575 publications from this institution
Climate observations and analyses by the Intergovernmental Panel on Climate Change and national meteorological organizations show that climate change is occurring. This chapter discusses the simulation of urban thermal microclimate with a focus on heat waves in urban areas, the simulation of overheating of buildings and the effects of adaptation measures to limit temperatures in buildings and urban areas during heat waves. In order to assess heat waves, urban heat islands and climate change adaptation measures for urban areas, different simulation tools can be employed. The spatial scales are the meteorological microscale and the building scale; the methods are computational fluid dynamics and building energy simulation. Adaptation measures investigated at the neighborhood scale are avenue trees, green facades and green roofs; adaptation measures at the building scale are increased thermal resistance, increased thermal mass, increased short-wave reflectivity of facades and roofs, peak ventilation, vegetated roofs and exterior solar shading.
Turbulence modeling is a key issue in computational wind engineering, particularly in the prediction of pollutant dispersion in cities. To be directly applicable, turbulence models need validation by comparison with experiments. This paper evaluates the performance of two different modeling approaches (RANS k-e and LES) for three test cases with varying complexity. For each case, wind tunnel experiments are used for validation. It is shown that the performance of the standard k-e model is very case-dependent and that it also depends on the turbulent Schmidt number, whose optimum value is a priori unknown. On the contrary, LES with the dynamic subgrid-scale model shows a good performance for all cases, without requiring any parameter input to solve the dispersion equation. For the test case of an actual urban environment, predicted concentration values with LES differ from experiments by less than a factor of 2, compared to less than a factor of 4 with the standard k-e model.
This paper provides a discussion of the results of two inter-model comparison exercises from IEA-Annex 41, where combined heat and moisture transfer was calculated for three variations of the BESTEST building (based on cases 600 and 900 from IEA- ECBCS SHC-Task 12 / IEA-ECBCS Annex 21). Firstly, basic statistical techniques were used to post- process some results of IEA-ECBCS Annex 41, providing confidence intervals in accordance to a certain confidence level. Secondly, the role of moisture (as well as some aspects of the exercises, such as initialization and material properties) was analysed using recently developed and validated prototype software for the coupling of BES and BEHAM. The paper concludes that combined heat and moisture transfer has a minor influence in the temperature distribution, energy demand and peak power load in the three variations of the BESTEST building simulated. However, combined heat and moisture transfer has large influence on the indoor relative humidity of the BESTEST.
This paper presents wind tunnel measurements of pedestrian wind conditions in passages between various configurations of two long narrow perpendicular buildings in open country exposure. The investigated parameters are passage width, building height and wind direction. The measurements were made along the passage centerline. The aim of this paper is to provide more insight in the pedestrian wind conditions in these basic building configurations, to address some contradictory statements reported in the literature and to provide detailed experimental data for computational fluid dynamics (CFD) validation. The results show that the wind speed amplification factors in diverging passages are generally larger than in converging passages. It is also shown that the maximum wind speed amplification factors increase monotonically with decreasing passage width, contrary to some general building design guidelines proposed in the past for such building configurations. Significant issues concerning the use of the experimental data for CFD validation are also discussed.
Spoked wheels are the most frequently used wheel type in road cycling competitions and their aerodynamic optimization is crucial for cyclist performance. The aerodynamic performance of wheels is generally analyzed by wind tunnel tests or CFD simulations for isolated wheels. There is a large number of options to model the wheel/ground contact in CFD simulations, including different clearances between tire and ground and different heights of solid contact patches (step). However, it is unclear to what extent these modeling options influence the CFD results. The present paper systematically analyzes the impact of these options on the computed forces and moments of an isolated cycling spoked wheel and elucidates the flow behavior around this wheel for zero yaw conditions. The wheel drag coefficient for the cases where the ground is included in the simulations using a clearance or a step is 1.0% and about 1.8% lower compared to the case without ground, respectively, whereas the rotational moment is about 2.0% lower for all the wheel/ground contact modeling approaches compared to the case without ground. The gap clearance (≤20 mm) and step height (≤10 mm) should be kept minimal to avoid a significant influence on the forces and moments. In addition, the presence of the ground influences the flow behavior in the lower section of the wheel including the pressure distribution on the exterior of the wheel. This study is intended to help researchers and manufacturers to perform accurate CFD simulations of cycling spoked wheels and to optimize their aerodynamics.
ABSTRACT An urban park can exhibit a cooling effect not only in the park itself, but also in its wake with respect to the prevailing wind. This study investigates the cooling potential of an urban park in Antwerp, Belgium focusing on two parameters: (1) intensity of the cooling effect, indicating the maximum reduction in air temperature and (2) range of the cooling effect, indicating the maximum horizontal distance where a minimum of 0.1 °C cooling effect is present. Computational fluid dynamics (CFD) simulations are performed using the three‐dimensional unsteady Reynolds‐averaged Navier–Stokes equations. Coupled simulations of wind flow and heat transfer consider all the conductive, convective and radiative heat transfer processes but not the long‐wave radiation heat exchange between buildings, trees and ground. The vegetation model used in this study is evaluated with experimental data from an earlier study. Moreover, the simulated air temperatures inside the Antwerp city center are compared with available measurement data. Following the fairly‐good predictions from CFD simulations, results from the following cases are compared: (1) base case with the park, (2) a case with an open square instead of the park, and (3) a case with representative buildings instead of the park. The results indicate a maximum daytime intensity and range of the cooling effect of 3.4 °C and 498 m at 1500 LST (UTC + 2, Central European Summer Time). Further analysis shows that the cooling effect is more profound closer to the ground. Apart from the daytime cooling effect, the park investigated has the potential to lower wind velocities, resulting in increased air temperatures during the night‐time (at 0000 LST) by 0.9 °C, although this number might be slightly different given the neglect of long‐wave radiation heat exchange between buildings, trees and ground.
We applied data-based recurrence CFD (rCFD) to model pollutant dispersion in near-field flow configurations. In case of complex topologies, the global-domain version of rCFD fails to account for local recurrent flow features. We therefore developed a novel island-based version of rCFD, which partitions the computational domain to isolate islands of high recurrence prominence, and subsequently defines a distinct recurrence path for each of these islands. We applied island-based rCFD to pollutant dispersion for two side-by-side cubical buildings with three different gap widths in between them and a real urban environment. We showed that numerical predictions of pollutant dispersion by island-based rCFD were in excellent agreement with full CFD simulations, thus outperforming the global-domain version of rCFD. In both applications, island-based rCFD simulations ran three orders of magnitude faster than corresponding full CFD simulations. In the second application, this speed-up enabled real-time simulations on a computational grid of 10 million cells.
As a result of climate change, projections show that the increasing urban population will be exposed to higher air temperatures (Hayes et al., 2014; Sheffield and Wood, 2008). In addition, due to climatic changes, the frequency and duration of heatwaves are expected to increase while the Urban Heat Island effect (UHI) is expected to become more severe (Emmanuel and Krüger, 2012; Kovats and Hajat, 2008; Mirzaei and Haghighat, 2010), causing increasing heat-related mortality and morbidity (Garssen et al., 2005; Xu et al., 2012). Meditteranean-type climate regions have been characterized as particularly vulnerable to climate change due to their high exposure to extreme weather phenomena (Diffenbaugh and Giorgi, 2012; Paz et al., 2016). The intensity of heat waves in these regions is expected to increase further, leading to other life-threatening consequences for urban populations in the Mediterranean region  (Legasa et al., 2020; Zittis et al., 2021). Under this context, the detailed investigation of the impact of climate change on the urban microclimate and human health is considered highly important. This study investigates the impact of climate change on urban microclimate and pedestrian thermal comfort through a series of numerical simulations. For this purpose, 3D Computational Fluid Dynamics (CFD) simulations are performed based on the Unsteady Raynolds-Average Navier-Stokes (URANS) equations for urban microclimate in a real compact heterogeneous urban area. The CFD validation is based on high-resolution field and laboratory measurements of the case study area in Nicosia, Cyprus (Antoniou et al., 2019, 2017). Simulations are performed for meteorological conditions based on field measurements performed in 2010, and for predicted meteorological conditions for 2050 derived from downscaled Regional Climate Models (RCMs). The RCM data are derived from the European Coordinate Regional Downscaling Experiment (EURO-CORDEX) database. CFD results are combined with radiation modeling results of the same urban area to calculate pedestrian thermal comfort using the Universal Thermal Comfort Index (UTCI). The results show that by 2050, a 2.3 °C  increase in the maximum air temperature is expected to occur, which can lead to a more than 240% increase in heat-related mortality. In addition, an increase of UTCI levels is also expected, which is more pronounced in the late afternoon hours, reaching up to 4.3 °C increase at 20:00. A significant change in the thermal stress categories is also identified between the 2010 and 2050 scenarios, where “very strong heat stress” conditions (UTCI: 38-46 °C) are expected to prevail for twice as long, increasing from 3.3 to 6.6 hours, and  “extreme heat stress” conditions (UTCI > 46 °C) appear at some locations of the area in 2050.    
