Studying orbital angular momentum(OAM) spectra is important for analyzing crosstalk in free-space optical(FSO)communication systems. This work offers a new method of simplifying the expressions for the OAM spectra...Studying orbital angular momentum(OAM) spectra is important for analyzing crosstalk in free-space optical(FSO)communication systems. This work offers a new method of simplifying the expressions for the OAM spectra of Laguerre-Gaussian(LG) beams under both weak/medium and strong atmospheric turbulences. We propose fixing the radius to the extreme point of the intensity distribution, review the expression for the OAM spectrum under weak/medium turbulence,derive the OAM spectrum expression for an LG beam under strong turbulence, and simplify both of them to concise forms.Then, we investigate the accuracy of the simplified expressions through simulations. We find that the simplified expressions permit accurate calculation of the OAM spectrum for large transmitted OAM numbers under any type of turbulence. Finally,we use the simplified expressions to analytically address the broadening of the OAM spectrum caused by atmospheric turbulence. This work should contribute to the concise theoretical derivation of analytical expressions for OAM channel matrices for FSO-OAM communications and the analytical study of the laws governing OAM spectra.展开更多
An experimental study on TBL (turbulent boundary layer) pressure fluctuation frequeny spectrum of a revolution body is presented. With the measured results, a relation of convective frequency fo of the models is...An experimental study on TBL (turbulent boundary layer) pressure fluctuation frequeny spectrum of a revolution body is presented. With the measured results, a relation of convective frequency fo of the models is obtained. Relations of turbuleat wall-pressure fluctuation spectrum in transition region and development region to frequency and speed are obtained also展开更多
The wind velocity spectra at Beijing Meteorological Tower are calculated using Hilbert-Huang transform and Fourier transform,respectively.A innovative model of wind velocity spectrum,which is accordant with the charac...The wind velocity spectra at Beijing Meteorological Tower are calculated using Hilbert-Huang transform and Fourier transform,respectively.A innovative model of wind velocity spectrum,which is accordant with the characteristics in both the inertial subrange and the large eddies range,is presented in this paper.The method of least squares is adopted to obtain the parameters in the model.Then the differences between the FFT spectrum and the HHT spectrum are compared.It is indicated that the values of the HHT spectrum in the energy containing range are slightly larger than those of the FFT spectrum while the values of the HHT spectrum in both inertial subrange and dissipation subrange are very close to that of Fourier spectrum.It is concluded that the HHT spectrum describes elaborately and accurately the spectrum values in the low frequencies and the fitted wind velocity model provides a reference for reconstructing the near-ground wind field of Beijing city in wind tunnel test and for numerical simulation.展开更多
Unlike previous studies on wind turbulence spectrum in the planetary boundary layer, this investigation focuses on high-altitude (1-5 km) wind energy spectrum and turbulence spectrum under various weather conditions...Unlike previous studies on wind turbulence spectrum in the planetary boundary layer, this investigation focuses on high-altitude (1-5 km) wind energy spectrum and turbulence spectrum under various weather conditions. A fast Fourier transform (FFT) is used to calculate the wind energy and turbulence spectrum density at high altitudes (1-5 km) based on wind profiling radar (WPR) measurements. The turbulence spectrum under stable weather conditions at high altitudes is expressed in powers within a frequency range of 2 × 10-5-10-3 s-1, and the slope b is between -0.82 and -1.04, indicating that the turbulence is in the transition from the energetic area to the inertial sub-range. The features of strong weather are reflected less obviously in the wind energy spectrum than in the turbulence spectrum, with peaks showing up at different heights in the latter spectrum. Cold windy weather appears over a period of 1.5 days in the turbulence spectrum. Wide-range rainstorms exhibit two or three peaks in the spectrum over a period of 15-20 h, while in severe convective weather conditions, there are two peaks at 13 and 9 h. The results indicate that spectrum analysis of wind profiling radar measurements can be used as a supplemental and helpful method for weather analysis.展开更多
This article aims to build a theory of atmospheric boundary layer turbulence under complex conditions. To achieve this goal, we constructed a multi-site observation and analysis method of atmospheric turbulence based ...This article aims to build a theory of atmospheric boundary layer turbulence under complex conditions. To achieve this goal, we constructed a multi-site observation and analysis method of atmospheric turbulence based on related principles.This method first requires verification for the ergodicity of the full-scale observation of surface-layer turbulence, which proves that eddies within a scale of 60 min during a four-site observation can easily meet ergodicity. Meanwhile, by applying the second-order structure function for the horizontal wind speed turbulence of a single site and upstream and downstream points, we verified the ergodicity of the turbulence observation. Comparing the turbulence spectrum to the second-order structure function for the horizontal wind speed from the four-site observation, a relatively high accordance was observed, proving the reasonability of the multi-site observation. Moreover, compared to the single-site observation, the four-site observation can improve the estimation accuracy of the surface-layer turbulence spectrum and vertical turbulent flux. As a result, we can describe the threedimensional structure of turbulence more accurately and comprehensively by combining analytical data from single-site and four-site observations. In summary, the multi-site turbulence observation method shows that the horizontal and vertical wind turbulence of the Baimiao plateau has a typical structure of a turbulence spectrum with clear spectral gaps. The result is in accordance with the scale of the turbulence spectral gaps obtained from the 6 h data. The horizontal wind speed is under the influence of the terrain, so its spectrum of large-scale eddies has higher fluctuations, but its spectral gaps can still be clearly distinguished. Although the spectral gaps of the temperature spectrum are not distinguishable, they still have the same scale as the spectral gap of the vertical and horizontal turbulence spectrum. Moreover, the temperature spectrum possesses typical structure characteristics of the boundary-layer turbulence spectrum.展开更多
To display the distribution characteristics of turbulence eddy under condition of complex terrain in Loess Plateau,and to enhance the precision of turbulence measurements,the research of turbulence ergodicity is consi...To display the distribution characteristics of turbulence eddy under condition of complex terrain in Loess Plateau,and to enhance the precision of turbulence measurements,the research of turbulence ergodicity is considered to be the prior section of the experiment.With the statistics of single-site turbulence measurements obtained in Baimiao Tableland,Ping Liang,Gan Su,analysis result shows that not only the turbulence with scale less than 10 min,which can easily satisfy the ergodicity,the tableland terrain can also be a major cause of the coherent structure of periodic,large-scale turbulence;compared to which for the turbulence above the flat underlying surface,the distribution of 10–40 min,large-scale turbulence in the tableland region tends to be more steady and,thus,can also satisfy the ergodicity easily.Under the condition of extremely unstable stratification,the wind is comparatively low in speed and features distinctly large-scale,periodical fluctuation,and with the trend of smooth increase in temperature,large-scale wind turbulence and temperature turbulence both tend to satisfy ergodicity.In comparison,under the condition of extremely stable stratification,the aperiodicity of large-scale wind turbulence and temperature turbulence caused by intermittency is comparatively strong,and the turbulence cannot satisfy ergodicity easily.展开更多
The paper computed the spectra of velocity and temperature,and the cospectra of velocity and temperature by the observational data of July,1990 in the suburbs of Tianjin.The results show that the characteristics of th...The paper computed the spectra of velocity and temperature,and the cospectra of velocity and temperature by the observational data of July,1990 in the suburbs of Tianjin.The results show that the characteristics of the atmospheric turbulence spectra over the suburbs are basically in accordance with some typical results over the flat terrain.But the scale,on which turbulence can satisfy the isotropic condition,over the suburbs is larger than over the flat terrain.The spectrum peak frequency range is a bit narrower.The feature of the spectrum range in low frequency(LF)is out of accordance with that of the flat terrain either.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61575027 and 61471051)
文摘Studying orbital angular momentum(OAM) spectra is important for analyzing crosstalk in free-space optical(FSO)communication systems. This work offers a new method of simplifying the expressions for the OAM spectra of Laguerre-Gaussian(LG) beams under both weak/medium and strong atmospheric turbulences. We propose fixing the radius to the extreme point of the intensity distribution, review the expression for the OAM spectrum under weak/medium turbulence,derive the OAM spectrum expression for an LG beam under strong turbulence, and simplify both of them to concise forms.Then, we investigate the accuracy of the simplified expressions through simulations. We find that the simplified expressions permit accurate calculation of the OAM spectrum for large transmitted OAM numbers under any type of turbulence. Finally,we use the simplified expressions to analytically address the broadening of the OAM spectrum caused by atmospheric turbulence. This work should contribute to the concise theoretical derivation of analytical expressions for OAM channel matrices for FSO-OAM communications and the analytical study of the laws governing OAM spectra.
文摘An experimental study on TBL (turbulent boundary layer) pressure fluctuation frequeny spectrum of a revolution body is presented. With the measured results, a relation of convective frequency fo of the models is obtained. Relations of turbuleat wall-pressure fluctuation spectrum in transition region and development region to frequency and speed are obtained also
基金supported by the National Natural Science Foundation of China (Grant Nos. 90815021, 51021140005, 50978025, 50938008)
文摘The wind velocity spectra at Beijing Meteorological Tower are calculated using Hilbert-Huang transform and Fourier transform,respectively.A innovative model of wind velocity spectrum,which is accordant with the characteristics in both the inertial subrange and the large eddies range,is presented in this paper.The method of least squares is adopted to obtain the parameters in the model.Then the differences between the FFT spectrum and the HHT spectrum are compared.It is indicated that the values of the HHT spectrum in the energy containing range are slightly larger than those of the FFT spectrum while the values of the HHT spectrum in both inertial subrange and dissipation subrange are very close to that of Fourier spectrum.It is concluded that the HHT spectrum describes elaborately and accurately the spectrum values in the low frequencies and the fitted wind velocity model provides a reference for reconstructing the near-ground wind field of Beijing city in wind tunnel test and for numerical simulation.
基金Supported by the National Natural Science Foundation of China(41075023 and 41475029)China Meteorological Administration Special Public Welfare Research Fund(GYHY201306004)Key Technology Integration and Application Fund of the China Meteorological Administration(CMAGJ2013M74)
文摘Unlike previous studies on wind turbulence spectrum in the planetary boundary layer, this investigation focuses on high-altitude (1-5 km) wind energy spectrum and turbulence spectrum under various weather conditions. A fast Fourier transform (FFT) is used to calculate the wind energy and turbulence spectrum density at high altitudes (1-5 km) based on wind profiling radar (WPR) measurements. The turbulence spectrum under stable weather conditions at high altitudes is expressed in powers within a frequency range of 2 × 10-5-10-3 s-1, and the slope b is between -0.82 and -1.04, indicating that the turbulence is in the transition from the energetic area to the inertial sub-range. The features of strong weather are reflected less obviously in the wind energy spectrum than in the turbulence spectrum, with peaks showing up at different heights in the latter spectrum. Cold windy weather appears over a period of 1.5 days in the turbulence spectrum. Wide-range rainstorms exhibit two or three peaks in the spectrum over a period of 15-20 h, while in severe convective weather conditions, there are two peaks at 13 and 9 h. The results indicate that spectrum analysis of wind profiling radar measurements can be used as a supplemental and helpful method for weather analysis.
基金supported by the National Natural Science Foundation of China(Grant Nos.42175104&41675014)。
文摘This article aims to build a theory of atmospheric boundary layer turbulence under complex conditions. To achieve this goal, we constructed a multi-site observation and analysis method of atmospheric turbulence based on related principles.This method first requires verification for the ergodicity of the full-scale observation of surface-layer turbulence, which proves that eddies within a scale of 60 min during a four-site observation can easily meet ergodicity. Meanwhile, by applying the second-order structure function for the horizontal wind speed turbulence of a single site and upstream and downstream points, we verified the ergodicity of the turbulence observation. Comparing the turbulence spectrum to the second-order structure function for the horizontal wind speed from the four-site observation, a relatively high accordance was observed, proving the reasonability of the multi-site observation. Moreover, compared to the single-site observation, the four-site observation can improve the estimation accuracy of the surface-layer turbulence spectrum and vertical turbulent flux. As a result, we can describe the threedimensional structure of turbulence more accurately and comprehensively by combining analytical data from single-site and four-site observations. In summary, the multi-site turbulence observation method shows that the horizontal and vertical wind turbulence of the Baimiao plateau has a typical structure of a turbulence spectrum with clear spectral gaps. The result is in accordance with the scale of the turbulence spectral gaps obtained from the 6 h data. The horizontal wind speed is under the influence of the terrain, so its spectrum of large-scale eddies has higher fluctuations, but its spectral gaps can still be clearly distinguished. Although the spectral gaps of the temperature spectrum are not distinguishable, they still have the same scale as the spectral gap of the vertical and horizontal turbulence spectrum. Moreover, the temperature spectrum possesses typical structure characteristics of the boundary-layer turbulence spectrum.
基金supported by the National Natural Science Foundation of China(Grant Nos.41675014&91437103)the Gansu Science and Technology Program(Grant No.18JR2RA005)。
文摘To display the distribution characteristics of turbulence eddy under condition of complex terrain in Loess Plateau,and to enhance the precision of turbulence measurements,the research of turbulence ergodicity is considered to be the prior section of the experiment.With the statistics of single-site turbulence measurements obtained in Baimiao Tableland,Ping Liang,Gan Su,analysis result shows that not only the turbulence with scale less than 10 min,which can easily satisfy the ergodicity,the tableland terrain can also be a major cause of the coherent structure of periodic,large-scale turbulence;compared to which for the turbulence above the flat underlying surface,the distribution of 10–40 min,large-scale turbulence in the tableland region tends to be more steady and,thus,can also satisfy the ergodicity easily.Under the condition of extremely unstable stratification,the wind is comparatively low in speed and features distinctly large-scale,periodical fluctuation,and with the trend of smooth increase in temperature,large-scale wind turbulence and temperature turbulence both tend to satisfy ergodicity.In comparison,under the condition of extremely stable stratification,the aperiodicity of large-scale wind turbulence and temperature turbulence caused by intermittency is comparatively strong,and the turbulence cannot satisfy ergodicity easily.
基金This study is supported by the National Natural Science Foundation of China.
文摘The paper computed the spectra of velocity and temperature,and the cospectra of velocity and temperature by the observational data of July,1990 in the suburbs of Tianjin.The results show that the characteristics of the atmospheric turbulence spectra over the suburbs are basically in accordance with some typical results over the flat terrain.But the scale,on which turbulence can satisfy the isotropic condition,over the suburbs is larger than over the flat terrain.The spectrum peak frequency range is a bit narrower.The feature of the spectrum range in low frequency(LF)is out of accordance with that of the flat terrain either.