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The transient lift and drag coefficients around a low rise cube of dimension 60mm and a portal building of dimensions 240 Â 130 Â 53mm with eaves height of 42mm, which arise from the numerical simulation of an impinging jet or downburst are investigated. The numerical results were validated against a experimental results from a laboratory impinging jet simulator operating at the same scale. Having found the CFD simulation to match well with the laboratory scale the CFD was then used to visualise and interpret the flow field around the buildings. Common transient atmospheric boundary layer flow features, such as conical vortices, vortices on the rear face of a building, flow separation and vortex shedding were observed and could be used to explain the lift and drag results obtained. In particular, motion of the primary vortex from the downburst and its effect on the transient pressures on the building were identified, with strong pressure gradients observed for a number of configurations. Aspects of the flow phenomena were identified, which along with the strong pressure changes on the building surfaces, indicate areas of further research due to their potential impact on building and cladding design.

This paper examines the best turbulence model to use when using computational fluid dynamics to simulate an impinging jet type flow. The IDDES, k-ω SST SAS, Smagorinsky and dynamic Smagorinsky models were used and compared to data collected from a laboratory impinging jet, developed to simulate thunderstorm downburst flow fields. From this it was found the dynamic Smagorinsky model performed best, especially at capturing the velocities and pressures in the near inlet region. A mesh dependency study was then performed for the dynamic Smagorinsky turbulence model. A small mesh dependency was demonstrated for the mesh densities studied but had issues in capturing the velocity height profile correctly in the near wall region. Despite this issue the model still closely matched the laboratory pressures around a 60 mm cube and demonstrated the suitability of this modelling approach for investigating thunderstorm downbursts.

Thunderstorm downbursts are transient, small-scale events which are, however, the cause of design wind speeds in many parts of the world. The difficulties in predicting when and where such events will occur make capturing full-scale wind loading data for buildings particularly difficult, and so attention has turned to physically simulating downburst flow fields in the laboratory. The University of Birmingham Transient Wind Simulator (UoB-TWS), a 1 m diameter, pulsed impinging jet facility, has been used to physically simulate a downburst and measure the wind-loading on building models which occurs. For the first time, pressure data have been recorded over both single and multiple building model arrangements, allowing interference effects to be quantified. It is demonstrated that the presence of another building nearby can increase drag coefficients by almost 40%, depending on the separation of the building models and the angle of the wind relative to the models.

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