The ejection-sweep cycle over bare and forested gentle hills: a laboratory experiment |
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| Authors: | D Poggi G Katul |
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| Institution: | (1) Dipartimento di Idraulica, Trasporti ed Infrastrutture Civili Politecnico di Torino, Torino, Italy;(2) Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC, USA |
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| Abstract: | Progress on practical problems such as quantifying gene flow and seed dispersal by wind or turbulent fluxes over nonflat terrain
now demands fundamental understanding of how topography modulates the basic properties of turbulence. In particular, the modulation
by hilly terrain of the ejection-sweep cycle, which is the main coherent motion responsible for much of the turbulent transport,
remains a problem that has received surprisingly little theoretical and experimental attention. Here, we investigate how boundary
conditions, including canopy and gentle topography, alter the properties of the ejection-sweep cycle and whether it is possible
to quantify their combined impact using simplified models. Towards this goal, we conducted two new flume experiments that
explore the higher-order turbulence statistics above a train of gentle hills. The first set of experiments was conducted over
a bare surface while the second set of experiments was conducted over a modelled vegetated surface composed of densely arrayed
rods. Using these data, the connections between the ejection-sweep cycle and the higher-order turbulence statistics across
various positions above the hill surface were investigated. We showed that ejections dominate momentum transfer for both surface
covers at the top of the inner layer. However, within the canopy and near the canopy top, sweeps dominate momentum transfer
irrespective of the longitudinal position; ejections remain the dominant momentum transfer mode in the whole inner region
over the bare surface. These findings were well reproduced using an incomplete cumulant expansion and the measured profiles
of the second moments of the flow. This result was possible because the variability in the flux-transport terms, needed in
the incomplete cumulant expansion, was shown to be well modelled using “local” gradient-diffusion principles. This result
suggests that, in the inner layer, the higher-order turbulence statistics appear to be much more impacted by their relaxation
history towards equilibrium rather than the advection-distortion history from the mean flow. Hence, we showed that it is possible
to explore how various boundary conditions, including canopy and topography, alter the properties of the ejection-sweep cycle
by quantifying their impact on the gradients of the second moments only. Implications for modelling turbulence using Reynolds-averaged
Navier Stokes equations and plausible definitions for the canopy sublayer depth are briefly discussed. |
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| Keywords: | Canopy turbulence Closure models Cumulant expansion Ejections and sweeps Flow over gentle hills Nonlocal transport |
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