arXiv subjects
Publications and source records attributed to Javier Jimenez.
By using new results from direct simulations of turbulent channels at moderate friction Reynolds numbers (Retau <= 1900) and in very large numerical boxes, we examine the corrections to the similarity assumptions in the overlap and outer regions of wall-bounded turbulence.
The dispersion of a passive scalar by wall turbulence, in the limit of infinite Peclet number, is analyzed using frozen velocity fields from the DNS by our group. The Lagrangian trajectories of fluid particles in those fields are integrated and used to compute the first and second-order moments of the distribution of fluid-particle displacements. It is shown that the largest scales in the flow dominate turbulent diffusion, and the computed dispersions are in good agreement with measurements in the atmospheric boundary layer. This agreement can be understood noting that the life times of the large strucutures are much longer than the time scale of the transition from linear to Gaussian particle spreading in the cross-stream plane. Numerical experiments performed computing the Lagrangian trajectories in reference frames moving at different velocities suggest that this transition is controlled by the difference between the mean streamwise velocity and the phase speed of the large-scale structures of the cross-stream velocities. In the streamwise direction, the effect of the mean shear dominates and produces elongated scalar patches, with dispersion exponents which are different from the transverse ones.
Over the last decades the knowledge on the small scales of turbulent wall flows has experienced a significant advance, especially in the near-wall region where the highest production of turbulent energy and the maximum turbulence intensity occur. The development of computers has played an important role in this progress, making direct numerical simulations affordable (Kim, Moin & Moser, 1987), and offering wider observational possibilities than most laboratory experiments. The large scales have received less attention, and it has not been until recently that their significance and their real size have been widely recognized, thanks in part to the experiments by Hites (1997) and Kim & Adrian (1999), and to the compilation of experimental and numerical data by Jimenez (1998). The requirements of both a very large box and a high Reynolds number has made direct numerical simulation of the VLAS unapproachable until today. The purpose of this report is to serve as a preliminary description of a newly compiled numerical database of the characteristics of the large scales in turbulent channel flow at moderate Reynolds numbers.