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From experimental data of a turbulent free jet we calculate the joint probability distribution p (v₁, L₁;v₂, L₂) for two velocity increments v₁, v₂ of different length scales L₁, L₂. We present experimental evidence that the conditional probability distribution p (v₂, L₂|v₁, L₁) obeys a Fokker-Planck equation. We calculate the corresponding drift and diffusion coefficients and discuss their relationship to universal behavior in the scaling region and to intermittency of the turbulent cascade. We explicitly present a stochastic process for the log-normal model of Kolmogorov and Oboukhov A. M. Oboukhov, J. Fluid Mech. 13, 77 (1962) ; A. N. Kolmogorov, J. Fluid Mech. 13, 82 (1962).
Friedrich et al. (Mon,) studied this question.