Comment on diffusion in biased turbulence

Recently Vlad et al. [Phys. Rev. E. 63, 066304 (2001)] applied the method of decorrelation trajectories to the transport of tracers in stochastic velocity fields with constant drift, and found that the average Lagrangian velocity is smaller than the Eulerian average. As this contradicts a theoretical result due to Lumley [in Me´canigue de la Turbulence, International Conference of the CNRS, Marseille, 1961 (Centre National de la Recherche Scientifique, Paris, 1962)], two-dimensional numerical simulations are performed to confirm that the average Lagrangian and Eulerian velocities are in fact equal when the velocity field is divergence free