT subjects （30.4%）,and increased with HTN duration （P〈0.000 1）.Compared with the scores of MMSE and CDT in the NT group,the declines were higher in the HTN patients （P 〈0.000 1）,and in the four HTN subgroups,both MMSE and CDT scores fall when the HTN duration increased （P 〈0.05）.Furthermore,in the HTN population without CI,the trend in alteration of CI was relatively mild but still existed （P 〈0.05）.Conclusion People with HTN are more likely to have CI and the possibility and aggravation increases the prolongation of HTN duration,both in amnesic and nonamnesic function.展开更多
基金supported by the Natural High Technology Research and Development of China(863 Program)(Grant No.2011AA05A112)the National Natural Science Foundation of China(Grant No.51377027)ABB(China)Ltd.
文摘Mining the inherent persistence property of the time series of wind power is crucial for forecasting and controlling wind power.Few common methods exist that can fully depict and quantify the persistence property.Based on the definition of the active power output state of a wind farm,this paper describes the statistical persistence property of the duration time and state transition.Based on the results of our analysis of significant amounts of wind power field measurements,it is found that the duration time of wind power conforms to an inverse Gaussian distribution.Additionally,the state transition matrix of wind power is discovered to yield a ridge property,the gradient of which is related to the time scale of interest.A systemaic methodology is proposed accordingly,allowing the statistical characteristics of the wind power series to be represented appropriately.
基金This work was financially supported by the National Science Foundation of China(No.49476270,49706067)
文摘Using the limit surface slope as a criterion of wave breaking, a simple model for estimating the spatial fraction of breaking surface of sea at an instant, which is regarded as the whitecap coverge in this paper, is analytically derived from the probability density of surface slope based on Gaussian statistics. The resulting fraction is found depending on the fourth moment of wave spectum, m(4), as well as the critical threshold of surface slope. By expressing the fourth moment in terms of the Neumann spectrum, a formula linking the fraction and wind speed for fully developed sea states is obtianed. Another formula relating the fraction to both wind speed and fetch (or duration) is achieved by expressing m, in terms of the Krylov spectrum and applying the empirical relationships used in the SMB ocean wave predicting technique. A comparison between these results and the field data of whitecap coverage collected by Monahan and O'Muircheartuigh shows an encouraging agreement.
文摘Background Hypertension （HTN） is a very prevalent public health problem and as the population ages,cognitive impairment （CI） is also going to be a public health burden.However,the relationship between hypertension duration and cognitive function declination worldwide is still unknown.The aim of the study was to investigate the effects of HTN duration on CI in Chinese population.Methods At baseline,1 386 HTN patients and 293 normotensive （NT） people were enrolled.The HTN patients were further divided into four subgroups （duration of HTN 〈6 years,6-10 years,11-20 years,and 〉20 years） according to the HTN duration and were screened for cognitive function with neuropsychological tests including mini-mental-state-examination （MMSE） and clock-drawing-test （CDT） in comparison with the NT group.Results More HTN patients had CI （45.3%） than NT subjects （30.4%）,and increased with HTN duration （P〈0.000 1）.Compared with the scores of MMSE and CDT in the NT group,the declines were higher in the HTN patients （P 〈0.000 1）,and in the four HTN subgroups,both MMSE and CDT scores fall when the HTN duration increased （P 〈0.05）.Furthermore,in the HTN population without CI,the trend in alteration of CI was relatively mild but still existed （P 〈0.05）.Conclusion People with HTN are more likely to have CI and the possibility and aggravation increases the prolongation of HTN duration,both in amnesic and nonamnesic function.
