Abstract
Subcritical transition to turbulence in wall-bounded shear flow is known to be characterized in terms of turbulent patches bounded by laminar regions. In this study spatially localized turbulence structures are investigated in rectangular-duct flow of cross-sectional aspect ratios A_S = 1 - 9 at marginal Reynolds number Ret. The lowest Reynolds number Ret for transient turbulence is determined to demonstrate its dependence on the aspect ratio As. It is found that Ret monotonically decreases from Ret = 730 for As = 1 with increasing As, and for As = 5 it nearly attains to a minimal value Re_T ≒ 670 which is consistent with the Reynolds number for the appearance of a turbulent puff in a plane channel (As = ∞). Streamwise-localized (turbulent puff), or streamwise- and spanwiselocalized (turbulent spot) turbulence are observed at the Reynolds numbers Re = Re_T for distinct As. The localized turbulence exhibits structural 'transition' between a turbulent puff and a turbulent spot around As = 4.
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