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Turbulence structure and scales in canopy-wake reattachment

Hayoon Chung* and Jeffrey Koseff

  • The Bob and Norma Street Environmental Fluid Mechanics Laboratory, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA

  • *hayoonch@stanford.edu
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Phys. Rev. Fluids 6, 114605 – Published 30 November, 2021

DOI: https://doi.org/10.1103/PhysRevFluids.6.114605

Abstract

The reattachment of turbulent flow in canopy wakes is studied in laboratory flume experiments with model submerged aquatic vegetation. Velocity profiles using acoustic Doppler velocimeters were taken in the wakes of homogeneous (single-patch) canopies and within long gaps of discontinuous canopies (two patches). Analysis of the velocity records for mean flow and turbulence statistics suggests the presence of two dominant dynamics and scales: canopy-shear dynamics (CSD) and backward-facing step dynamics (BFSD). The mean streamwise velocities near the bed of the canopy wakes indicate the presence of an adverse pressure gradient, as found in the wake of BFS or other flow expansions. Using this observation, we redefine the reattachment for canopy wakes. Spectral analysis of the streamwise velocity near the bed in the immediate wake of the canopy shows that there is a significant peak centered about a frequency comparable to the streamwise periodicity found in the immediate wake of a BFS. Further from the bed, the vertical velocity spectra maintain a strong spectral peak that corresponds to the coherent structures formed by the canopy-shear instability from the upstream canopy. While different canopy systems have different contributions from the two dynamics, our canopy systems are dominated by canopy-shear dynamics about the canopy height in the wake. For example, CSD has a strong influence on the turbulence in the wake of the canopy because the length of the canopy controls the turbulence development. This in turn has a strong influence on the reattachment length. (However, the BFSD modifies the dominant CSD-induced signal by introducing both larger and smaller scales to the flow.) We find that the reattachment length decreases with increasing canopy length of turbulence, with an asymptotic limit for the canopy length required for fully developed turbulence.

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Phys. Rev. Fluids 6, 114605– Published 30 November, 2021
  • Received 21 July 2021
  • Accepted 15 November 2021
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