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Localized turbulence structures in transitional rectangular-duct flow
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DOI：10.1017/jfm.2015.546.png)
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Abstract
En 中文
Direct numerical simulations of transitional flow in a rectangular duct of cross-sectional aspect ratio A equivalent to s/h = 1-9 (s and h being the duct half-span and half-height, respectively) have been performed in the Reynolds number range Re equivalent to u(b)h/nu = 650-1500 (u(b) and nu being the bulk velocity and the kinematic viscosity, respectively) in order to investigate the dependence on the aspect ratio of spatially localized turbulence structures. It was observed that the lowest Reynolds number Re-T, estimated in a specific way, for localized (transiently sustaining) turbulence decreases monotonically from Re-T = 730 for A = 1 (square duct) with increasing aspect ratio, and for A = 5 it nearly attains a minimal value Re-T approximate to 670 that is consistent with the onset Reynolds number of turbulent spots in a plane channel (A = infinity). Turbulent states consist of localized structures that undergo a fundamental change around A = 4. At Re = Re-T turbulence for A = 1-3 is streamwise-localized similar to turbulent puffs in pipe flow, while for A = 5-9 turbulence at Re = Re-T is also localized in the spanwise direction, similar to turbulent spots in plane channel flow. This structural change in turbulent states at Re = Re-T is attributed to the exclusion of turbulence from the vicinity of the duct sidewalls in the case of a wide duct with A greater than or similar to 4: here the friction length on the sidewalls is so long that the size (around 100 times the friction length) of a self-sustaining minimal flow unit of streamwise vortices and streaks is larger than the duct height and, therefore, it cannot be accommodated.
Keywords:
instability
transition to turbulence
turbulent flows
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