The drag length is key to quantifying tree canopy drag
File(s)
Published version
Author(s)
Majumdar, Dipanjan
Vita, Giulio
Ramponi, Rubina
Glover, Nina
van Reeuwijk, Maarten
Type
Journal Article
Abstract
The effects of trees on urban flows are often determined in computational fluid dynamics simulations using a
quadratic drag formulation based on the leaf-area density 𝑎 and a volumetric drag coefficient 𝐶
𝑉
𝑑
. We develop
an analytical model for the flow within a vegetation canopy and identify the drag length 𝓁𝑑 = (𝑎𝐶𝑉
𝑑
)
−1 as the
key metric to describe the local tree drag, which represents the adjustment lengthscale for the mean velocity
inside the canopy due to tree drag. A detailed literature survey suggests that the median 𝓁𝑑 observed in
field experiments is 21 m for trees and 0.7 m for low vegetation (crops). Total 168 large-eddy simulations
are conducted to obtain a closed form of the analytical model which allows determining 𝑎 and 𝐶
𝑉
𝑑
from the
wind-tunnel experiments that typically present the drag characteristics in terms of the classical drag coefficient
𝐶𝑑 and the aerodynamic porosity 𝛼𝐿
. We show that geometric scaling of 𝓁𝑑
is the appropriate scaling of trees
in wind tunnels. Evaluation of 𝓁𝑑
for numerical simulations and wind-tunnel experiments (assuming geometric
scaling 1 ∶ 100) in literature shows that the median 𝓁𝑑
in both these cases is about 5 m, suggesting a potential
overestimation of vegetative drag.
quadratic drag formulation based on the leaf-area density 𝑎 and a volumetric drag coefficient 𝐶
𝑉
𝑑
. We develop
an analytical model for the flow within a vegetation canopy and identify the drag length 𝓁𝑑 = (𝑎𝐶𝑉
𝑑
)
−1 as the
key metric to describe the local tree drag, which represents the adjustment lengthscale for the mean velocity
inside the canopy due to tree drag. A detailed literature survey suggests that the median 𝓁𝑑 observed in
field experiments is 21 m for trees and 0.7 m for low vegetation (crops). Total 168 large-eddy simulations
are conducted to obtain a closed form of the analytical model which allows determining 𝑎 and 𝐶
𝑉
𝑑
from the
wind-tunnel experiments that typically present the drag characteristics in terms of the classical drag coefficient
𝐶𝑑 and the aerodynamic porosity 𝛼𝐿
. We show that geometric scaling of 𝓁𝑑
is the appropriate scaling of trees
in wind tunnels. Evaluation of 𝓁𝑑
for numerical simulations and wind-tunnel experiments (assuming geometric
scaling 1 ∶ 100) in literature shows that the median 𝓁𝑑
in both these cases is about 5 m, suggesting a potential
overestimation of vegetative drag.
Date Issued
2025-06-01
Date Acceptance
2025-03-08
Citation
Journal of Wind Engineering and Industrial Aerodynamics, 2025, 261
ISSN
0167-6105
Publisher
Elsevier BV
Start Page
106084
End Page
106084
Journal / Book Title
Journal of Wind Engineering and Industrial Aerodynamics
Volume
261
Copyright Statement
© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
License URL
Identifier
10.1016/j.jweia.2025.106084
Publication Status
Published
Article Number
106084
Date Publish Online
2025-03-23