Spectra of the very large anisotropic scales in turbulent channels

The spectra of numerically simulated channels at Re-tau = 180 and Re-tau = 550 in very large boxes are described and analyzed. They support a model in which the u- structures can be decomposed in two components. The first one is formed by structures of size lambda(x) greater than or similar to 5 h, lambda(z) approximate to 2 h, which span most of the channel height, and penetrate into the buffer layer. The second one has maximum intensity in the near- wall region, where it is highly anisotropic and scales in inner units. It widens, lengthens, and becomes more isotropic in the outer layer, where it scales with h. The cospectrum exhibits an analogous quasi- isotropic range, whose width grows linearly with wall distance. At the present Reynolds numbers, nothing can be said about a possible streamwise similarity, due to limited scale separation. An extensive set of statistics from the simulations is downloadable from ftp:// torroja. dmt. upm. es/ channels. (C) 2003 American Institute of Physics.