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

下击暴流是一种下降并撞击地面的空气柱，导致通常强烈的外流，其中包含一个占主导地位的漩涡，其底部与可能损坏结构的最强风速有关，特别是输电线路系统。在风工程领域，下击暴流的模型已被采用，该模型基于从喷嘴发出的脉冲启动的动量射流，然后冲击地面。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“下击暴流”事件都可以从单个轴对称风场实现（来自数值或实验建模）的强度和大小进行线性缩放。本申请人最近的工作已经表明，该模型从根本上是有缺陷的，因为它不包含真实的事件的物理学（特别是缺少涡度的斜压生成），结果是流出风速廓线形状和大小被错误地预测。此外，冲击射流模型是完全解耦的任何物理的母风暴。最后，真实的下击暴流远非轴对称的，因此从冲击射流模型中得出的任何“设计”风速都不能代表该事件的峰值风速。研究的目的是：（1）建立一个简单但物理上真实的模型，描述单个雷暴下击暴流事件以及形成下击暴流线的多个事件的时间和空间变化的近地面出流风速，并考虑当地环境的影响。（2）利用该模型为输电线路结构物的设计提供峰值风速信息。这些目标将通过耦合来自世界领先的、分辨率良好的、完整的雷暴云模拟的结果来实现，包括降水微物理学，这些结果产生基于大涡模拟的具有更简单的数值模型的下击暴流，并得到从一个新的大型水槽设施中的单个和多个密度驱动的平移下击暴流事件获得的实验数据的支持。

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.