An adaptive Fourier Transform （FT） with an optimal window has been proposed for the time-frequency analysis of nonstationary time series. The method allows for a good estimation of both frequency and amplitude of th...An adaptive Fourier Transform （FT） with an optimal window has been proposed for the time-frequency analysis of nonstationary time series. The method allows for a good estimation of both frequency and amplitude of the spectrum and can be easily applied to the general case of time-varying signals. The evaluation of the proposed approach has been performed on measured time-varying signals from a suspension bridge model and a steel frame model whose data have the typical non-stationary characteristics. The numerical results show that the proposed approach can overcome some of the difficulties encountered in the classic Fourier transform technique and can achieve higher computation accuracy.展开更多
In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbul...In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbulence models with hydraulic performance experiment, SST k-co model was confirmed to study the rational determination of sampling frequency and time better. The Fast Fourier Transform （FFT） technology was then adopted to process those fluctuating pressure signals obtained. On these bases, the characteristics of pressure fluctuations acting on the tongue were discussed. It is found that aliasing errors decrease at higher sampling frequency of 17 640 Hz, but not at a lower sampling frequency of 1 764 Hz. Correspondingly, an output frequency range ten-times wider is obtained at 17 640 Hz. Compared with 8R, when the sampling time is shorter, the amplitudes may be overvalued, and the frequencies and amplitudes of low-frequency fluctuations can not be well predicted. The frequencies at the tongue are in good agreement with the values calculated by formula and the frequency compositions less than the blade passing frequency are accurately predicted.展开更多
During the last 30 years, China has witnessed rapid economic growth and dramatic urbanization, with about 1.2 x 107 rural people migrating annually into urban areas. Meanwhile, especially since 1995, the rural populat...During the last 30 years, China has witnessed rapid economic growth and dramatic urbanization, with about 1.2 x 107 rural people migrating annually into urban areas. Meanwhile, especially since 1995, the rural population has been declining, which is closely linked to land circulation and the increase in farm size in many villages. Increasing scale of farming operations is often regarded as a key to avoiding the abandonment of farmland and to increasing the income of rural farmers. However, until now, there has been little research on the spatial and temporal variability of farm size at the national level in China. Using data from the national agricultural cen- sus and rural household surveys, this study examines the characteristics of land use circulation and the consequent changes in the area of farmland per household. The results show that： 1） 12.2% of rural households were involved in land circulation at the national level. The highest amounts of land circulation have occurred in those provinces where the farmland per capita is more than 0.2 ha or less than 0.1 ha; 2） over 80% of households operate less than 0.6 ha of farmland; 3） the proportion of mid-sized farms （between 0.2 ha and 0.6 ha per household） has decreased while the smallest and the largest farms have increased. This bears some similarity with the phenomenon known as the ＇disappearing middle＇, referring to the changes in farm size. This study establishes a framework for interpreting the factors affecting the changes in farm size in China, which include two promoting factors （urbanization and agriculture） and four hindering fac- tors （agricultual land system, household registration, stable clan system, and farmland loss）.展开更多
The authors state briefly the possibility of various simulators to handle propagation of electromagnetic waves along some interconnections, in 3D RF （Radio Frequency） circuits. The studies are first derived in the t...The authors state briefly the possibility of various simulators to handle propagation of electromagnetic waves along some interconnections, in 3D RF （Radio Frequency） circuits. The studies are first derived in the time domain： a Finite-Difference Time-Domain method is applied, taking spectra via FFTs （Fast Fourier Transform） as post-processors. Electric and magnetic field distributions, pulse propagations along stripline structures or vias are highlighted. The scattering parameters for various cases are extracted and compared. Some original issue of this work is an insight on crosstalk or shielding phenomena between lines.展开更多
This paper deals with the backscatter coefficients known as sigma0 at Ku band and C band based on the GDR-M (Merged Geophysical Data Record) of TOPEX/Poseidon (T/P) through Jan.1993 to Dec.2004 over land surface of Ch...This paper deals with the backscatter coefficients known as sigma0 at Ku band and C band based on the GDR-M (Merged Geophysical Data Record) of TOPEX/Poseidon (T/P) through Jan.1993 to Dec.2004 over land surface of China.After smoothing and interpolating the backscatter coefficients for both bands, we achieve the 5′×5′ grid data and the time series of backscatter coefficients in 12 years.The spatial distribution of sigma0 over typical areas (wetland, desert, mountainous area, agriculture base, etc.) of Chinese territory is analyzed and discussed.The fast Fourier transformation (FFT) is used to detect the cycles of seasonal variations of sigma0 time series and gives that the annual period is the major cycle.Meanwhile a semiannual period is also found in some places.We use the least squares method on both periods and find that the amplitude of annual period is obviously greater than that of semi-annual period.The relationship among the anomalousness of time series, variations of environment and climate change, and the serious natural calamity (flood, drought) is also discussed.Data of topography slope extracted from SRTM are used to do correlation analysis with the backscatter coefficients in parts of China to quantify the impact of slope on backscatter coefficients in Ku and C bands, and the results show that they all have a negative correlation but the magnitudes are different in different places with different coverages.Such as the area of Liaoning and Jilin has the maximum correlation -0.56, the Taklimakan Desert has the minimum correlation -0.11, and the other places commonly have correlations in (-0.3, -0.5).展开更多
基金Sponsored by the National Natural Science Foundation of China (Grant No. 50378041) the Specialized Research Fund for the Doctoral Program ofHigher Education (Grant No. 20030487016).
文摘An adaptive Fourier Transform （FT） with an optimal window has been proposed for the time-frequency analysis of nonstationary time series. The method allows for a good estimation of both frequency and amplitude of the spectrum and can be easily applied to the general case of time-varying signals. The evaluation of the proposed approach has been performed on measured time-varying signals from a suspension bridge model and a steel frame model whose data have the typical non-stationary characteristics. The numerical results show that the proposed approach can overcome some of the difficulties encountered in the classic Fourier transform technique and can achieve higher computation accuracy.
基金Project supported by the Priority Academic Development Program of Jiangsu Higher Education Institutions, ChinaProject(CXZZ12_0680) supported by Postgraduate Innovation Foundation of Jiangsu Province, ChinaProject(12JDG082) supported by the Advanced Talent Foundation of Jiangsu University, China
文摘In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbulence models with hydraulic performance experiment, SST k-co model was confirmed to study the rational determination of sampling frequency and time better. The Fast Fourier Transform （FFT） technology was then adopted to process those fluctuating pressure signals obtained. On these bases, the characteristics of pressure fluctuations acting on the tongue were discussed. It is found that aliasing errors decrease at higher sampling frequency of 17 640 Hz, but not at a lower sampling frequency of 1 764 Hz. Correspondingly, an output frequency range ten-times wider is obtained at 17 640 Hz. Compared with 8R, when the sampling time is shorter, the amplitudes may be overvalued, and the frequencies and amplitudes of low-frequency fluctuations can not be well predicted. The frequencies at the tongue are in good agreement with the values calculated by formula and the frequency compositions less than the blade passing frequency are accurately predicted.
基金Under the auspices of National Natural Science Foundation of China(No.4097111241161140352)Ministry of Environmental Protection of the People's Republic of China(No.STSN-09-04)
文摘During the last 30 years, China has witnessed rapid economic growth and dramatic urbanization, with about 1.2 x 107 rural people migrating annually into urban areas. Meanwhile, especially since 1995, the rural population has been declining, which is closely linked to land circulation and the increase in farm size in many villages. Increasing scale of farming operations is often regarded as a key to avoiding the abandonment of farmland and to increasing the income of rural farmers. However, until now, there has been little research on the spatial and temporal variability of farm size at the national level in China. Using data from the national agricultural cen- sus and rural household surveys, this study examines the characteristics of land use circulation and the consequent changes in the area of farmland per household. The results show that： 1） 12.2% of rural households were involved in land circulation at the national level. The highest amounts of land circulation have occurred in those provinces where the farmland per capita is more than 0.2 ha or less than 0.1 ha; 2） over 80% of households operate less than 0.6 ha of farmland; 3） the proportion of mid-sized farms （between 0.2 ha and 0.6 ha per household） has decreased while the smallest and the largest farms have increased. This bears some similarity with the phenomenon known as the ＇disappearing middle＇, referring to the changes in farm size. This study establishes a framework for interpreting the factors affecting the changes in farm size in China, which include two promoting factors （urbanization and agriculture） and four hindering fac- tors （agricultual land system, household registration, stable clan system, and farmland loss）.
文摘The authors state briefly the possibility of various simulators to handle propagation of electromagnetic waves along some interconnections, in 3D RF （Radio Frequency） circuits. The studies are first derived in the time domain： a Finite-Difference Time-Domain method is applied, taking spectra via FFTs （Fast Fourier Transform） as post-processors. Electric and magnetic field distributions, pulse propagations along stripline structures or vias are highlighted. The scattering parameters for various cases are extracted and compared. Some original issue of this work is an insight on crosstalk or shielding phenomena between lines.
基金supported by National Natural Science Founda-tion of China(Grant Nos.40974004 and 40774009)International Sci&Tech Cooperation Program of China(Grant No.2009DFB00130)+1 种基金Key Laboratory of Mapping from Space of NASMG,China(Grant No.K201103)the R&I Team Supporting Program and Graduate Sci.&Tech Innovation Fund of SDUST,China(Grant No.YCB110010)
文摘This paper deals with the backscatter coefficients known as sigma0 at Ku band and C band based on the GDR-M (Merged Geophysical Data Record) of TOPEX/Poseidon (T/P) through Jan.1993 to Dec.2004 over land surface of China.After smoothing and interpolating the backscatter coefficients for both bands, we achieve the 5′×5′ grid data and the time series of backscatter coefficients in 12 years.The spatial distribution of sigma0 over typical areas (wetland, desert, mountainous area, agriculture base, etc.) of Chinese territory is analyzed and discussed.The fast Fourier transformation (FFT) is used to detect the cycles of seasonal variations of sigma0 time series and gives that the annual period is the major cycle.Meanwhile a semiannual period is also found in some places.We use the least squares method on both periods and find that the amplitude of annual period is obviously greater than that of semi-annual period.The relationship among the anomalousness of time series, variations of environment and climate change, and the serious natural calamity (flood, drought) is also discussed.Data of topography slope extracted from SRTM are used to do correlation analysis with the backscatter coefficients in parts of China to quantify the impact of slope on backscatter coefficients in Ku and C bands, and the results show that they all have a negative correlation but the magnitudes are different in different places with different coverages.Such as the area of Liaoning and Jilin has the maximum correlation -0.56, the Taklimakan Desert has the minimum correlation -0.11, and the other places commonly have correlations in (-0.3, -0.5).
