Integrated vegetation model for studying the cooling potential of trees in urban street canyons

Vegetation in cities provides natural cooling of the climate and is therefore increasingly integrated as an essential part of Urban Heat Island (UHI) mitigation strategies. In the present study, the influence of trees on the local climate in a street canyon is studied using an integrated vegetation model in OpenFOAM. Vegetation is modeled as porous medium providing the necessary source terms for the heat, mass and momentum fluxes. Additionally, a radiation model is developed to model the short-wave and long-wave radiative heat flux exchanges between vegetation and the surroundings. The study investigates the influence of transpirative and shaded cooling due to vegetation on the pedestrian comfort inside a street canyon. The study shows that both shading and transpiration have a direct positive influence on the temperatures measured in the street canyon. Moreover, the cooling due to shading is seen to be larger than the transpirative cooling, especially under the tree

Aerodynamic characterization of model vegetation by wind tunnel experiments

A better prediction of turbulent airflow around porous vegetation is required for urban environment studies as vegetation is being increasingly utilized to mitigate Urban Heat Islands (UHI). Trees in urban areas impact ventilation by disturbing the flow and lead to cooling by evapotranspiration. Furthermore, trees have been shown to play a role in pollutant removal. A first step in properly accounting for the impact of vegetation on UHI is to accurately determine the heat and mass exchange between vegetation and the environment. For such determination, an accurate model of the turbulent flow field around vegetation and an improved parameterisation of the turbulent momentum deficit using drag coefficient must be obtained. The aim of this paper is to investigate the drag profile and turbulent flow field of flexible and inflexible model trees. The drag coefficients of model trees are measured using a force balance and the turbulent flow fields are measured using a stereo-PIV setup. This paper provides a mean to better predict the turbulent airflow within and around porous vegetation by studying the relation of drag coefficients with turbulent flow fields for model trees. The drag coefficients of inflexible model trees are found to be nearly independent of wind speed whereas, for the flexible model with leaves and branches that streamline to the flow field, the drag coefficients decrease with increasing wind speed. These finding agree with the literature. The normalized mean velocity is related to the drag coefficient, where velocity deficit increases with the drag. Investigating the mean Reynolds stress component does not yield a definite correlation with drag coefficient

A study on diurnal microclimate hysteresis and plant morphology of a Buxus sempervirens using PIV, infrared thermography, and X-ray imaging

Plants modify the climate and provide natural cooling through transpiration. However, plant response is not only dependent on the atmospheric evaporative demand due to the combined effects of wind speed, air temperature, humidity, and solar radiation, but is also dependent on the water transport within the plant leaf-xylem-root system. These interactions result in a dynamic response of the plant where transpiration hysteresis can influence the cooling provided by the plant. Therefore, a detailed understanding of such dynamics is key to the development of appropriate mitigation strategies and numerical models. In this study, we unveil the diurnal dynamics of the microclimate of a Buxus sempervirens plant using multiple high-resolution non-intrusive imaging techniques. The wake flow field is measured using stereoscopic particle image velocimetry, the spatiotemporal leaf temperature history is obtained using infrared thermography, and additionally, the plant porosity is obtained using X-ray tomography. We find that the wake velocity statistics are not directly linked with the distribution of the porosity but depends mainly on the geometry of the plant foliage which generates the shear flow. The interaction between the shear regions and the upstream boundary layer profile is seen to have a dominant effect on the wake turbulent kinetic energy distribution. Furthermore, the leaf area density distribution has a direct impact on the short-wave radiative heat flux absorption inside the foliage where 50% of the radiation is absorbed in the top 20% of the foliage. This localized radiation absorption results in a high local leaf and air temperature. Furthermore, a comparison of the diurnal variation of leaf temperature and the net plant transpiration rate enabled us to quantify the diurnal hysteresis resulting from the stomatal response lag. The day of this plant is seen to comprise of four stages of climatic conditions: no-cooling, high-cooling, equilibrium, and decaying-cooling stages

A lightweight convolutional neural network to reconstruct deformation in BOS recordings

We introduce a Convolutional Neural Network (CNN) that is specifically designed and trained to post-process recordings obtained by Background Oriented Schlieren (BOS), a popular technique to visualize compressible and convective flows. To reconstruct BOS image deformation, we devised a lightweight network (LIMA) that has comparatively fewer parameters to train than the CNNs that have been previously proposed for optical flow. To train LIMA, we introduce a novel strategy based on the generation of synthetic images from random-irrotational deformation fields, which are intended to mimic those provided by real BOS recordings. This allows us to generate a large number of training examples at minimal computational cost. To assess the accuracy of the reconstructed displacements, we consider test cases consisting of synthetic images with sinusoidal displacement as well as images obtained in the experimental studies of a hot plume in air and a flow past and inside a heated hollow hemisphere. By comparing the reconstructed deformation fields using the LIMA or conventional post-processing techniques used in Direct Image Correlation (DIC) or conventional image cross-correlation, we show that LIMA is more accurate and robust in the synthetic test case. When applied to experimental BOS recordings, all methods provide similar and consistent deformation fields. As LIMA is capable of achieving a comparable or better accuracy at a fraction of the computational costs, it represents a valuable alternative to conventional post-processing techniques for BOS experiments.ISSN:0723-4864ISSN:1432-111

Parametric study of the influence of environmental factors and tree properties on the transpirative cooling effect of trees

ISSN:0168-1923ISSN:1873-224

Comparative study of flow field and drag coefficient of model and small natural trees in a wind tunnel

The influence of trees in urban areas is typically assessed using urban microclimate models. These models rely on wind tunnel experiments using small-scale tree models to verify and validate their predictions of the flow field. However, it is not known sufficiently to which extent small model trees used in wind tunnel studies can recreate the behavior of large trees found in cities. In the present study, the drag coefficient and the turbulent flow downstream of model trees are compared with the ones of natural trees of a similar size to determine whether both types of tree provide similar aerodynamic characteristics. Therefore, measurements of the drag force and the flow field, using particle image velocimetry, are performed. The aerodynamic characteristics of the small trees are compared with the ones measured on larger mature trees from previous studies. The present study shows that the drag coefficients of model and natural trees are similar only if both types have a similar aerodynamic porosity and if the model tree can undergo an aerodynamic reconfiguration similar to that of a natural tree. Such reconfiguration implies the reorientation of the branches and leaves due to wind. A study on the influence of seasonal foliar density variation shows that the foliage configuration plays a critical role on the drag coefficient and the flow field. A defoliated tree, such as a deciduous tree in winter, is shown to have a substantially lower drag coefficient and a negligible influence on the flow.ISSN:1618-866