The drag length is key to quantifying tree canopy drag
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arXiv
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| Main Authors: | , , , , |
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| Format: | Preprint |
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2024
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| _version_ | 1866910682238156800 |
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| author | Majumdar, Dipanjan Vita, Giulio Ramponi, Rubina Glover, Nina van Reeuwijk, Maarten |
| author_facet | Majumdar, Dipanjan Vita, Giulio Ramponi, Rubina Glover, Nina van Reeuwijk, Maarten |
| contents | The effects of trees on urban flows are often determined using computational fluid dynamics approaches which typically use a quadratic drag formulation based on the leaf-area density $a$ and a volumetric drag coefficient $C_{d}^V$ to model vegetation. In this paper, we develop an analytical model for the flow within a vegetation canopy and identify that the drag length $\ell_d = (a C_d^V)^{-1}$ is the key metric to describe the local tree drag characteristics. A detailed study of the literature suggests that the median $\ell_d$ observed in field experiments is $21$ m for trees and $0.7$ m for low vegetation (crops). A total of $168$ large-eddy simulations are conducted to obtain a closed form of the analytical model. The model allows determining $a$ and $C_d^V$ from wind-tunnel experiments that typically present the drag characteristics in terms of the classical drag coefficient $C_d$ and the aerodynamic porosity $α_L$. We show that geometric scaling of $\ell_d$ is the appropriate scaling of trees in wind tunnels. Evaluation of $\ell_d$ for numerical simulations and wind-tunnel experiments (assuming geometric scaling $1:100$) in literature shows that the median $\ell_d$ in both these cases is about $5$ m, suggesting possible overestimation of vegetative drag. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2411_01570 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | The drag length is key to quantifying tree canopy drag Majumdar, Dipanjan Vita, Giulio Ramponi, Rubina Glover, Nina van Reeuwijk, Maarten Fluid Dynamics The effects of trees on urban flows are often determined using computational fluid dynamics approaches which typically use a quadratic drag formulation based on the leaf-area density $a$ and a volumetric drag coefficient $C_{d}^V$ to model vegetation. In this paper, we develop an analytical model for the flow within a vegetation canopy and identify that the drag length $\ell_d = (a C_d^V)^{-1}$ is the key metric to describe the local tree drag characteristics. A detailed study of the literature suggests that the median $\ell_d$ observed in field experiments is $21$ m for trees and $0.7$ m for low vegetation (crops). A total of $168$ large-eddy simulations are conducted to obtain a closed form of the analytical model. The model allows determining $a$ and $C_d^V$ from wind-tunnel experiments that typically present the drag characteristics in terms of the classical drag coefficient $C_d$ and the aerodynamic porosity $α_L$. We show that geometric scaling of $\ell_d$ is the appropriate scaling of trees in wind tunnels. Evaluation of $\ell_d$ for numerical simulations and wind-tunnel experiments (assuming geometric scaling $1:100$) in literature shows that the median $\ell_d$ in both these cases is about $5$ m, suggesting possible overestimation of vegetative drag. |
| title | The drag length is key to quantifying tree canopy drag |
| topic | Fluid Dynamics |
| url | https://arxiv.org/abs/2411.01570 |