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Lateral Dispersion in Vegetated Flows: Reynolds-Number Dependence at Transitional Reynolds Numbers

Author(s): Yukie Tanino; Heidi M. Nepf

Linked Author(s): Heidi M. Nepf

Keywords: Dispersion; Turbulence; Transport; Aquatic vegetation

Abstract: Lateral dispersion in an array of randomly distributed cylinders, a model for emergent aquatic plants, at Reynolds numbers lower than in fully turbulent flow is considered. Theory developed by Tanino and Nepf (2008) implies that, for cylinder volume fractions φ > 0.19, the asymptotic dispersion coefficient normalized by the mean pore velocity and cylinder diameter, ▲, does not depend strongly on the Reynolds number, Red. In contrast, for φ < 0.19, the theory implies that ▲ decreases with decreasing Re din transitionally turbulent flows. At lower Red, cylinder wakes may exhibit an unsteady laminar regime, in which the Reddependence of ▲ is expected to be complex. To evaluate these conjectures, laboratory measurements of lateral dispersion at φ = 0.060 and φ = 0.35at Red= 38-170 and Re d= 48-390, respectively, are examined in this paper. K yy/ (u d) at (φ, Red) = (0.35,300-390), which is prior to the onset of full turbulence, was indeed the same as that predicted for fully turbulent flows. In contrast, at φ = 0.060, ▲ was16% smaller than that predicted for fully turbulent flow at Red= 160-170. As Re dwas reduced, ▲ increased by 60% between Re d= 160-170 and Re d= 97-99, then remained constant between Red= 97-99 and Re d= 59-61.


Year: 2009

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