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

下击暴流是下降并冲击地面的空气柱，导致经常强烈的外流，其中包含一个主导涡旋，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脉冲启动的动量喷流的下击暴流模型，该喷流从喷嘴发出，然后撞击地面。这个模型的吸引力在于，通过改变喷嘴(强度)或喷嘴直径(空间尺度)处的急流速度，可以从单一轴对称风场实现(无论是从数值模拟还是从实验模拟)线性地刻画强度和大小的任何“下暴流”事件。本申请人最近的工作表明，该模型是有根本缺陷的，因为它不包含真实事件的物理(特别是缺少斜压涡度的产生)，结果是不正确地预测了外流风速剖面的形状和大小。此外，撞击喷流模型与其母风暴的任何物理模型完全分离。最后，真正的下击暴流远不是轴对称的，因此从撞击喷流模型中提取的任何“设计”风速都不能代表该事件的峰值风速。研究的目的是：(1)开发一个简单但物理现实的模型，描述单个雷暴事件以及形成一条下暴线的多个事件在时间和空间上的变化，包括当地环境(大气)条件、地形粗糙度和地形的影响；(2)使用该模型为输电线路结构设计提供峰值风速信息。这些目标将通过将世界领先的、分辨率良好的全雷暴云模拟的结果与更简单的基于大涡模拟的数值模式相结合来实现，这些模拟包括降水微物理，并得到从新的大型水槽设施中的单个和多个密度驱动的转换下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.