"It is now an accepted fact that the disruption and economic losses arising as a result of extreme storms are increasing at a significant rate. There is also tentative evidence to suggest that these storms are increasing in frequency and magnitude due primarily to climate change effects, although it is acknowledged that such evidence is far from conclusive. Any increases in magnitude and frequency of extreme storms are likely to result in serious damage to the urban infrastructure, the world economy and society as a whole. In European terms, it has been suggested that by 2080, there will be an increase in wind-related insured losses from extreme European storms by at least....25-30bn Euro. However, it is perhaps worth noting that these estimates do not take into account society's increasing exposure to extreme storms, due to growing populations, wealthier populations and increasing assets at risk.
Over the last few years there has been renewed interest in the effects of extreme wind events, since in a number of cases these events are the most important with respect to wind loading (i.e., the design of buildings/infrastructure). One particular set of extreme wind events which has received little attention in the past are those associated with thunderstorm downbursts. During a downburst a column of air moves vertically downwards and impinges on the ground. This causes the resultant air to be displaced radially outwards from the point of impingement, with a ring vortex travelling away from the stagnation point. The effect of this is to alter the velocity field significantly. In other words, the velocity field which was assumed when the building was designed may no longer occur, and a new, very different field may exist. The effect that this new wind field has on typical structures has yet to be addressed.
Hence, there is a need to undertake a comprehensive examination of the structure of thunderstorm downbursts and to investigate the corresponding wind induced forces which can arise. The scarcity of full-scale data and the difficulty of predicting such events ensure that at present, modelling is a sensible way forward. Furthermore, the uncertainties associated with both physical and numerical modelling strongly suggest that a combined physical/numerically modelling programme supplemented by (limited) full-scale data is the best way forward. Without such an examination of the wind field associated with thunderstorm downbursts, the suitability of existing design methods remains an open question. This is of importance since in many parts of the world wind speeds of this origin constitute the design wind speeds. Even in areas where these events are not dominant, the continued investment and development in society and its related infrastructure ensures that society as a whole is more vulnerable to the effects of such an event irrespective of how frequently they current occur."
The strong winds created by thunderstorm downbursts a very different to those normally experienced, and are characterised by a much higher peak windspeed along with a very rapid increase in speeds up to the peak value. This research project has enhanced the understanding of the profile of these winds, giving more understanding of how they interact with the urban environment. Measurements and predictions on how the rapidly changing wind speeds and directions interact with buildings will influence future building design and possibly in the future, design codes and methodology. The traditional computational tools for assessing these wind flows do not work very well with the strong downflow and resulting vortical flows as the wind interacts with the ground plane. The research undertaken provides enhanced models to deliver more accurate predictive tools to allow more understanding of the flow field to be ascertained.