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

Due to the complicated flow phenomenon in urban \nareas, the assessment of wind pressure forces as well as \nthe rates of natural ventilation for groups of low rise \nbuildings is complex. As a result, the current design \nmethods for the prediction of these forces are oversimplified \nand lead to inaccurate estimates of wind forces \nand ventilation rates in buildings. \nA survey of previous studies regarding wind \nproperties and their influence on pressure forces along with \nwork related to natural ventilation, wind loading and air \nflow round buildings was carried out. The survey revealed \nthat no general relationship exists which defines the \ninteraction between the various aspects of flow. This \nthesis, therefore, attempts to enhance our knowledge about \nthe flow around groups of buildings and suggests a means of \nquantifying the interaction between building shape, group \ngeometry, flow properties and the resulting pressure forces. \nThe present study has been carried out in a \nsimulated urban terrain atmospheric boundary layer flow. \nA series of model scale experiments were performed for \ndifferent building shapes. The study starts with the \ninvestigation of the influence of upstream fetch on the \ncentral model drag before going on to the detailed measurements \non various models covering a wide range of building shapes, \ngroup form and plan area density. The detailed measurements \nof mean pressure forces on model buildings situated within a \nvariety of groups of similar form indicated three different \ntrends in the behaviour of these forces, corresponding to \nthe three flow regimes known to exist for flow over general \nroughness elements. The existence of these flow regimes \nwas confirmed by velocity profile measurements. A general \ncorrelation between group geometry, flow properties and the \nresulting pressure forces has been suggested. \nIn order to apply the foregoing results to full \nscale building arrays, a method has been proposed to yield \nthe pressure difference across low rise buildings for the \nprediction of natural ventilation rates in an urban built \nform. This method takes into account the relevant built \nform and flow parameters which are ignored in the current \nIHVE design guide method (1970), (applicable to high rise \nbuildings only). The suggested method includes the \nprediction of ventilation rates from the openings in the \nwalls as well as in the roof. Suggestions have also been \nmade to revise the British Standard Code of Practice for \nwind loading to incorporate the trends which have been \nfound to be different to those currently recommended.