Outer-layer structure arrangements based on the large-scale zero-crossings at moderate Reynolds number

Abstract

The structural arrangements in the outer layer of turbulent boundary layer flows were explored with large-field time-resolved particle image velocimetry measurements at moderate Reynolds number. The large- and small-scale structures were reconstructed by the modes of multiscale proper orthogonal decomposition. The association between hairpin packets and uniform momentum zones (UMZs) was examined by the conditional averaging results based on the large-scale positive-to-negative/negative-to-positive (PN/NP) zero-crossings. The scale arrangements provided the spatial evidence that the intense small-scale swirling motions are aligned in the confined internal shear layers along the backside of the large-scale, low-speed region, which was characterized by hairpin vortex packets. The uniform momentum zones (UMZs) conditioned on the large-scale PN/NP zero-crossings were detected from the histograms of the instantaneous streamwise velocity. The attached eddy behavior was consolidated based on the conditional events, by presenting the joint probability of UMZs thickness and wall-normal location. A close agreement of the conditional averaging raw velocity and modal velocity was examined. Moreover, the conditional averaging results of the UMZs interface probability exhibited a similar spatial distribution as the small-scale turbulent kinetic energy and swirling strength, which manifests the coincidence between the hairpin heads and the UMZs interfaces. This result was confirmed by the distribution of the wall-normal locations corresponding to the maximum value of interface probability and small-scale representations, which performs a streamwise inclination angle of 15°. The statistical spatial feature demonstrated the association between hairpin packets and uniform momentum as proposed by Adrian et al. [“Vortex organization in the outer region of the turbulent boundary layer,” J. Fluid Mech. 422, 1–54 (2000)].