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Impact of height heterogeneity on canopy turbulence

Published online by Cambridge University Press:  27 January 2017

A. M. Hamed ,
M. J. Sadowski ,
H. M. Nepf  and
L. P. Chamorro
A. M. Hamed
Affiliation:
Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801, USA
M. J. Sadowski
Affiliation:
Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801, USA
H. M. Nepf
Affiliation:
Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
L. P. Chamorro*
Affiliation:
Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801, USA Department of Civil and Environmental Engineering, University of Illinois, Urbana, IL 61801, USA Department of Aerospace Engineering, University of Illinois, Urbana, IL 61801, USA
*
†Email address for correspondence: lpchamo@illinois.edu
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Abstract

The flow development above and within homogeneous and heterogeneous canopies was experimentally studied using particle image velocimetry in a refractive-index-matching 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 $h_{1}=h+(1/3)h$ and $h_{2}=h-(1/3)h$ 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 $Re_{H}\simeq 6500$, 11 300 and 12 300 and nearly constant Froude number $Fr\simeq 0.1$. 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.

JFM classification

Geophysical and Geological Flows: River dynamics Turbulent Flows: Shear layer turbulence Turbulent Flows: Turbulent boundary layers
Type
Papers
Information
Journal of Fluid Mechanics , Volume 813 , 25 February 2017 , pp. 1176 - 1196
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Copyright
© 2017 Cambridge University Press 

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