Development of a physically-realistic model for thunderstorm downburst outflows for wind-resistant design of transmission line structures

下爆是空气柱下降并撞击地面，导致经常强烈的流出，其中包含一个主要的漩涡，其底部与可能损坏建筑物的最强风速有关，特别是输电线路系统。在风工程领域，已经采用了一种基于冲激启动动量射流的下爆模型，该射流从喷嘴发出，然后撞击地面。该模型的吸引力在于，通过改变喷嘴处的射流速度（强度）或喷嘴直径（空间缩放），任何“下爆”事件都可以从单个轴对称风场实现（通过数值或实验建模）的强度和大小上线性缩放。本申请人最近的工作表明，该模型从根本上是有缺陷的，因为它没有包含任何真实事件的物理特性（特别是缺少斜压涡度的产生），结果是流出风的风速剖面形状和大小被错误地预测了。此外，撞击射流模型与母风暴的物理特性完全分离。最后，真正的下爆远不是轴对称的，所以从撞击射流模型中获得的任何“设计”风速都不能代表该事件的峰值风速。本研究的目的是：(1)考虑当地环境（大气）条件、地形粗糙度和地形的影响，建立一个简单但物理逼真的模型，描述单个雷暴下暴事件以及形成下暴线的多个事件的近地流出风速的时空变化；(2)利用该模型为输电线路结构设计提供峰值风速信息。目标将通过将世界领先的、分辨率高的、完整的雷暴云模拟（包括降水微物理）的结果耦合在一起来实现，这些模拟使用基于大涡模拟的更简单的数值模型产生下暴，并得到来自新型大型水槽设施中单个和多个密度驱动的翻译下暴事件的实验数据的支持。

Downbursts are columns of air that descend and impact upon the ground, resulting in often-intense outflows containing a dominant vortex, whose underside is associated with the strongest wind speeds that may damage structures, notably electricity transmission line systems. In the field of wind engineering a model of downbursts has been adopted based upon an impulsively-started momentum jet, issuing from a nozzle, which then impinges upon the ground. The attraction of this model is that any "downburst" event may be linearly scaled, in both intensity and size, from a single axi-symmetric wind field realization (either from numerical or experimental modelling) by varying the jet velocity at the nozzle (intensity) or the nozzle diameter (spatial scaling). Recent work by the present applicant has shown that this model is fundamentally flawed because it contains none of the physics of a real event (in particular the baroclinic generation of vorticity is missing), with the result that the outflow wind velocity profile shape and magnitudes are incorrectly predicted. In addition, the impinging jet model is completely de-coupled from any of the physics of its parent storm. Finally, real downbursts are far from axi-symmetric and so any "design" wind speed taken from an impinging jet model will not represent the peak wind speed for that event. The objectives of the research are; (1) To develop a simple but physically-realistic model that describes the temporally and spatially varying near-ground outflow wind velocities of single thunderstorm downburst events, as well as multiple events that form a downburst line, incorporating the influence of local environmental (atmospheric) conditions, terrain roughness and topography; (2) To use this model to provide peak wind speed information for the design of transmission line structures. The objectives will be achieved by coupling results from world-leading, well-resolved, full thunderstorm cloud simulations, including precipitation microphysics, that produce downbursts with simpler numerical models based on Large-Eddy Simulations, supported by experimental data obtained from single and multiple density-driven translating downburst events in a novel large flume facility.