Impact of height heterogeneity on canopy turbulence
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H_S_N_C_2017.pdf
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Author(s)
Hamed, A. M.
Sadowski, M. J.
Chamorro, L. P.
Nepf, Heidi
Date Issued
January 2017
Journal
Journal of Fluid Mechanics
Publisher
Cambridge University Press
Citation
Hamed, A. M. et al. “Impact of Height Heterogeneity on Canopy Turbulence.” Journal of Fluid Mechanics 813 (January 2017): 1176–1196 © 2017 Cambridge University Press
Version
Author's final manuscript
Abstract
The flow development above and within homogeneous and heterogeneous canopies was experimentally studied using particle image velocimetry in a refractive-indexmatching channel. The experiments were designed to gain insight into the effect of height heterogeneity on the structure and spatial distribution of the turbulence. The homogeneous model (base case) is constituted of elements of height h arranged in a staggered configuration; whereas the heterogeneous canopy resembled a row canopy and consisted of elements of two heights h₁ = h + (1/3)h and h₂ = h - (1/3)h
alternated every two rows. Both canopies had the same density, element geometry and mean height. The flow was studied under three submergences H/h = 2, 3 and 4, where H denotes the flow depth. The experiments were performed at Reynolds number Re[subscript H] ≃ 6500, 11 300 and 12 300 and nearly constant Froude number Fr ≃ 0:1. Turbulence statistics complemented with quadrant analysis and proper orthogonal decomposition reveal richer flow dynamics induced by height heterogeneity. Topography-induced spatially periodic mean flows are observed for the heterogeneous canopy. Furthermore, and in contrast to the homogeneous case, non-vanishing vertical velocity is maintained across the entire length of the heterogeneous canopy with increased levels at lower submergence depths. Further alternations were induced in the magnitude and distribution of the turbulent kinetic energy, Reynolds shear stress and characteristics of the canopy mixing layer, evidencing enhanced mixing and turbulent transport for the heterogeneous canopy especially at lower submergence depths. Overall, the results indicate that heterogeneous canopies exhibit greater vertical turbulent exchange at the canopy interface, suggesting a potential for greater scalar exchange and a greater impact on channel hydraulic resistance than a homogeneous canopy of similar roughness density. Key words: river dynamics; shear layer turbulence; turbulent boundary layers
MIT Department
Massachusetts Institute of Technology. Department of Civil and Environmental Engineering
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Creative Commons Attribution-Noncommercial-Share Alike
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DOI of Published Version
https://doi.org/10.1017/jfm.2017.22
