In this article, the Von-Karman model of turbulence spatial correlation function which contains the parameter of outer scale is analyzed. Then, the experimental data of air refractive index variation obtained from a high quality fiber optical turbulence sensing array are used to evaluate the outer scale of atmospheric optical turbulence as well as its diurnal variation through the algorithm of nonlinear fitting. The results validate the suitability of the Von-Karman model. By combining the theory of correlation function with the spatially distributed and simultaneously measured data, three kinds of turbulence spatial scales described by correlation function are revealed as clearly as possible. Results show that the values of outer scale in atmospheric optical turbulence 1.8 m above the grassland tend to be larger in the daytime and smaller in the night. The mean value around noon is 0.44 m, while in the night it becomes 0.3 m. Here, three of the important views should be noted. Firstly, when the displacement of two points is just equal to the outer scale, their correlation coefficient is 0.26, and when it exceeds the outer scale, there is still a certain value of correlation coefficient. Secondly, the integral scale represents the averaged value of scale in the vortex of optical turbulence. And, it is slightly smaller than the outer scale. Thirdly, when the distance of two points equals the biggest scale of vortex in optical turbulence, the correlation coefficient tends to zero, and the value of biggest scale is slightly bigger than the outer scale. It is easy to find that the diurnal variation tendencies of the three spatial scales are similar to that of intensity in optical turbulence. The method of obtaining the characteristic scales by spatially arranged and simultaneously measured optical turbulence is direct, and the results can be considered as the evidence to prove the models of correlation function including the Von-Karman model. So, it promotes the research on the property of spatial structure to a certain extent.
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