Downburst dynamics and the implications for engineering structures

极端风暴造成的破坏和经济损失正在以显著的速度增加，这已是一个公认的事实。也有初步证据表明，这些风暴的频率和强度正在增加，主要是由于气候变化的影响，尽管人们承认，这种证据远非结论性的。极端风暴强度和频率的任何增加都可能对城市基础设施、世界经济和整个社会造成严重损害。在欧洲方面，有人提出，到2080年，欧洲极端风暴造成的与风有关的保险损失将至少增加...... 250 - 300亿欧元。然而，也许值得注意的是，这些估计数没有考虑到由于人口增长、人口富裕和风险资产增加，社会面临的极端风暴风险越来越大。在过去的几年里，人们对极端风事件的影响重新产生了兴趣，因为在许多情况下，这些事件对于风荷载是最重要的（即，建筑物/基础设施的设计）。过去很少受到关注的一组特定的极端风事件是与雷暴下击暴流相关的事件。在下击暴流期间，一股空气垂直向下移动并撞击地面。这导致所产生的空气从冲击点径向向外移位，并且环形涡流远离停滞点行进。这样做的效果是显著地改变速度场。换句话说，设计建筑物时假设的速度场可能不再出现，而可能存在一个新的、非常不同的场。这种新的风场对典型结构的影响还有待解决。因此，有必要对雷暴下击暴流的结构进行全面的检查，并调查可能出现的相应的风致力。全面数据的稀缺和预测此类事件的困难确保了目前建模是一个明智的前进方向。此外，与物理和数值模拟有关的不确定性强烈表明，以（有限的）全尺寸数据为补充的物理/数值模拟综合方案是最佳的前进方向。如果不对雷暴下击暴流相关的风场进行这样的检查，现有设计方法的适用性仍然是一个悬而未决的问题。这一点很重要，因为在世界许多地方，这一来源的风速构成了设计风速。即使在这些事件不占主导地位的地区，对社会及其相关基础设施的持续投资和发展也会确保整个社会更容易受到这种事件的影响，无论这些事件目前发生的频率如何。

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.