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 $\overline{Y^2}$ , the r.m.s. value of fluctuating velocity v ′, the velocity autocorrelation coefficient R L (τ), and the integral scale Λ L , 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 Re λ = 20 − 70, the ratio β(= Λ L /Λ f ) is within the values of 0.6 – 0.3, becoming lower as Re λ increases, and (ii) the distribution of R L against time lag τ is analogous to that of f against β r / v ′. Further, it was confirmed by both theoretical analysis and experiments that the growth of $\overline{Y^2}(t)$ was weakened by the decay of turbulent energy, particularly so for long diffusion times.