Turbulent transport mechanics within and above tall ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

Turbulent transport mechanics within and above tall canopies on gentle hills
Author:Gabriel Katul <gaby@duke.edu> (Duke University)
Davide Poggi <poggi@polito.it> (Dipartimento di Idraulica, Trasporti ed Infrastrutture Civili, Politecnico di Torino, Torino, Italy)
Presenter:Gabriel Katul <gaby@duke.edu> (Duke University)
Date: 2006-06-18     Track: Special Sessions     Session: Keynote speakers (reserved for keynotes only)
DOI:10.4122/1.1000000280

Progress on many practical problems in eco-hydrology and eco-hydraulics are now requiring fundamental understanding of how topography modulates the basic properties of turbulence. In particular, how hilly terrain alters the ejection-sweep cycle, which is the main coherent transporting motion, remains a problem that 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 were conducted over a bare surface while the second set of experiments were conducted over a modelled vegetated surface composed of tall and densely arrayed rods. Using this 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 entire 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 agreement partly stems from the fact that the imbalance in the flux-transport terms, needed in the incomplete cumulant expansion, were well modelled using "local" gradient-diffusion principles. This finding 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 to numerical modelling of turbulent flows using Reynolds-Averaged Navier Stokes equations are briefly discussed.