Abstract
Subcritical transition to turbulence is investigated experimentally and numerically in rectangular ducts of crosssectional aspect ratios As = 1 - 9. The lowest Reynolds number Re_T for transient turbulence is determined to demonstrate its dependence on the aspect ratio As. The numerical Rer is shown to be in good agreement with the experimental Re_T for As = 3 despite the difference in their strict definitions. It is found that Rer monotonically decreases from Re_T = 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 spanwise-localized (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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