Scaling Of Downburst Outflows Simulated Using A Full-Scale Cooling Source Model With Different Ambient Conditions

C. Kondo, Leigh Orf, E. Savory
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Abstract

A downburst is a natural phenomenon that occurs during thunderstorms, creating hazards and damage to infrastructure, notably electricity transmission lines, due to the strong winds produced and the fact that those winds are, laterally, highly correlated compared to everyday synoptic winds. Downbursts are characterized by divergent and generally straight-line winds near the ground, which are naturally observed when several trees in a forest are pushed in one direction. A downburst may be modelled experimentally and also numerically using computational fluid dynamics. One of the experimental methods is the liquid release approach, which consists of releasing a volume of fluid that is slightly denser than the ambient fluid contained in a tank or flume, creating a downdraft that descends to the ground surface. Previous researchers used a solution of water and glycerol as the ambient fluid and an aqueous solution of potassium dihydrogen phosphate solution as the heavier fluid. They proposed length, time, and velocity scaling parameters, R 0 , T 0 , and V 0 , respectively, that allowed comparison of liquid release experiments carried out with different fluid densities, as well as with real events in nature. In addition, these scaling parameters allow comparison of experimental liquid release model data with numerical simulation results. The present project uses these parameters to scale numerical model data obtained by the simulation of a full-scale downburst using a cooling source model. The resulting non-dimensional velocity profiles have similar characteristics to the scaled non-dimensional profiles observed in full-scale field downburst measurements, and similar profiles to data from density-driven downburst experiments. Furthermore, the scaling preserved the vertical location of the near-ground peak wind speed, which is of great interest to wind and structural engineers. These results demonstrated that the scaling parameters, R 0 , T 0 , and V 0 , work well for cooling source downburst simulations. The time scaling parameter T 0 is determined by using the density percentage difference Dr / r obtained from the spatial average of densities within the thunderstorm cloud. This project also examines the influence of the ambient lapse rate on downburst events. Results showed that downbursts are affected by the ambient lapse rate, producing weak peak wind speed when the ambient lapse rate is lower than the dry adiabatic lapse rate, 9.8 K/km. Also, there is a delay in the evolution of the outflow when the ambient lapse rate is lower than the dry adiabatic lapse rate. However, the structure of the downflow and the outflow does not change.
不同环境条件下全尺寸冷却源模型模拟下爆流出的尺度
降暴是雷暴期间发生的一种自然现象,会对基础设施造成危害和破坏,尤其是输电线路,因为产生了强风,而且这些风在横向上与日常天气风高度相关。下爆的特点是在地面附近发散的、通常是直线的风,当森林中的几棵树被推向一个方向时,就会自然地观察到这种风。下爆可以用实验模拟,也可以用计算流体力学模拟。其中一种实验方法是液体释放法,它包括释放体积略大于水箱或水槽中所含环境流体的液体,产生下降气流,下降到地面。以前的研究人员使用水和甘油的溶液作为环境流体,磷酸二氢钾溶液的水溶液作为较重的流体。他们分别提出了长度、时间和速度缩放参数r0、t0和v0,以便比较不同流体密度下进行的液体释放实验,以及与自然界中的真实事件。此外,这些尺度参数允许将实验液体释放模型数据与数值模拟结果进行比较。本项目使用这些参数对使用冷却源模型模拟全尺寸下爆所获得的数值模型数据进行缩放。由此得到的无量纲速度剖面与在全尺寸野外降爆测量中观察到的缩放无量纲剖面具有相似的特征,也与密度驱动降爆实验数据相似。此外,该尺度保留了近地面峰值风速的垂直位置,这是风和结构工程师非常感兴趣的。这些结果表明,r0、t0和v0这三个尺度参数可以很好地用于冷却源下突模拟。时间尺度参数t0由雷暴云内密度的空间平均得到的密度百分比差Dr / r确定。本项目还研究了环境递减率对降暴事件的影响。结果表明:降暴受环境递减率的影响,当环境递减率低于干绝热递减率9.8 K/km时,降暴产生弱峰值风速;此外,当环境递减率低于干绝热递减率时,流出流的演化有延迟。然而,下行和流出的结构没有改变。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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