Scaling and similarity of the anisotropic coherent eddies in near-surface atmospheric turbulence

Khaled Ghannam, Gabriel G. Katul, Elie Bou-Zeid, Tobias Gerken, Marcelo Chamecki

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend's "attached" eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k-1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k-1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

Original languageEnglish (US)
Pages (from-to)943-964
Number of pages22
JournalJournal of the Atmospheric Sciences
Volume75
Issue number3
DOIs
StatePublished - Mar 1 2018

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eddy
turbulence
canopy
roughness
surface layer
velocity structure
kinetic energy
turbulent boundary layer
vegetation
energy dissipation
dissipation
momentum
wind velocity
experiment

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

Cite this

Ghannam, Khaled ; Katul, Gabriel G. ; Bou-Zeid, Elie ; Gerken, Tobias ; Chamecki, Marcelo. / Scaling and similarity of the anisotropic coherent eddies in near-surface atmospheric turbulence. In: Journal of the Atmospheric Sciences. 2018 ; Vol. 75, No. 3. pp. 943-964.
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Scaling and similarity of the anisotropic coherent eddies in near-surface atmospheric turbulence. / Ghannam, Khaled; Katul, Gabriel G.; Bou-Zeid, Elie; Gerken, Tobias; Chamecki, Marcelo.

In: Journal of the Atmospheric Sciences, Vol. 75, No. 3, 01.03.2018, p. 943-964.

Research output: Contribution to journalArticle

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T1 - Scaling and similarity of the anisotropic coherent eddies in near-surface atmospheric turbulence

AU - Ghannam, Khaled

AU - Katul, Gabriel G.

AU - Bou-Zeid, Elie

AU - Gerken, Tobias

AU - Chamecki, Marcelo

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AB - The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend's "attached" eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k-1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k-1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

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