DNS of the very large anisotropic scales in a turbulent channel

The large structures in the outer layer of turbulent wall flows are of great physical importance, because they contain a substantial fraction of the streamwise kinetic energy and of the Reynolds stresses. Nevertheless, the organization of the outer region of wall turbulence has historically received less attention than that of the inner region, and as a consequence, it is still the subject of many open experimental and theoretical questions. In order to address some of those questions, we have performed direct numerical simulations of the turbulent incompressible flow in plane channels at Reynolds numbers 185 - 550. This paper presents some of the results of this simulation, focusing on the statistical description of the size of the large structures of the streamwise velocity and on its scaling with Reynolds number.