Large-eddy simulations of ventilation for thermal comfort — A parametric study of generic urban configurations with perpendicular approaching winds
Introduction
The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) has noted that warming of the climate system is unequivocal; 1983–2012 was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere, and it is likely that the frequency of heat waves has increased in large parts of Europe, Asia, and Australia (IPCC, 2013). Meanwhile, according to the World Health Organization, the urban population in 2014 accounted for 54% of the total global population, up from 34% in 1960, and it continues to grow. The urban heat island effect further intensifies large-scale high temperatures in high-density cities and threatens the inhabitants' health (Wang et al., 2016). In high-density Hong Kong, for example, the rate of warming increased by 0.37 °C per decade between 1989 and 2005, based on the observed temperature (Lam, 2006). The temperature is also projected to rise by 4.4 °C in 2090–2099 for the case of urbanization frozen at its 2006 level, and to rise by 5.2 °C for the case of a constant rate of urbanization (Leung et al., 2007). Mean mortality associated with heat stroke would experience a twofold per unit rise in net effective temperature beyond 26 °C (Leung et al., 2008).
Rapid urbanization in the tropical and subtropical regions means that a better understanding of how to design and plan a city with good ventilation performance is needed. To achieve neutral thermal sensation in an urban environment, a wind speed of 0.9–1.3 m/s is needed for a person wearing light clothing under shaded conditions (Ng and Cheng, 2012). Hence, thermal comfort can be achieved by capturing the natural wind. Meanwhile, good air ventilation is also important for pollutant dispersion in street canyons (Lo and Ngan, 2015, Mirzaei and Haghighat, 2010, Yuan et al., 2014). Outdoor air quality can further affect indoor air quality via natural as well as artificial ventilation, as indoor air will be replaced by outdoor air eventually (Chen, 2009). Therefore, providing good urban air ventilation is very important for quality and healthy living in high-density cities in tropical and subtropical regions (Ng et al., 2011, Yuan and Ng, 2014). However, a distinction should be made between ventilation for air quality and ventilation for thermal comfort. When the purpose is to study ventilation for air quality, the main parameters are flow rate, which provides dilution capacity for contaminants, and turbulent transport at rooftop level, which removes contaminants from street canyons. When the aim is to study ventilation for thermal comfort, the main parameter is wind velocity at the pedestrian level. This study focuses on ventilation for thermal comfort, so the main parameter to be investigated is the wind velocity ratio at the pedestrian level.
Urban ventilation is strongly influenced by wind speed and direction, which in turn are affected by three-dimensional urban morphology (Skote et al., 2005, Yang et al., 2013). As a combination of the individual shapes and dimensions of buildings and their arrangement in the city, urban density can be described by geometric parameters in planning like ground coverage ratio (λp), frontal area density (λf), and plot ratio (P). So-called parametric studies, which simplify complex actual urban geometries into simple morphological models, are widely applied in urban ventilation studies for their advantages of linking specific geometric parameters to air ventilation performance. Using a κ–ω shear stress transport turbulence model, Yuan and Ng (2012) carried out a parametric study with a focus on building porosity for better urban ventilation and evaluated the effects of wind speed on outdoor thermal comfort. Using a standard κ–ε turbulence model, Buccolieri et al. (2015) investigated the breathability in dense building arrays with λp values similar to those of typical European cities. Yang and Li (2011) modeled turbulence effects in two simple Hong Kong city models with relatively complex terrain under different atmospheric conditions, and the importance of thermal stratification was highlighted under a weak wind background. Hang et al. (2013) investigated neutral ventilation assessment in two idealized urban models with various approaching wind directions, while Lin et al. (2014) investigated urban canopy layer ventilation under neutral atmospheric conditions with the same λp (0.25) and λf (0.25) but with various urban sizes, building height variations, overall urban forms, and wind directions. Ramponi et al. (2015) provided a review of the literature for computational fluid dynamics (CFD) studies of outdoor ventilation for generic urban configurations and indicated that there is a lack of studies of urban configurations where not all parallel streets have equal widths. This initiated their CFD simulation of ventilation in generic urban configurations with different urban densities and equal and unequal street widths. Ho et al. (2015) examined flows over idealized two-dimensional street canyons of different building aspect ratios and urban boundary layer depths and utilized the friction factor and the air-exchange rate to parameterize aerodynamic resistance and street-level urban ventilation. Using large-eddy simulation (LES), Nazarian and Kleissl (2016) studied realistic solar heating in a three-dimensional idealized urban environment and investigated mean flow and turbulence statistics as determinants for urban canyon ventilation. However, comprehensive parametric studies considering a number of varying practical parameters are rarely found in the literature.
Associated with investigations of ventilation in idealized urban models or generic configurations, CFD techniques, such as the Reynolds-averaged Navier-Stokes (RANS) model and LES, are needed. As reviewed above, RANS models have commonly been used in previous CFD studies, mainly due to their low computational cost. However, there is debate regarding the performance of different kinds of RANS models (Hang et al., 2013, Yuan and Ng, 2012). LES overcomes the deficiencies of RANS by explicitly resolving large, energy-containing turbulent eddies and parameterizing only small (subgrid) scale turbulence (Rodi et al., 1997, Tamura, 2008). This advantage of LES comes at a much higher computational cost than RANS. But today's ever-rising computational power makes urban LES applications feasible (Tamura, 2008). The dimensionality, spatial resolution, and turbulence intensity that an LES model can handle are superior to those of most other methodologies (Castillo et al., 2011). What affects pedestrian comfort directly is the wind flow within cities, and the local turbulence level in particular (Britter and Hanna, 2003). LES provides not only mean flow fields but also instantaneous turbulences, which are especially important for human comfort at the pedestrian level in the urban canopy layer. We therefore use an LES model to produce CFD simulations of air flow and ventilation performance in a set of comprehensive parametric urban scenarios in this study.
Section snippets
The Parallelized LES Model (PALM)
The LES model used in this study is the Parallelized LES Model (PALM), which was developed in 1997 (Raasch and Schröter, 2001), when it was one of the first parallelized LES models for atmospheric research (Maronga et al., 2015). The governing equations of PALM are based on the non-hydrostatic, filtered, incompressible Navier-Stokes equations with Boussinesq approximation and are filtered implicitly using the volume-balance approach of Schumann (Schumann, 1975). The first law of thermodynamics
Parametric urban scenarios
Parametric scenarios of generic urban configurations are defined in a practical way. Building dimensions and street layouts are calculated from practical geometric parameters for urban planning. Nomenclature of all involved parameters and their values is given in Table 1. The first three parameters are to be investigated and their values are prescribed in Table 2. The plot ratio P is obtained by dividing the gross floor area of the building by the area of the site on which the building is
Identification of the most important factor
Taking scenarios HM/IM01 and HM/IM04 as examples, Fig. 6 shows the vertical distributions of streamwise horizontal velocity and some general flow features in the main runs. Frontal views of streamwise velocity in a y-z section are shown for HM01 (Fig. 6a) and IM01 (Fig. 6b). Differential canopy heights in the spanwise direction are found. Lateral views of streamwise velocity in an x-z section are shown for HM04 (Fig. 6c) and IM04 (Fig. 6d). Velocity rotations behind buildings are prominent,
Conclusions
This study investigates ventilation performance in parametric urban scenarios using an LES model—PALM. The PALM codes used in this study are first validated using the AIJ guidelines for CFD building simulations before being utilized in simulations of parametric scenarios. Four morphological parameters in urban design and planning, including ground coverage ratio (λp), frontal area density (λf), plot ratio (P), and building height differential, are used to construct the parametric scenarios.
Acknowledgments
This study was supported by the Research Grants Council of the Hong Kong Special Administrative Region (Project No. 14408214), and Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong (Project ID: 1907002).
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