In this article, we describe numerical multilevel methods applied to the Navier–Stokes equations (homogeneous isotropic turbulence and channel flow problem), to the shallow water equations (geophysical problems), and to the renormalization of small eddies. The first difficulties with multilevel methods is to separate the scales. In this work, various cases are considered: the case of spectral methods, the case of finite difference methods, and the case of finite element methods. Then the numerical treatment is adapted to the size of the computed scales. The aim is to obtain new schemes, with better modeling properties and improved numerical stability. The spectral case is studied in detail, and results of numerical simulations are reported in this case.
Multilevel numerical methods are indispensable for the approximation of phenomena involving a large number of scales, such as turbulence. Although the methods described in this article are generally limited to two levels, they can, as explained in the text, be extended to a more important number of levels, thus extending significantly their performances.