An investigation of wall-layer dynamics using a combined temporal filtering and correlation

Abstract

The relative role of outer- and wall-layer structures in the dynamics of the near-wall region of a turbulent boundary layer was explored by examining the scaling of the spanwise correlation coefficient between the wall shear stress and the streamwise velocity fluctuation, Rτu(z). within narrow frequency bands spanning the entire turbulence spectrum. The scaling characteristics of Rτu(z) within the individual frequency bands indicate that one can separate the contribution to the streamwise velocity fluctuations, in the buffer and logarithmic regions, due to outer-layer structures from those due to wall-layer events. Results provide insight into the lack of wall scaling for the conventional correlation coefficient, Rτu(z), in the near-wall region. Moreover, the ‘negative dip’ in Rτu(z+), which has often been associated with the low-speed streaks, was found to exist within certain frequency bands for all Reynolds numbers investigated (1579 [Lt ] Reθ [Lt ] 5961). More interestingly, it is shown that, although the energy of the outer-layer structures increases with Reynolds number to overwhelm the streamwise velocity fluctuations in the near-wall region, the production of the Reynolds shear stress is dominated by wall-layer eddies. The findings of the current investigation provide strong support for Townsend's hypothesis of ‘active’ and ‘inactive’ motions.