On Lagrangian single-particle statistics

In turbulence, ideas of energy cascade and energy flux, substantiated by the exact Kolmogorov relation, lead to the determination of scaling laws for the velocity spatial correlation function. Here we ask whether similar ideas can be applied to temporal correlations. We critically review the relevant theoretical and experimental results concerning the velocity statistics of a single fluid particle in the inertial range of statistically homogeneous, stationary and isotropic turbulence. We stress that the widely used relations for the second structure function, D-2(t) equivalent to <[nu(t) - nu(0)](2)> proportional to epsilon t, relies on dimensional arguments only: no relation of D-2(t) to the energy cascade is known, neither in two- nor in three-dimensional turbulence. State of the art experimental and numerical results demonstrate that at high Reynolds numbers, the derivative dD(2)(t)/dt has a finite non-zero slope starting from t approximate to 2 tau(eta). The analysis of the acceleration spectrum Phi(A)(omega) indicates a possible small correction with respect to the dimensional expectation Phi(A)(omega) similar to omega(0) but present data are unable to discriminate between anomalous scaling and finite Reynolds effects in the second order moment of velocity Lagrangian statistics. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4711397]