شبیه سازی CFD ضرایب فشار آیرودینامیک در مورد ساختمان های با و بدون بالکن: اعتبار سنجی و تجزیه و تحلیل حساسیت

Knowledge of the pressure distribution on building walls is important for the evaluation of wind loads and natural ventilation. Wind-induced pressure distributions are influenced by a wide range of factors including approach-flow conditions, urban surroundings and building geometry. Computational Fluid Dynamics (CFD) can be a valuable tool for determining mean wind pressure coefficients on building facades. However, while many CFD studies of mean wind pressure on buildings have been performed in the past, the vast majority of these studies focused on simple building geometries without facade details such as balconies. These details however can drastically influence the flow pattern and the overall pressure distribution on the facade. This paper presents a systematic evaluation of 3D steady Reynolds-Averaged Navier–Stokes (RANS) CFD for predicting mean wind pressure distributions on windward and leeward surfaces of a medium-rise building with and without balconies. The evaluation is based on a grid-sensitivity analysis and on validation with wind-tunnel measurements. It is shown that building balconies can lead to very strong changes in wind pressure distribution, because they introduce multiple areas of flow separation and recirculation across the facade. The results show that steady RANS, in spite of its limitations, can accurately reproduce the mean wind pressure distribution across the windward facade of the building. The average deviations from the wind-tunnel measurements are 12% and 10% for the building with and without balconies, respectively. In addition, also the important impact of the reference static pressure and the turbulence model are demonstrated.

Knowledge of the pressure distribution on building walls is essential to evaluate wind-induced natural ventilation and to assess wind loads on building walls and building components (e.g. [1], [2], [3], [4], [5], [6], [7] and [8]). As an example, Building Energy Simulation (BES) programs require pressure coefficient data as input for analysing ventilation and infiltration flow rates [2]. Similarly, design standards need data with a high accuracy for effective-cost designs and reduction of wind damage and cost to building components [9] and [10]. The pressure distribution on building walls is influenced by a wide range of factors including approach-flow conditions [11], [12] and [13], urban surroundings [14], building geometry [1] and wind direction [15]. In particular, building facade details such as balconies and other protrusions can affect the peak and mean surface pressure distributions on buildings walls and roofs [16], [17] and [18]. Pressure coefficients can be determined using full-scale on-site measurements [15], [19], [20], [21], [22], [23], [24], [25] and [26], reduced-scale wind-tunnel measurements [27], [28], [29], [30], [31] and [32] or numerical simulation with Computational Fluid Dynamics (CFD) [13], [16], [33], [34], [35], [36] and [37]. Full-scale measurements offer the advantage that the real situation is studied and the full complexity of the problem is taken into account. However, full-scale measurements are usually only performed in a limited number of points in space. In addition, there is no or only limited control over the boundary conditions [38]. Reduced-scale wind-tunnel measurements allow a strong degree of control over the boundary conditions, however at the expense of – sometimes incompatible – similarity requirements. Furthermore, wind-tunnel measurements are usually also only performed in a limited set of points in space [13]. CFD on the other hand provides whole-flow field data, i.e. data on the relevant parameters in all points of the computational domain [5], [39] and [40]. Unlike wind-tunnel testing, CFD does not suffer from potentially incompatible similarity requirements because simulations can be conducted at full scale. CFD simulations easily allow parametric studies to evaluate alternative design configurations, especially when the different configurations are all a priori embedded within the same computational domain and grid (see e.g. [41]). CFD is increasingly used to study a wide range of atmospheric and environmental processes. Examples are pedestrian wind comfort and wind safety around buildings [40], [42], [43], [44], [45] and [46], natural ventilation of buildings [5], [41], [47], [48], [49], [50], [51], [52] and [53], air pollutant dispersion [54], [55], [56], [57] and [58], convective heat transfer [59], [60] and [61], etc. In some of these studies, CFD was applied and evaluated in detail, including verification, validation and sensitivity analyses. CFD has also been used on many occasions in the past to determine mean wind-induced pressure distributions on building facades. However, the vast majority of these studies focused on relatively simple building shapes and plane, smooth facades without protrusions or recessions (e.g. [34], [35], [36], [62] and [63]). Nevertheless, many historical and contemporary building facades are characterized by protrusions and recessions. To the best of our knowledge, a detailed evaluation of steady Reynolds-averaged Navier–Stokes (RANS) CFD has not yet been performed for mean wind pressure distributions on such building facades. This paper therefore presents a systematic and detailed evaluation of 3D steady RANS CFD for predicting mean wind pressure distributions on building facades with and without balconies for both normal and obliquely approach-flow conditions. The evaluation is based on a grid-sensitivity analysis and on validation with wind-tunnel measurements by Chand et al. [17]. The impact of several computational parameters is also investigated, including the resolution of the computational grid, the reference static pressure and the turbulence model. In Section 2, the wind tunnel experiments by Chand et al. [17] are briefly outlined. Section 3 presents the computational settings and parameters for the reference case, and the validation of the CFD results with the wind-tunnel measurements. In Section 4, the sensitivity analysis is performed, including the influence of building balconies on the wind pressure distribution. A discussion on the limitations of the study is given in Section 5. The main conclusions are presented in Section 6.

This paper has presented a systematic evaluation of 3D steady RANS CFD for the prediction of the mean wind pressure distribution on windward and leeward surfaces of a medium-rise building with and without balconies. The evaluation is based on a grid-sensitivity analysis and on validation with wind-tunnel measurements. The study was motivated by the lack of knowledge on the accuracy and reliability of CFD for determining mean wind pressure coefficients on building facades with balconies. Although indeed many CFD studies of mean wind pressure distributions on buildings have been performed in the past, the vast majority of these studies focused on simple building geometries without facade details such as balconies. These details however are important because they can drastically change the flow pattern and the overall pressure distribution on the facade. In addition, many historical and contemporary building facades are characterized by protrusions and recessions. The present study has shown that 3D steady RANS CFD, in spite of its limitations, is suitable to predict the wind-induced mean pressures at windward building facades with (and without) balconies. It has also been shown that the presence of building balconies can indeed lead to very strong changes in wind pressure distribution on these windward facades, because the balconies introduce multiple areas of flow separation, recirculation and reattachment. 3D steady RANS CFD has also been shown to provide accurate predictions of the mean wind pressure at the leeward wall in case of a perpendicular approach flow wind direction. This however is not the case for oblique flow, where large discrepancies with the wind-tunnel measurements have been found. Finally, also the impact of the turbulence model, the reference static pressure and the wind direction have been investigated, and it has been shown that a careful selection of these parameters is very important for accurate and reliable results.