Lagrangian statistics of Navier-Stokes and MHD turbulence

We report on a comparison of high-resolution numerical simulations of Lagrangian particles advected by incompressible turbulent hydro- and magnetohydrodynamic (MHD) flows. Numerical simulations were performed with up to 1024³ collocation points and 10 million particles in the Navier-Stokes case and 512³ collocation points and 1 million particles in the MHD case. In the hydrodynamics case our findings compare with recent experiments from Mordant et al. (2004 New J. Phys. 6, 116) and Xu et al. (2006 Phys. Rev. Lett. 96, 024503). They differ from the simulations of Biferale et al. (2004 Phys. Rev. Lett. 93, 064502) due to differences of the ranges chosen for evaluating the structure functions. In Navier-Stokes turbulence intermittency is stronger than predicted by the multifractal approach of Biferale et al. (2004 Phys. Rev. Lett. 93, 064502) whereas in MHD turbulence the predictions from the multifractal approach are more intermittent than observed in our simulations. In addition, our simulations reveal that Lagrangian Navier-Stokes turbulence is more intermittent than MHD turbulence, whereas the situation is reversed in the Eulerian case. Those findings can not consistently be described by the multifractal modeling. The crucial point is that the geometry of the dissipative structures have different implications for Lagrangian and Eulerian intermittency. Application of the multifractal approach for the modeling of the acceleration probability density functions works well for the Navier-Stokes case but in the MHD case just the tails are well described.