A study of the wind forces on low rise building arrays and their application to natural ventilation design methods.

Due to the complicated flow phenomenon in urban areas, the assessment of wind pressure forces as well as the rates of natural ventilation for groups of low rise buildings is complex. As a result, the current design methods for the prediction of these forces are oversimplified and lead to inaccurate estimates of wind forces and ventilation rates in buildings. A survey of previous studies regarding wind properties and their influence on pressure forces along with work related to natural ventilation, wind loading and air flow round buildings was carried out. The survey revealed that no general relationship exists which defines the interaction between the various aspects of flow. This thesis, therefore, attempts to enhance our knowledge about the flow around groups of buildings and suggests a means of quantifying the interaction between building shape, group geometry, flow properties and the resulting pressure forces. The present study has been carried out in a simulated urban terrain atmospheric boundary layer flow. A series of model scale experiments were performed for different building shapes. The study starts with the investigation of the influence of upstream fetch on the central model drag before going on to the detailed measurements on various models covering a wide range of building shapes, group form and plan area density. The detailed measurements of mean pressure forces on model buildings situated within a variety of groups of similar form indicated three different trends in the behaviour of these forces, corresponding to the three flow regimes known to exist for flow over general roughness elements. The existence of these flow regimes was confirmed by velocity profile measurements. A general correlation between group geometry, flow properties and the resulting pressure forces has been suggested. In order to apply the foregoing results to full scale building arrays, a method has been proposed to yield the pressure difference across low rise buildings for the prediction of natural ventilation rates in an urban built form. This method takes into account the relevant built form and flow parameters which are ignored in the current IHVE design guide method (1970), (applicable to high rise buildings only). The suggested method includes the prediction of ventilation rates from the openings in the walls as well as in the roof. Suggestions have also been made to revise the British Standard Code of Practice for wind loading to incorporate the trends which have been found to be different to those currently recommended.