Immersed boundary method applied to flow past a tree skeleton

Niels Troldborg,Niels N Sørensen,Ebba Dellwik,Horia Hangan

doi:10.1016/j.agrformet.2021.108603

Niels Troldborg, Niels N Sørensen + Show 2 more

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https://doi.org/10.1016/j.agrformet.2021.108603

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Abstract

We present numerical predictions and experimental observations of the drag force on the major branches of a model tree as well as the wind field in its wake. The tree model was mounted on a force sensor in a wind tunnel. The numerical simulations solved both the Reynolds Averaged Navier Stokes and detached eddy simulations, in combination with an immersed boundary model representation of the tree, while the measurements were obtained in the WindEEE Dome. Contrary to previous studies on trees or fractal structures, we find that the simulated forces on the tree skeleton were strongly dependent on the roughness of its surface. Upon setting a proper surface roughness, the simulations were capable of predicting both forces and wake fields, which were in good agreement with the observations both in terms of mean wind speed, mean turbulence and spectra. Furthermore, both simulations and observations showed that turbulence and mean wind speed were reduced for up to a distance of two tree heights downwind of the tree skeleton, underlining the efficiency of trees to remove momentum from a flow.

Highlights

Small-scale tree configurations, such as solitary trees, alleys, wind breaks, and small forests occur frequently in both urban and rural landscapes
How ever, whereas we find that the two models predict similar forces on the tree, it is well known that Reynolds aver aged Navier-Stokes (RANS) is often inadequate for representing wakes in the atmospheric boundary layer as shown by (Laan et al, 2015)
From a validation point of view, a smoother model tree would have been preferable since Fig. 7 suggests that the friction drag starts to play a minor role for roughness heights below 10− 4 m

Summary

Introduction

Small-scale tree configurations, such as solitary trees, alleys, wind breaks, and small forests occur frequently in both urban and rural landscapes. Their effect on the immediate environment is significant, both in terms of wind, temperature and moisture level. Trees can pose a severe danger, since breakage can cause material damage and death. For these reasons, it is important to accurately represent the effect of trees in wind and meteorological models. This is challenging because of their complex multi-scale structure (Gosselin, 2019), which requires proper treatment of a wide range of length scales

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