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On the surface expression of a canopy-generated shear instability

Published online by Cambridge University Press:  26 March 2019

Tracy L. Mandel Open the ORCID record for Tracy L. Mandel [Opens in a new window] ,
Saksham Gakhar ,
Hayoon Chung ,
Itay Rosenzweig Open the ORCID record for Itay Rosenzweig [Opens in a new window]  and
Jeffrey R. Koseff Open the ORCID record for Jeffrey R. Koseff [Opens in a new window]
Tracy L. Mandel*
Affiliation:
Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA School of Natural Sciences, University of California, Merced, CA 95343, USA
Saksham Gakhar
Affiliation:
Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
Hayoon Chung
Affiliation:
Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
Itay Rosenzweig
Affiliation:
Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
Jeffrey R. Koseff
Affiliation:
Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
*
†Email address for correspondence: [email protected]
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Abstract

Results are presented from a laboratory study on the free-surface signal generated over an array of submerged circular cylinders, representative of submerged aquatic vegetation. We aim to understand whether aquatic ecosystems generate a surface signature that is indicative of both what is beneath the water surface as well as how it is altering the flow. A shear layer forms over the canopy, generating coherent vortex structures which eventually manifest in the free-surface slope field. We connect the vortex properties measured at the surface with measurements of the bulk flow, and show that correlations between these quantities are adequate to create a parameterized model in which the interior velocity profile can be predicted solely from measurements taken at the free surface. Experimental surface observations yield a Strouhal number that is twice the most amplified mode predicted by linear stability theory, suggesting that vortices may evolve between generation at the canopy height and their manifestation at the water surface. Additionally, the surface signal continues evolving with distance downstream, with vortices becoming spread farther apart and the passage frequency gradually decreasing. By the trailing edge of the canopy, surface-impacting boils emerge for canopies with higher submergence ratios, suggesting a transition from coherent two-dimensional rollers to transversely varying structures.

JFM classification

Waves/Free-surface Flows: Channel flow Geophysical and Geological Flows: Shallow water flows Wakes/Jets: Shear layers
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 867 , 25 May 2019 , pp. 633 - 660
Copyright
© 2019 Cambridge University Press 

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