Investigation on the velocity field near the ground during downburst impingement

作为下击暴流的一个实验室模型，研究了垂直重力流湍流速度场的统计特性。通过机械地破坏固定在圆柱形容器底部的薄膜，有限质量的高密度液体开始落入静止的低密度液体中，形成垂直的热冲击，冲击到水平地面上并径向向外发散。注意保持高再现性，使用PIV对百个样品进行整体平均。得到了子午面内速度矢量、方位涡度和湍流应力的整体平均图。在此基础上，分析了下降段、撞击段和扩散段的统计特性。循环流的性质和作用被证明。结果表明，相关的循环流是不是在高密度块的边界处的剪切层卷起的结果，但它是纯粹的无粘性和希尔的涡理论很好地描述。计算结果与大气中观测到的真实的下击暴流有较好的一致性。根据这些结果，飞机遇到下击暴流的风险程度进行了评估。

As a laboratory model of downburst, statistics of turbulent velocity field of a vertical gravitational flow is investigated. By mechanically breaking a thin film fixed at the bottom of a cylindrical container, a finite mass of a high-density liquid begins to fall into a stationary low-density liquid, forming a vertical thermal, impinging onto a horizontal ground and diverging radially outward. Taking care of keeping high reproducibility, the ensemble average was performed over hundred samples employing PIV. The ensemble-averaged maps of velocity vectors, azimuthal vorticity and turbulent stresses in a meridian plane are obtained. Based on them, the statistical characteristics in the downdraft stage, impinging stage and diverging stage are examined. The nature and the role of the circulatory flow are demonstrated. It was revealed that the associated circulatory flow is not the result of rolling-up of the shear layer at the boundary of the high density parcel, but it is purely inviscid and well described by Hill's vortex theory. The results shows reasonable agreement with the real downburst observed in the atmosphere. Based on these results, the degree of risk for aircraft encountering a downburst is evaluated.