Journal
JOURNAL OF FLUID MECHANICS
Volume 844, Issue -, Pages 635-668Publisher
CAMBRIDGE UNIV PRESS
DOI: 10.1017/jfm.2018.160
Keywords
low-dimensional models; turbulent boundary layers
Categories
Funding
- National Natural Science Foundation of China [11490552, 11672020, 11721202]
- Fundamental Research Funds for the Central Universities of China [YWF-16-JCTD-A-05]
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The present study is devoted to characterizing the coherent organization of vortical structures, which can be fitted into the paradigm of the hairpin-packet model, in the streamwise-wall-normal plane of a canonical turbulent boundary layer at Re-tau = 377-1093. Proper orthogonal decomposition (POD) of the planar velocity fields measured via two-dimensional particle image velocimetry, together with a spatio-temporal coherence analysis, shows that the first four leading-order POD modes share both geometric similarity and dynamic coherence and jointly depict the downstream convection of the large-scale Q2/Q4 events, which can be regarded as the low-order imprints of the hairpin packets. A simple low-order indicator is then proposed to extract the inclined interfaces of the hairpin packets, based on which a two-point conditional correlation analysis forms a statistical picture of the spatial organization of multiple prograde vortices aligned along the interface within one packet. A saturation of the self-similar growth of the streamwise gap between two neighbouring vortices is seen. This implies a detachment of the hairpin packets from the inner layer. Both the detachment height and the saturated streamwise spacing are found to scale as Re-tau(1)/2.
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