Transitional-turbulent spots and turbulent-turbulent spots in boundary layers [Engineering]
![Transitional-turbulent spots and turbulent-turbulent spots in boundary layers [Engineering]](/image/eyJ1cmkiOiJodHRwOi8vd3d3LnBuYXMub3JnL2NvbnRlbnQvZWFybHkvMjAxNy8wNi8xMy8xNzA0NjcxMTE0L0YxLm1lZGl1bS5naWYiLCJmb3JtYXQiOiJ3ZWJwIiwicXVhbGl0eSI6MTAwLCJub0NhY2hlIjp0cnVlfQ==.webp)
Two observations drawn from a thoroughly validated direct numerical simulation of the canonical spatially developing, zero-pressure
gradient, smooth, flat-plate boundary layer are presented here. The first is that, for bypass transition in the narrow sense
defined herein, we found that the transitional–turbulent spot inception mechanism is analogous to the secondary instability
of boundary-layer natural transition, namely a spanwise vortex filament becomes a
Λ vortex and then, a hairpin packet. Long streak meandering does occur but usually when a streak is infected by a nearby existing
transitional–turbulent spot. Streak waviness and breakdown are, therefore, not the mechanisms for the inception of transitional–turbulent
spots found here. Rather, they only facilitate the growth and spreading of existing transitional–turbulent spots. The second
observation is the discovery, in the inner layer of the developed turbulent boundary layer, of what we call turbulent–turbulent
spots. These turbulent–turbulent spots are dense concentrations of small-scale vortices with high swirling strength originating
from hairpin packets. Although structurally quite similar to the transitional–turbulent spots, these turbulent–turbulent spots
are generated locally in the fully turbulent environment, and they are persistent with a systematic variation of detection
threshold level. They exert indentation, segmentation, and termination on the viscous sublayer streaks, and they coincide
with local concentrations of high levels of Reynolds shear stress, enstrophy, and temperature fluctuations. The sublayer streaks
seem to be passive and are often simply the rims of the indentation pockets arising from the turbulent–turbulent spots.
Publisher URL: http://feedproxy.google.com/~r/Pnas-RssFeedOfEarlyEditionArticles/~3/O-U_N5cduhM/1704671114.short
DOI: 10.1073/pnas.1704671114
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