This paper presents preliminary results of on-going research on the integration of building energy simulation (BES) and building envelope heat, air and moisture transfer (HAM) programs. The paper contrasts the capabilities of two BES and HAM programs, and presents the theory and preliminary results of one-way coupling between them.
Wind-induced convection at the exterior surface of specific building components or constructions has a noticeable effect on the heat transfer in the building envelope. Hence a proper knowledge of the convective heat transfer coefficients (CHTC) can improve the accuracy of numerical heat transfer modeling. In this paper, the forced CHTC on the surfaces of a cubic building is studied with CFD. Steady RANS (Reynolds-Averaged Navier-Stokes) simulations provide, for each facade, correlations of the surface-averaged transfer coefficient with the wind speed at a height of 10 m. These are compared with existing correlations. Results show a distinct variation of the values of the coefficient over the facade and significant differences are found between the surfaceaveraged transfer coefficients of the facades. RANS simulations are also compared with unsteady simulations with the Detached-Eddy Simulation (DES) model. The predicted transfer coefficients for DES and RANS are found to differ both in magnitude and distribution over the facades. Significant differences are found for the value of the CHTC on the windward facade which is partially attributed to the rather simplified inlet conditions that are used for DES. Nevertheless, a good similarity is found in the distribution over the windward facade for all models. In contrast to RANS, DES does incorporate the unsteady nature of the separated flow in the distribution of the heat transfer coefficient. This is clearly manifested in a more uniform and relative high CHTC on the leeward facade.
The role of research in the Department of Architecture, Building and Planning is shifting. Design must become more scientific and research more international - and above all, interdisciplinary. In search of greater perspective on this new interpretation of research, this article presents the views of the Department’s Pieter van Wesemael and Bert Blocken. Designing, developing, researching - on change, trends, innovation, tradition and the scientification of design.
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. 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. This paper presents a systematic evaluation of 3D steady 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 windtunnel measurements. The impact of several computational parameters is also investigated, including the resolution of the computational grid, the turbulence model and building balconies. The results show that steady RANS can accurately reproduce the mean wind pressure distribution across the windward facade. 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.
Wind flow modeling in urban areas is influenced by many uncertainties, such as geometric detailing, inflow and boundary conditions, numerical approach (RANS, LES, and DNS) and turbulence model. This study aims to investigate how the different inflow conditions that are usually adopted to simulate urban wind flows may affect the accuracy of the results. CFD simulations were performed on a selected urban area - “Quartiere La Venezia” in Livorno (Italy) - using a steady-state RANS approach. Two types of inflow profiles were considered for one wind direction (α = 240° from the North). Wind tunnel tests performed on the same urban model were used as a benchmark to validate the numerical simulations. Mean wind profiles at 25 locations were compared and the statistical performance in terms of four different metrics was quantified for both inflow conditions. The results show that slightly different inflow conditions can greatly affect the results in terms of mean wind speed and turbulent kinetic energy.
Façade geometrical details can substantially influence the near-façade airflow patterns and pressures. This is especially the case for building balconies as their presence can lead to multiple separation and recirculation areas near the façades and hence large changes in surface pressure distribution. Computational fluid dynamics (CFD) has been widely used to investigate the impact of building balconies, mainly based on the steady Reynolds-averaged Navier-Stokes (RANS) approach. The objective of the present study is to evaluate the performance of steady RANS and large-eddy simulations (LES) in predicting the near-façade airflow patterns and mean surface pressure coefficients (Cp) for a building with balconies for three wind directions θ = 0°, 90°, 180°, where 0° is perpendicular to the façade under study. The evaluation is based on validation with wind-tunnel measurements of Cp. The results show that both RANS and LES can accurately predict Cp on the windward façade for θ = 0° with average absolute deviations of 0.113 and 0.091 from the measured data, respectively. For the other two wind directions, LES is clearly superior. For θ = 90°, the average absolute deviations for RANS and LES are 0.302 and 0.096, while these are 0.161 and 0.038 for θ = 180°. Large differences are found in the computed flow fields on the balcony spaces. Because RANS systematically underestimates the absolute values of both Cp and mean wind speed on the balconies, it is suggested that building design based on RANS might result in excessive ventilation and in too high wind nuisance level.
This paper describes how the energy performance of single storey multiple-skin facades can be optimized by changing the settings of the facades and HVACsystem. The energy performance is analyzed with a yearly whole building energy analysis under Belgian climatic conditions. Three multiple-skin facades are scrutinised: a mechanically ventilated airflow window, a naturally ventilated double-skin facade and a mechanically ventilated supply window. Their performance is compared against the performance of a traditional cladding with exterior and interior shading device. It is shown that both the heating and cooling demand may significantly be improved by implementing control strategies such as controlling the airflow rate and recovery of air returning from multiple-skin facades.
17 18 Computational Fluid Dynamics; turbulence model; cyclist; aerodynamics; wind tunnel 19 20 Word count (Introduction to conclusions): 3494 words 21 22 23 Abstract 24 This study aims at assessing the accuracy of Computational Fluid Dynamics (CFD) for applications in sports 25 aerodynamics, for example for drag predictions of swimmers, cyclists or skiers, by evaluating the applied 26 numerical modelling techniques by means of detailed validation experiments. In this study, a wind-tunnel 27 experiment on a scale model of a cyclist (scale 1:2) is presented. Apart from three-component forces and 28 moments, also high-resolution surface pressure measurements on the scale model's surface, i.e. at 115 locations, 29 are performed to provide detailed information on the flow field. These data are used to compare the performance 30 of different turbulence-modelling techniques, such as steady Reynolds-averaged Navier-Stokes (RANS), with 31 several k-� and k-� turbulence models, and unsteady Large-Eddy Simulation (LES), and also boundary-layer 32 modelling techniques, namely wall functions and low-Reynolds number modelling (LRNM). The commercial 33 CFD code Fluent 6.3 is used for the simulations. The RANS shear-stress transport (SST) k-� model shows the 34 best overall performance, followed by the more computationally expensive LES. Furthermore, LRNM is clearly
Knowledge of the convective heat transfer coefficient (CHTC) on building walls is important for research on building energy and building component durability. In building aerodynamics, steady RANS is frequently used to model air flow, rather than unsteady RANS (URANS) or Large-Eddy Simulations (LES). To gain insight into the performance of LES compared to steady RANS, this paper presents LES and RANS CFD simulations of the temperature distributions at the surfaces of a reduced-scale cubic model measured in turbulent channel flow. The evaluation is based on a grid-sensitivity analysis. The results show that LES can accurately predict the surface temperature distributions of the cube walls. Steady RANS, however, indicates a satisfactory agreement with the experiments only for the windward surface.
The majority of numerical studies of room airflow using Computational Fluid Dynamics (CFD) are conducted with the steady Reynolds-averaged Navier-Stokes (RANS) approach. In this approach the averaged quantities are computed, and the effect of turbulence is modelled. Furthermore, the standardgradient diffusion hypothesis is often used to model the turbulent mass transport, which relates the turbulent mass flux to the mean concentration derivative. In this paper, a CFD analysis of pollutant dispersion in an enclosure ventilated by a transitional wall jet (Re ≈ 2,500) is presented, using validated high-resolution RANS and Large Eddy Simulations (LES). Although the LES computations show that a counter-gradient turbulent mass flux is present, indicating that the standard gradient-diffusion hypothesis used in RANS is not valid in the entire flow domain, it is shown that the convective mass fluxes dominate over the turbulent mass fluxes, and that therefore the pollutant concentrations predicted by RANS do not differ significantly.
