Abstract: Wind pressure coefficients (cp) are important inputs for analytical calculations of wind load. The aim of this research is to investigate wind pressure coefficients on a test house located in Norway in order to pave the way for improved analysis of wind-driven roofing ventilation. The large-scale test measurements show that the wind pressure coefficient along the eaves of the house varies with different wind approach angles. Assuming wind-driven air flow through the air cavity beneath the roofing, an average Δ c p ¯ value of 0.7 is derived for practical engineering purposes. The results from the study are applicable for single or two-storey houses with pitched roofs at different roof angles.

Abstract: Facade geometrical details can substantially influence the near-facade 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 facades 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-facade 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 facade 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 facade 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.

33 citations

Cites background from "Wind pressure coefficients for roof..."

...evaluation of wind-induced natural ventilation [6,7], wind comfort on balcony spaces [8], pollutant dispersion [9], wind loads on building walls and building components [10] and convective heat transfer at building surfaces [11–13]....

Abstract: Wind characteristics on building surfaces are used to evaluate natural ventilation of a building. As a type of building component, external shading louvers are applied in hot climatic regions to block solar radiation and provide better visual environments. The structure of external louvers can affect wind-induced characteristics, such as convective heat transfer coefficient, wind pressure and pollutant dispersion around building envelopes. This paper aims to analyze the potential ventilation capacity of a multi-storey building with shading louvers, based on wind pressure coefficient by the numerical method. A reference case was established and a previous study was applied to validate the numerical results. The rotation angle of horizontal louvers is taken from 0° to 75° in the simulation cases. The results show that average wind pressure has the greatest reduction for all floors when rotation angle turns from 60° to 75°. Ventilation openings on the stagnation zone contribute to higher ventilation rates for the windward facade with louvers. The analysis, based on multi-floor and multi-row buildings under shaded conditions, will provide a greater perspective for engineers to make optimal natural ventilation routes in multi-storey buildings with external shading louvers.

Abstract: Pitched wooden roofs are ventilated through an air cavity beneath the roofing in order to remove heat and moisture from the roof construction. The ventilation is driven by wind pressure and thermal buoyancy. This paper studies ventilation driven by thermal buoyancy in the air cavity of inclined roofs. The influence of air cavity design and roof inclination on the airflow is investigated. Laboratory measurements were carried out on an inclined full-scale roof model with an air cavity heated on one side in order to simulate solar radiation on a roof surface. Equipment to measure temperature was installed in the roof model, while air velocity in the cavity was determined by smoke tests. Combinations of different roof inclinations, air cavity heights and applied heating power on the air cavity top surface were examined. The study showed that increased air cavity height led to increased airflow and decreased surface temperatures in the air cavity. Increased roof inclination and heating power applied to the roofing also increased the airflow. The investigations imply that thermal buoyancy in the air cavity of pitched roofs could be a relevant driving force for cavity ventilation and important to consider when evaluating the heat and moisture performance of such a construction.

Abstract: To meet the ventilation requirement, an engineer needs to design the cantilevered roof of a stadium as a layered one with gaps between the layers. In order to further understand the wind resistance...

4 citations

Cites background from "Wind pressure coefficients for roof..."

...Gullbrekken et al. (2018) studied the sloped roof with air cavities below the roof and showed that the difference of mean wind pressure coefficients between the air inlet and the outlet of eaves could reach a value of 0.7....

Abstract: The building sector is an energy-intensive sector, consuming over 40% of the total energy use in Norway. Energy efficiency improvement of the building sector is crucial to fulfilling the Norwegian obligations to the UN. Zero Emission Building, ZEB, are constructed to achieve an on-site production of renewable energy compensating any greenhouse gas emissions occurring throughout the lifespan of the building. ZEB Laboratory is a 2000 square meters office and education building currently under construction located in Trondheim, Norway. ZEB Laboratory strives to act as an example for future office and educational buildings aiming to achieve a level of ZEB. The object of this Master’s Thesis is to find the optimal use of mechanical and natural ventilation in ZEB Laboratory regarding energy demand, without compromising the indoor environment of the building. An extensive literature review was performed. The definitions, recommendations, and requirements of a good indoor environment in offices and educational building were reviewed. Further, the different methods and strategies of ventilation were evaluated, in addition to a state of the art review of energy efficient ventilation strategies. There is a lack of literature regarding how well-functioning office and educational buildings only supplied with only natural ventilation are in colder climates, so studies performed in southern regions were reviewed. Provided information regarding ZEB Laboratory, including building structure, zonal division, external openings, and planned available mechanical ventilation, has been reviewed. This information formed a basis for the composition of a simulation model of ZEB Laboratory. A basic building model of the ZEB Laboratory was created, including different controllers of the natural and mechanical ventilation systems. The basic building model was simulated in three different ventilation modes: A) Clean natural mode, B) Clean mechanical mode, and C) Hybrid mode, during three different cases: Case 1) Winter, Case 2) Transition, and Case 3) Summer. Some corrections of the modes were performed to minimize the energy demand while not compromising the indoor environment. The results from the simulations show that the largest amount of demanded energy is due to the heating requirement of cold ambient air entering the building. Hence, the most energy efficient ventilation mode during the winter is a clean mechanical mode. During the transition season, a hybrid ventilation mode is the most energy efficient solution due to lower requirements of fan power. The building should be implemented with passive cooling outside the occupied hours during the summer season. This will lead to a satisfactory indoor environment when the building is supplemented with clean natural ventilation. Further, the simulations show that a clean natural ventilation mode is substantial to ensure a good indoor environment during the entire year. However, it’s important to note that the simulations doesn’t simulate internal heat gains, heat transfer, or the resulting temperature change. The simulation results may therefore deviate from the real resulting energy demand and indoor environment.

3 citations

Cites background from "Wind pressure coefficients for roof..."

...in 2018, Tokyo Polytechnic University has produced a detailed database correctly describing the wind load of low rise buildings (Gullbrekken et al. 2018)....

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...Wind pressure coefficient The wind pressure coefficient, Cp, is a dimensionless coefficient describing the ratio of the wind pressure at a given point x (Gullbrekken et al. 2018)....

Abstract: Wind pressure coefficients (Cp) are influenced by a wide range of parameters, including building geometry, facade detailing, position on the facade, the degree of exposure/sheltering, wind speed and wind direction. As it is practically impossible to take into account the full complexity of pressure coefficient variation, Building Energy Simulation (BES) and Air Flow Network (AFN) programs generally incorporate it in a simplified way. This paper provides an overview of pressure coefficient data and the extent to which they are currently implemented in BES-AFN programs. A distinction is made between primary sources of Cp data, such as fullscale measurements, reduced-scale measurements in wind tunnels and computational fluid dynamics (CFD) simulations, and secondary sources, such as databases and analytical models. The comparison between data from secondary sources implemented in BES-AFN programs shows that the Cp values are quite different depending on the source adopted. The two influencing parameters for which these differences are most pronounced are the position on the facade and the degree of exposure/sheltering. The comparison of Cp data from different sources for sheltered buildings shows the largest differences, and data from different sources even present different trends. The paper concludes that quantification of the uncertainty related to such data sources is required to guide future improvements in Cp implementation in BES-AFN programs.

Abstract: The air flow around isolated gable-roof buildings with different roof pitches was investigated by wind tunnel experiments and computational fluid dynamics (CFD) simulations based on a steady Reynolds-averaged Navier–Stokes equations (RANS) model. Firstly, wind tunnel experiments on the air flow around building models with three different pitches, specifically, 3:10, 5:10, and 7.5:10, were conducted to create a measurement database of the time-averaged velocity, turbulent kinetic energy, and pressure coefficient around the building. Next, sensitivity analyses for the grid resolutions and turbulence models of the CFD simulations were performed for the 5:10 roof pitch model. The performance of the CFD simulation with the selected grid resolution and turbulence model was examined and validated by comparing the results of the simulation with the measured data for all the roof pitches. Generally, for the streamwise velocity, the simulation results were found to be in good agreement with the measured values, with an average deviation of less than 15%. For points behind the building, however, the prediction accuracy showed as much as 30% deviation. This discrepancy was closely related to the fact that the transient fluctuations caused by vortex shedding around the building are not reproduced by the steady-RANS simulations used in this study. Finally, the effect of the roof pitch on the flow field around the building was investigated using the CFD simulations. We clarified that the difference in the flow fields of the 3:10 and 5:10 roof pitches is large, relative to the difference between the 5:10 and 7.5:10 pitches.

Abstract: Data gathered from a number of field and laboratory experiments concerned with wind pressures acting on low-rise buildings are reviewed, and selected experimental results are presented in this paper. Particular attention is paid to works related to cladding design. Only either full-scale studies or those done under conditions simulating the atmospheric boundary layer have been considered. Comparisons of the data from various sources are made for the characteristics of the mean and fluctuating wind pressures. The results indicate that the statistical properties of fluctuating pressures on the roof edges and corners can be predicted by a quasi-steady approach. Furthermore, the peak-factor approach is found to perform adequately in evaluating the design wind loads. The relation between the spatial and time averages is also discussed.

Abstract: In this paper, the influences of two important problems in computational wind engineering (CWE), which are the modeling of equilibrium atmosphere boundary layer (ABL) and the specification of turbulence model parameters, on the numerical simulations of wind pressure distributions on a typical low-rise building are investigated sequentially through detailed comparisons between the numerical results and the wind tunnel test data. The capability of the proposed inflow turbulence boundary conditions in constructing equilibrium ABL is verified, and the effect of the turbulence parameters on the numerical results is illustrated. The combination of the carefully considered inflow boundary conditions, and the turbulence parameters can improve the numerical simulation accuracy of the wind pressures of the low-rise building. The present work seems to be helpful to further realize the importance of these two problems as well as to provide a referential study to the accuracy improvement of CWE.

Abstract: The building wind pressure coefficient (Cp) is an important quantity which is used in many fields of building engineering including heating and cooling load calculations, ventilation design, and structural design. Cp is a dimensionless quantity that represents the proportionality between the wind velocity and the pressure generated on the surface of the building. Values for Cp can be obtained from full-scale building tests, wind tunnel tests, or, more commonly, from parametric equations derived from tests. The purpose of this paper is to analyze a set of wind tunnel tests and to present a new set of surface-averaged wind pressure coefficient values for low-rise buildings and a new parametric equation determined from a curve fit to the surface-averaged data. The resulting equations are compared to another popular low-rise parametric equation and another popular wind pressure coefficient database. The new parametric equation is found to fit both databases better than the older parametric equation.