Showing papers by "Juan C. del Álamo published in 2006"

Self-similar vortex clusters in the turbulent logarithmic region

Abstract: The organization of vortex clusters above the buffer layer of turbulent channels is analysed using direct numerical simulations at friction Reynolds numbers up to Re τ = 1900. Especial attention is paid to a family of clusters that reach from the logarithmic layer to the near-wall region below y + = 20. These tall attached clusters are markers of structures of the turbulent fluctuating velocity that are more intense than their background. Their lengths and widths are proportional to their heights Ay and grow self-similarly with time after originating at different wall-normal positions in the logarithmic layer. Their influence on the outer region is measured by the variation of their volume density with Δ y . That influence depends on the vortex identification threshold, and becomes independent of the Reynolds number if the threshold is low enough. The clusters are parts of larger structures of the streamwise velocity fluctuations whose average geometry is consistent with a cone tangent to the wall along the streamwise axis. They form groups of a few members within each cone, with the larger individuals in front of the smaller ones. This behaviour is explained considering that the streamwise velocity cones are 'wakes' left behind by the clusters, while the clusters themselves are triggered by the wakes left by yet larger clusters in front of them. The whole process repeats self-similarly in a disorganized version of the vortex-streak regeneration cycle of the buffer layer, in which the clusters and the wakes spread linearly under the effect of the background turbulence. These results characterize for the first time the structural organization of the self-similar range of the turbulent logarithmic region.

Linear energy amplification in turbulent channels

Abstract: We study the temporal stability of the Orr-Sommerfeld and Squire equations in channels with turbulent mean velocity profiles and turbulent eddy viscosities. Friction Reynolds numbers up to Re τ =2×10 4 are considered. All the eigensolutions of the problem are damped, but initial perturbations with wavelengths λ x > λ z can grow temporarily before decaying. The most amplified solutions reproduce the organization of turbulent structures in actual channels, including their self-similar spreading in the logarithmic region. The typical widths of the near-wall streaks and of the large-scale structures of the outer layer, λ + z = 100 and λ z /h = 3, are predicted well. The dynamics of the most amplified solutions is roughly the same regardless of the wavelength of the perturbations and of the Reynolds number. They start with a wall-normal v event which does not grow but which forces streamwise velocity fluctuations by stirring the mean shear (uv < 0). The resulting u fluctuations grow significantly and last longer than the v ones, and contain nearly all the kinetic energy at the instant of maximum amplification.

The near-wall structures of turbulent wall flows

Abstract: Models for the viscous and buffer layers over smooth walls are reviewed. It is shown that there is a family of numerically-exact nonlinear structures which account for about half of the energy production and dissipation in the wall layer. The other half can be modelled by the unsteady bursting of those structures. Many of the best-known characteristics of the wall layer, such as the lateral spacing among the streaks, are well predicted by these models. The limitations of minimal models are then discussed, and it is noted that a better approximation is to represent the velocity streaks as 'semi-infinite' wakes of the wall-normal velocity structures, both in the buffer and in the logarithmic layer. The consequences of this characterization on the causal relation between bursting structures are also briefly discussed. This paper reviews current theories about the flow in the immediate vicinity of smooth walls. We will see that, although this part of the flow is geometrically very thin in the high-Reynolds number limit, it is the seat of a large fraction of the total velocity difference across the turbulent boundary layer. The same is true for the total energy production and dissipation, and for many of the features that distinguish wall-bounded turbulent shear flows from those away