Buoyancy flows and pollutant dispersion through different scale urban areas: CFD simulations and wind-tunnel measurements

Highlights

• A coupled CFD model is built to study the multiscale problems in urban areas.

• Two turbulence models with two near-wall treatments are considered.

• The optimal value of Sc_t is 0.7 for the studied multiscale models.

• The wind-tunnel measurements are meaningful when Re_H,crit ≥ 7.57E+03.

• The effects of the outdoor environment on the IAQ are investigated.

Abstract

In this paper, a coupled CFD model was established to study the multiscale problems on the mixed force and buoyancy flow and dispersion passing neighborhood scale – street scale – indoor scale models, and the numerical results were validated by wind-tunnel measurements with Richardson numbers (Ri) from 0 to 4.77 with SF₆ being the tracer gas. The basic flow, heat and pollutant transfer were solved with the 3-D steady RANS (Reynolds-Averaged Navier-Stokes) equations. The results show that when Ri ≤ 0.85, the standard k-ε model (SKE) can better predict the flow and temperature fields. When Ri > 0.85, the realizable k-ε model (RLKE) performs better. For the same turbulence model equation, with the increasing Ri the effects of the two near-wall functions (standard and non-equilibrium wall functions, SWFs and NEWFs) on the flow structures and temperature distributions become more and more significant. The specific value of Sc_t has a significant effect on predicting the pollutant dispersion and the optimal value is 0.7 for the studied cases. It is also found that for indoor flow caused by an outdoor street flow there also exists the Re-independence region. For the model studied only when Re_H ≥ 7.57E+03, the wind-tunnel measured results can represent the realistic cases meaningfully for both flow and pollutant distributions in street canyon and inside the room.

Introduction

The physical phenomena that mixed forced and buoyancy flow with harmful pollutants passes over different geometry scale urban areas and enters into indoor environment to affect the health of residents belong to multiscale physical problems [1], [2]. Recently, the frequent haze weather caused by the increasing level of air pollutants such as PM₁₀ and PM_2.5 has caught many people's attentions in many big cities of China, which is mainly resulted from the coal combustion, industrial and traffic exhausts [3], [4], [5], [6]. Besides, the city managers and planners also pay attentions to the diffusion of air pollutants due to the incidents such as terrorist attack and accidental leakage of toxicity fluids in different scale urban areas [7], [8]. The studied regions can be divided into several ranges of length scales: regional scale (up to 100 or 200 km), city scale (up to 10 or 20 km), neighborhood scale (up to 1 or 2 km) and street scale (less than 100–200 m) [9], indoor scale (less than 10–20 m) and even body scale (less than 1–2 m). However, most of the previous studies mainly concentrated on the same geometry scales, and three approaches have been used: (1) on-site full-scale experiments; (2) reduced-scale physical modeling (wind-tunnel or water-channel experiments); and (3) numerical simulations based on CFD techniques.

There are pros and cons to these three research methods for studying flow and dispersion within urban environment. CFD method can provide detailed information under well-controlled conditions, but the model and method should be validated by some test results. Reduced-scale model can give some accurate experimental data, but is required to overcome the similarity constraints, especially when the forced flow and buoyancy effects are combined together. More intuitive and real data can be obtained from on-site experiments. However, for this method, apart from huge sources required, the experimental repeatability is quite small due to the uncontrollability of the meteorological conditions. van Hooff and Blocken [10] carried out the full-scale measurements of indoor environmental conditions and natural ventilation in a large semi-enclosed stadium and discussed four issues: (1) the advantage and disadvantage of full-scale on-site measurements versus reduced-scale wind tunnel measurements; (2) the limited number of measurement positions inside the stadium; (3) the repeatability of the full-scale measurements; and (4) the possibilities and limitations of the full-scale measurements for CFD validation. Given the above factors, most researchers usually conducted their studies by coupling CFD methods with other two methods for the CFD validation. Gousseau et al. [11] evaluated the performance of two different modeling methods (RANS standard k-ε model and LES) applied to predict near-field pollutant dispersion in an actual urban environment: downtown Montreal, validated by the detailed wind-tunnel experiments. They found that LES with the dynamic subgrid-scale model shows a better performance without requiring any parameters input to solve the dispersion equation. Allegrini et al. [12] used the wind-tunnel measurements to validate the scaled CFD model for the mixed buoyancy flow in urban street canyons, and the numerical results showed that a significantly better agreement can be achieved by using a standard k-ε model with the NEWFs (non-equilibrium wall functions) than LRNM (low-Reynolds number modeling). Stavrakakis et al. [13] applied CFD method coupled with the on-site building-scale experiments to investigate the natural cross-ventilation and evaluate the thermal comfort inside the building. They concluded that the RNG (renormalization group) k-ε model performed relatively better, especially for temperature predictions and hence they chosen it for further thermal comfort estimation purposes.

Obviously, the above studies mainly focused on one geometric scale (neighborhood scale, street scale or building scale). However, the flow and dispersion within urban environment belong to multiscale phenomena, the geometry scales of which are always across more than three orders of magnitude, taking the pollutant transport in Lujiazui region of Pudong district, Shanghai (see Fig. 1) as an example. The pollutant transport process in such an area could be described as: the pollutants are released into the ambient air in somewhere of the upwind direction and diffuse with the air flows that are affected by many factors, then pass through building groups in the neighborhood scale, and arrive at a high-rise building located in a street canyon, finally enter into a room through the open windows, and affect the health of residents. In general, there are three numerical methods to solve the above multiscale problem to obtain the detailed information in the smallest scale geometry. The first one is called “coupled” CFD simulation or full-scale simulation, in which grid generation and equation solving are conducted simultaneously within the whole computational domain. This method allows the proper calculation of air flow in the smallest scale geometry, but needs a high-resolution grid system and a relatively high computational cost for a large grid number [14]. The second method is called “de-coupled” simulation, in which the computational domain is divided into several regions according to the different geometry scales, then separate simulations are conducted, for example, one for the outdoor flow and other for the indoor flow, each in their own computational domain [15], [16], [17]. In the outdoor simulation, the information (generally pressure coefficients) is obtained at the closed ventilation openings to be used as the boundary conditions for the indoor simulation. The accuracy of this method can easily be compromised due to the simplifications involved. To improve the second approach, another “de-coupled” method called “from top-to-down solution procedure” is proposed with increasing grid fineness [18], [19]. Both the two “de-coupled” methods require an accurate full-scale simulation to validate and evaluate their simulation results.

It is well-known that the wind-tunnel experiment is an important tool to investigate the basic flow, heat and mass transfer within urban environment and can also provide data for the validation of the CFD method. However, to the authors' knowledge, the above complex multiscale problems have been only investigated by on-site full-scale measurements coupled with CFD simulations by van Hooff and Blocken [14] and not yet performed by reduced-scale wind tunnel measurements. Their study presented a coupled CFD modeling method for urban wind flow and indoor natural ventilation on a high-resolution body-fitted grid using realizable k-ε model and the CFD model was validated by the on-site full-scale measurements [14]. Then, they applied this coupled CFD modeling to investigate the effect of direction and urban surroundings on the ACH of a large semi-enclosed stadium [20]. For the reduced-scale wind-tunnel measurements in order to obtain meaningful results of the multiscale problems mentioned above there are two important challenges. The first challenge is the satisfaction of flow similarity between prototype and wind tunnel. In this regard previous studies [21], [22] have shown that for flows over buildings once Reynolds number of the oncoming flow is larger than a certain values, denoted by Re_crit, the values of Re will not affect the flow structure too much. The value of Re_crit is usually 2–3 orders of magnitude less than Re of the prototype flow. When Re is larger than Re_crit the flow is in the Re-independence region. The existence of Re-independence region creates a very favorable condition for adopting small-scale wind tunnel test. Second, it should be guaranteed that the smallest scale geometry of the multiscale modeling satisfies the Re-independence requirement. Thus whether the flow in the smallest scale geometry can satisfy Re-independence is an important condition for wind-tunnel measurements to reproduce the real flow and dispersion through different scale urban areas.

However, most previous studies on the Re-independence have been focused on the external flow, such as the flow passing over a single building or building groups [21], [22], [23]. For the internal flow, such as the flow passing the indoor scale model through the open windows in this study, the concept of Re-independence of the flow structure is no more applicable. The air flow passing through the open windows and entering the rooms will be the turbulent flow in the reality but would be likely laminar flow in wind-tunnel tests. According to the above analysis, when the reduced scale model is used, it is necessary to confirm the flow-structure independence for the studied multiscale models, including the street and indoor scales.

The above discussion has set forth the background of the present study. The first aim of the present work is to construct a coupled CFD model for accurately simulating the multiscale problems on the flow and pollutant dispersion within different scale urban areas. The feasibility of the adopted turbulence models are examined by comparing their predictions with the wind-tunnel measurements, which were reported in our previous work, where a simple multiscale model with three geometry scales (neighborhood scale – street scale – indoor scale) was constructed by considering the thermal effects with the range of Ri from 0 to 4.77 [24]. The influence of turbulent Schmidt number (Sc_t) on the pollutant dispersion is considered. The second purpose of this study is to clarify whether there exists Re-independence for the flow structures in the scaled indoor environment. The third one is to examine the effects of the outdoor environment on the IAQ (indoor air quality) by numerical and experimental methods.