pressure coefficient

The accurate estimation of the local wind pressure coefficient is crucial in the numerical modeling of natural or mixed ventilation in buildings subjected to wind. Building ventilation modeling typically relies on average wind pressure coefficient values specific to the building façade and wind direction. While the literature provides some correlations and standards for building wall-average pressure coefficients, these values are only useful in the absence of additional information or a database, as they can vary significantly based on urban forms.

The paper presents a numerical methodology to assess the natural ventilation. UrbaWind is an automatic computational fluid dynamics code. It was developed to model the wind in urban environments. The turbulence modelling, namely the dependence of turbulence length on the distance from wall, and the model constants were calibrated in order to reproduce with good agreements flow separation around buildings walls and pressure coefficient field on façades. Numerical results match well with the experiments: separation patterns and pressure field on walls in dense urban areas.

Global Information Systems (GIS) are being used to provide data on a wide range of environmental issues, and in the area of climate studies they are being used to determine zones of towns and cities where there are good or poor external environments. From this analysis, planning guidelines can be drawn up to ensure that developments either enhance or do not deter from the environmental quality of the surrounding area.

Wind loading Codes of Practice are usually presented in the form of tables of pressure and force coefficients corresponding to buildings of typical dimensions and shapes. Pressure and force coefficients are obtained from wind tunnel tests on small scale models placed in a turbulent boundary layer of thickness greater than the building height. Most of the wind tunnel studies of low or medium rise buildings which have been undertaken to date consider only the case of an isolated building.

The effect of wind on building ventilation is determined by pressure coefficients that depend on many factors such as building geometry, wind direction, mean and turbulent atmospheric boundary layer velocity profiles, and thefactor to be considered here: shelter by upwind obstacles. Pressure coefficients on the exterior of building are defined by normalising the pressure difference between that measured on the surface and a reference pressure by an appropriate stagnation (or velocity) pressure.

As part of the Air Infiltration & Ventilation Centre's Numerical Database development programme, Wind Pressure Coefficient data are being collected from many published sources, much of which will contribute to the Centre's own Pressure Coefficient database. To ensure the compatibility and validity of the various data sets, a preliminary comparison has been carried out of the data in use. The work has concentrated on an analysis of the effects of Cp values, for vertical walls, on calculated wind driven infiltration and ventilation rates.