DOI: 10.1615/TSFP4
CHARACTERISTICS OF VORTEX ORGANIZATION IN THE OUTER LAYER OF WALL TURBULENCE
ABSTRAKT
Recent experimental and computational studies support the notion that the outer layer of wall turbulence is populated by hairpin-like vortices which tend to organize themselves into larger-sale coherent groups. The present effort attempts to evaluate the characteristics of this large-scale organization. Wide-field-of-view (2.5h × h) velocity fields are acquired by PIV in the streamwise-wall-normal plane of turbulent channel flow for 744 < Reτ < 2099 in order to study the imprint of this organization upon multi-point statistics as well as its average characteristics. Comparisons of unconditional and conditional two-point correlations of streamwise velocity indicate that outer-layer vortex organization leaves a definitive imprint upon multi-point statistics, highlighting its statistical significance. Its average streamwise extent, estimated from two-point velocity correlations, is found to increase with Reτ, even when scaled in outer units. Further, its streamwise extent grows substantially within the log layer. In contrast, the average inclination angle of the packets appears to be independent of Reτ and relatively insensitive to wall-normal position outside the near-wall region.