Spectral Short-circuiting and Wake Production within the Canopy Trunk Space of an Alpine Hardwood Forest |
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| Authors: | Daniela Cava Gabriel G Katul |
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| Institution: | (1) CNR - Institute of Atmosphere Sciences and Climate, National Research Council (CNR), Lecce, Italy;(2) Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC, USA;(3) Department of Civil and Environmental Engineering, Duke University, Durham, NC, USA |
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| Abstract: | Using synchronous multi-level high frequency velocity measurements, the turbulence spectra within the trunk space of an alpine
hardwood forest were analysed. The spectral short-circuiting of the energy cascade for each velocity component was well reproduced
by a simplified spectral model that retained return-to-isotropy and component-wise work done by turbulence against the drag
and wake production. However, the use of an anisotropic drag coefficient was necessary to reproduce these measured component-wise
spectra. The degree of anisotropy in the vertical drag was shown to vary with the element Reynolds number. The wake production
frequency in the measured spectra was shown to be consistent with the vortex shedding frequency at constant Strouhal number
given by f
vs
= 0.21ū/d, where d can be related to the stem diameter at breast height (dbh) and ū is the local mean velocity. The energetic scales, determined from the inflection point instability at the canopy–atmosphere
interface, appear to persist into the trunk space when  , where C
du
is the longitudinal drag coefficient, a
cr
is the crown-layer leaf area density, h
c
is the canopy height, and β is the dimensionless momentum absorption at the canopy top. |
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| Keywords: | Canopy turbulence Foliage drag Spectral short-circuiting Strouhal instabilities Trunk space Wake production Von Karman streets |
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