Lagrangian measurement of fluid-particle motion in an isotropic turbulent field

By means of an optical tracer-particle tracking method, measurements for Lagrangian characteristics of turbulence, including the mean-square value of lateral diffusion from a point source Y²̄ , the r.m.s. value of fluctuating velocity v′, the velocity autocorrelation coefficient RL(τ), and the integral scale ΛL, were made in approximately isotropic turbulent flow behind a grid. Comparison of these Lagrangian data with the Eulerian integral scale Λ f and the double velocity correlation coefficient f(r) led to the following results: (i) for a moderate turbulent Reynolds-number range of Reλ = 20 − 70, the ratio β(= ΛL/Λ f ) is within the values of 0.6 – 0.3, becoming lower as Reλ increases, and (ii) the distribution of RL against time lag τ is analogous to that of f against βr/v′. Further, it was confirmed by both theoretical analysis and experiments that the growth of Y²̄(t) was weakened by the decay of turbulent energy, particularly so for long diffusion times.