Spatial-spectral characteristics of momentum transport in a turbulent boundary layer

Spectral content and spatial organization of momentum transport events are investigated in a turbulent boundary layer at the Reynolds number (Re_τ) = 2700, with time-resolved planar particle image velocimetry. The spectral content of the Reynolds-shear-stress fluctuations reveals that the largest range of time and length scales can be observed in proximity to the wall, while this range becomes progressively more narrow when the wall distance increases. Farther from the wall, longer time and larger length scales exhibit an increasing spectral content. Wave velocities of transport events are estimated from wavenumber-frequency power spectra at different wall-normal locations. Wave velocities associated with ejection events (Q2) are lowerthan the local average streamwise velocity, while sweep events (Q4) are characterized by wave velocities larger than the local average velocity. These velocity deficits are almost insensitive to the wall distance, which is also confirmed from time tracking the intense transport events. The vertical advection velocities of the intense ejection and sweep events are on average a small fraction of the friction velocity U_τ, different from previous observations in a channel flow. In the range of wall-normal locations 60 < y⁺ < 600, sweeps are considerably larger than ejections, which could be because the ejections are preferentially located in between the legs of hairpin packets. Finally,it is observed that negative quadrant events of the same type tend to appear in groups over a large spatial streamwise extent.