The structure of turbulence over smooth and rough walls

The statistics and structure of turbulence are investigated experimentally over smooth and rough walls. In the smooth-wall investigation, particle image velocimetry (PIV) measurements are performed in the horizontal plane at several y locations with Re_&thetas; = 1015 and 7705. The dominant motions of the flow are shown to be large-scale regions of momentum deficit elongated in the streamwise direction. Throughout the logarithmic layer, the regions are consistently bordered by vortices with significant vertical rotation organized in the streamwise direction, offering strong support for the vortex packet theory of Adrian et al. (2000). Additionally, evidence is presented for the existence and organization of hairpin vortices in the region y+ < 60. Statistical evidence is also presented for two important aspects of the vortex packet theory for the first time: vortex organization in the streamwise direction, and the clear association of the hairpin signature with local minima in streamwise velocity.; The importance of the large-scale motions, in terms of the streamwise turbulent kinetic energy, is established. At y+ = 21, it is shown that the well established streak spacing mode of λ_z + = 100 contains surprisingly little energy relative to modes in the range λ_z+ = 200–400, providing complementary results to the channel flow data of Liu et al (2000). However, comparison with the channel also reveals that the near-wall spanwise energy distribution is not universal. At the high Reynolds number, the energetic dominance of the organized outer-region flow structures of large spanwise scale is demonstrated throughout the logarithmic region.; Several spanwise lengthscales are shown to vary linearly with distance from the wall, revealing self-similar growth of spanwise structure in an average sense. However, inspection of the data suggests individual structures do not grow strictly self-similarly in time. It is proposed that additional scale growth occurs by the merging of vortex packets on an eddy-by-eddy basis via a vortex leg-annihilation mechanism similar to that suggested by Perry and Chong (1982). The proposed mechanism provides a link between vortex-pairing concepts and the observed coalescence of streaky low-speed regions in the inner layer.