Anomalous scaling in random shell models for passive scalars

A shell-model version of Kraichnan's [Phys. Rev. Lett. 72, 1016 (1994)] passive scalar problem is introduced which is inspired by the model of Jensen, Paladin, and Vulpiani [Phys. Rev. A 45, 7214 (1992)]. As in the original problem, the prescribed random velocity field is Gaussian and delta correlated in time, and has a power-law spectrum proportional to k(m)(-xi), where k(m) is the wave number. Deterministic differential equations for second- and fourth-order moments are obtained and then solved numerically. The second-order structure function of the passive scalar has normal scaling, while the fourth-order structure function has anomalous scaling. For xi=2/3 the anomalous scaling exponents zeta(p) are determined for structure functions up to p=16 by Monte Carlo simulations of the random shell model, using a stochastic differential equation scheme, validated by comparison with the results obtained for the second- and fourth-order structure functions.