Modal time correlation functions for homogeneous isotropic turbulence in a projection operator method

Applying a projection operator method to homogeneous isotropic turbulence in the Lagrangian and Eulerian specifications of the flow field, we obtain a closure equation, which is called the similarity approximation (SA) equation, for the time correlation function. We compare its solutions with the time correlation function obtained from direct numerical simulation in the Eulerian specification and with that obtained from direct interaction approximation in the Lagrangian specification. Results of the comparison show that the SA equation can express time correlation for homogeneous isotropic turbulence in the Lagrangian and Eulerian specifications.