Parameterisation of the drag effect of climbers depending on wind speed and LAD

Abstract

The implementation of green walls is increasingly seen as a strategy to tackle urban air pollution and to make cities more climate resilient. The correct description of the vegetation–wind interaction is key in describing the effect of vegetation in computational fluid dynamics (CFD) models. The accuracy of the modelled wind flow is highly linked to the uncertainty about the drag coefficient (C d ). In addition, at low wind speeds viscous drag (K) is not negligible and it should be regarded in CFD models. This research aims to address the uncertainty related to C d and K by including the effect of climbers on both the momentum and turbulence equations in the Wilcox revised k − ω model. The change of K with increasing Reynolds number showed an increase from 5 ⋅ 10 −8 m 2 up to the dynamic viscosity of air ( ≈ 10 −5 m 2 ) following a logistic function. Beyond the transition region from viscous to form drag, C d , in the range of 0.1-1.1, declined with increasing Reynolds number following a power law function. Furthermore, the plant morphological parameters determining permeability and drag coefficient were identified. This study showed that the knowledge of viscous and shape resistance is necessary to obtain accurate statistics for air flow through vegetation. • Optimisation of vegetation effects on momentum and turbulence equations in CFD models. • Permeability of vegetation cannot be neglected at low wind speeds. • Drag coefficient declines following a power law with increasing Reynolds number. • Smaller, complex leaves and higher LAD resist air flow more at lower wind speed. • Elongated leaves and higher LAD give more resistance to air flow at higher wind speed.