Advances in Meteorology

Volume 2018, Article ID 2189234, 16 pages

https://doi.org/10.1155/2018/2189234

## CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

^{1}Sapienza University of Rome, DAEEE, Via Eudossiana 18, Rome 00184, Italy^{2}Sapienza University of Rome, DICEA, Via Eudossiana 18, Rome 00184, Italy

Correspondence should be addressed to Paolo Monti; ti.1amorinu@itnom.oloap

Received 19 March 2018; Accepted 2 July 2018; Published 29 July 2018

Academic Editor: Jorge E. Gonzalez

Copyright © 2018 Fabio Nardecchia et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Computational fluid dynamics (CFD) is currently used in the environmental field to simulate flow and dispersion of pollutants around buildings. However, the closure assumptions of the turbulence usually employed in CFD codes are not always physically based and adequate for all the flow regimes relating to practical applications. The starting point of this work is the performance assessment of the V2F (i.e., − *f*) model implemented in Ansys Fluent for simulating the flow field in an idealized array of two-dimensional canyons. The V2F model has been used in the past to predict low-speed and wall-bounded flows, but it has never been used to simulate airflows in urban street canyons. The numerical results are validated against experimental data collected in the water channel and compared with other turbulence models incorporated in Ansys Fluent (i.e., variations of both *k*-*ε* and *k*-*ω* models and the Reynolds stress model). The results show that the V2F model provides the best prediction of the flow field for two flow regimes commonly found in urban canopies. The V2F model is also employed to quantify the air-exchange rate (ACH) for a series of two-dimensional building arrangements, such as step-up and step-down configurations, having different aspect ratios and relative heights of the buildings. The results show a clear dependence of the ACH on the latter two parameters and highlight the role played by the turbulence in the exchange of air mass, particularly important for the step-down configurations, when the ventilation associated with the mean flow is generally poor.

#### 1. Introduction

The continuous growth of large cities occurred in the last decades has prompted the scientific community towards the understanding of the urban environment [1, 2]. Great attention has been paid especially in predicting the flow field within and outside the urban street canyon, which is the space delimited by the street and the facades of the surrounding buildings. Knowledge on wind and temperature distributions within the street canyon is crucial, for example, in the design of the urban geometry with the aim of achieving an energy-optimized architecture of the city [3–5] as well as determining the concentration of pollutants emitted at the street level by vehicular traffic [6–8].

One of the parameters that mostly influence the gross features of the flow over urban canopies is the aspect ratio, AR, which is defined as the ratio between the average height of the buildings, *H*, and the spacing, *W*, between two consecutive buildings. Oke [3] introduced three kinds of flow regimes as a function of AR: isolated obstacle, wake interference, and skimming flow. In the isolated-obstacle regime (AR < 0.4), the flow around each building is not affected by disturbances coming from other obstacles. In the wake-interference flow (0.4 < AR < 0.67), two counterrotating vortices form within the canyon, and the wake of each building interacts with the subsequent building. The skimming flow (AR > 0.67) corresponds to narrow urban canyons, where the wind circulation is characterized by a vortex that occupies a large part of the canyon. Besides the three flow regimes defined by Oke, there is a fourth flow pattern, the multivortex flow regime (AR > 1.54), which is a variant of the skimming flow [9]. Another important parameter influencing the street canyon is the relative height of the buildings, *H*_{2}/*H*_{1}, where *H*_{1} and *H*_{2} are the heights of the leeward and the windward buildings, respectively.

Thanks to the increasing computational power of computers, computational fluid dynamics (CFD) has recently supported laboratory and field experiments, improving the knowledge of street-canyon flows. Much effort has been done in recent years to analyze urban canopy flows by means of CFD, often using Reynolds-averaged Navier–Stokes (RANS) simulations of two-dimensional (2D) arrays of buildings. The interest of the scientific community for such a simplified building arrangement is justified by the fact that the 2D array can be considered as an archetype for more complex geometries [10–13]. Huang et al. [14] carried out 2D simulations to investigate the effect of wedge-shaped roofs on the flow in an urban street canyon and found that they have significant influence on the vortex structure and pollutant distribution pattern. Memon et al. [15] analyzed heating in 2D isolated street canyons applying the RNG *k*-*ɛ* model (here, *k* is the turbulent kinetic energy, while *ɛ* is its rate of dissipation). Those authors compared their results with wind-tunnel data and showed that the nighttime and daytime air temperature difference between urban and rural areas closely resembles each other. Murena and Mele [16] analyzed an ideal deep street canyon with 2D unsteady RANS simulations using the shear stress transport (SST) *k*-*ω* model. They observed that short-time variations of wind velocity can greatly influence the mass transfer rate between the canyon and the overlying boundary layer. Allegrini et al. [17] carried out 2D steady RANS simulations with different near-wall treatments in order to validate numerical results for buoyant flows in urban street canyons by comparison with wind-tunnel measurements. They compared the results of different turbulence models (STD *k*-*ε*, realizable *k*-*ε*, *k*-*ω*, Spalart–Allmaras, and Reynolds stress model (RSM)), showing a better agreement of the STD *k*-*ε* model with the NEWFs (nonequilibrium wall functions) than the LRNM (low-Reynolds number modeling). Ho et al. [18] studied idealized 2D urban street canyons of different ARs and urban boundary-layer depths using the RNG *k*-*ɛ* model. They found that the atmospheric turbulence contributes most to street-level ventilation because the turbulent component of the air-exchange rate (ACH) dominates the transport process. Xie et al. [19] investigated the impact of the urban street layout on the local atmospheric environment through numerical simulation and wind-tunnel experiments. The authors found that the vortex structure in the canyon and, consequently, the street layout strongly influence the wind field and the pollutant dispersion in the canopy.

A well-known CFD approach alternative to RANS simulation is the large eddy simulation (LES), which explicitly resolves the larger structures of the turbulence, while it models the finer ones by adopting suitable closure assumptions [20–22]. It is believed that the RANS approach provides reasonable accurate predictions of mean flow quantities and that it is still an appropriate methodology considering the low CPU cost. However, in some applications such as the analysis of transient features of the flow like vortex shedding in the wake, LES performs generally better than RANS simulation [6]. In any case, LES resolves the large-scale turbulent eddies, which are 3D by nature. Therefore, since in this work a 2D simulation has been used, the most suitable CFD approach is the RANS one.

Based on the previous literature, the *k*-*ε* turbulence model appears to be the most widely employed one in CFD simulations of urban canopy flows. However, uncertainties still exist regarding the capability of CFD codes in simulating velocity and turbulence fields in different flow regimes. For this reason, a comparison between numerical results obtained through Ansys Fluent v.14.5 [23] and experimental data taken in the water channel has been carried out in this work. In addition to the most known turbulence models, the comparison has also taken into account the V2F model, based on the closure developed by Durbin [24]. The V2F model is similar to the STD *k*-*ε* model but includes an additional transport equation that models the velocity scale, , and its source term, *f* [25]. Since the V2F model incorporates both near-wall turbulence anisotropies and nonlocal pressure-strain effects, it is usually employed for low-speed and wall-bounded flows. This implies that wall functions are not required, and consequently, lower computational costs are needed. The V2F model has been developed for attached or mildly separated boundary layers and used mainly for studying three-dimensional (3D) boundary layers [26, 27] and heat transfer problems in jet impingement [28–30] and in ribbed-channel flows [31, 32], subsonic and transonic flows for aerospace applications [33, 34], and flow physical phenomena in enclosed environments [35–37]. To the best of our knowledge, this paper is the first one to deal with numerical simulations of 2D street canyons by means of the V2F model. Here, the effectiveness of the V2F model in predicting the flow field for two typical building arrangements (AR = 0.5 and 1) has been investigated. The V2F model is also employed to analyze the air-exchange rate (ACH) for a series of two-dimensional building arrangements, such as step-up and step-down configurations, having different aspect ratios and relative heights of the buildings, a design quite underexplored in the literature.

This paper is organized as follows: firstly, the experimental setup used in the water channel and the numerical approach followed in the simulation are described. Secondly, tests of the V2F model through comparisons with the experimental data and results obtained employing other turbulence models are presented and discussed together with the analyses of several flow regimes referred to several ARs and *H*_{2}/*H*_{1}. Particular attention is also paid to the analysis of canyon ventilation as well as to its dependence on the canyon geometry. This paper concludes with a summary of the main results.

#### 2. The Water-Channel Experiments

The numerical simulations have been validated with a series of experiments conducted in the close-loop water channel located at the Laboratory of Hydraulics of the University of Rome “La Sapienza.” The water channel allows the reproduction of the atmospheric boundary layer with several advantages [38–41]. One of them is that image analysis techniques, such as particle tracking velocimetry, can be easily employed. These permit accurate spatial measurements, which generally allow a clearer understanding of complex flows such as the one under investigation.

The water channel has a rectangular cross section of 0.35 m height and 0.25 m width and 7.4 m length (Figure 1). The flow rate is set by a floodgate placed at the closing section of the channel, and the water depth, *h* = 0.16 m, is maintained constant throughout the experiments (more information about the facility can be found in [42]). The reference frame has been defined with the *x*-axis aligned with the streamwise velocity and the *z*-axis vertical. The water is seeded with nonbuoyant particles (2 *μ*m in diameter), which were assumed to be passively transported by the flow. Upwind of the buildings, the channel bottom is covered by unevenly spaced, roughness elements (pebbles with an average diameter of 5 mm) in order to reproduce the logarithmic vertical profile of the undisturbed streamwise velocity as well as the (nearly) constant Reynolds stress profile typically observed in the atmospheric boundary layer. The roughness Reynolds number, (here, is the friction velocity, = 10^{−6}·m^{2}·s^{−1} is the kinematic viscosity of water, and *H* is the obstacle height), ranges from 340 up to 470; that is, it is well above the critical value of 70 given by Snyder [43], which guarantees the independence of the investigated large-scale structures and the mean flow of Reynolds number effects [44]. Therefore, in our experiments, is large enough to ensure both the conditions of full turbulence of the simulated boundary layer and the dynamic similarity between experiments and real conditions.