Multiaffine Lagrangian subgrid model

We present the first phenomenological Lagrangian subgrid model for multi-particle dispersion in homogeneous isotropic turbulence that incorporates intermittency and predicts deviations from Richardson's self-similar scaling. Building upon the model of Mazzitelli, Toschi, and Lanotte [Phys. Fluids 26, 095101 (2014)], which reproduces second-order pair and tetrad dispersion statistics using self-similar Gaussian dynamics, we introduce multiaffine stochastic processes at each scale following Biferale et al. [Phys. Rev. E 57, R6261 (1998)]. This replaces the Ornstein-Uhlenbeck processes at each scale in the original model with multifractal fluctuations whose statistics are prescribed through scale-dependent multipliers. The distribution of these multipliers can be tuned to match anomalous scaling exponents. The resulting model reproduces non-Gaussian statistics in single particle trajectories, as well as pair and tetrad dispersion, providing a minimal phenomenological framework for intermittent Lagrangian turbulence that can also act as a subgrid model for under-resolved Eulerian-Lagrangian simulations.