Abstract
We conduct a direct numerical simulation of a high-Reynolds-number turbulent boundary and identify the hierarchy of vortices by applying a Gaussian filter to the simulated velocity fields. We quantitatively show how the hierarchy of vortices is generated by evaluating the contribution of the scale-dependent strain-rate to the scale-dependent enstrophy production rate. Largest-scale vortices, that is, eddies with the size in the order of the distance from the wall are stretched and amplified predominantly by the mean flow. In contrast, small-scale vortices, that is, eddies sufficiently smaller than the height are generated mainly by the vortices twice as large as themselves. The generation mechanism of small-scale vortices is similar to the one in fully developed turbulence in a periodic cube, and it also explains the disappearance of hairpin vortices observed in the turbulent boundary layer as the Reynolds number increases.
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