Investigation on the velocity field near the ground during downburst impingement

作为下爆的实验室模型，研究了垂直重力流湍流速度场的统计。通过机械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，形成垂直热，撞击水平地面并径向向外发散。为了保持较高的再现性，采用PIV对100个样品进行了集合平均。得到了子午平面上速度矢量、方位涡度和湍流应力的集合平均图。在此基础上，分析了下降气流阶段、撞击阶段和辐散阶段的统计特征。论证了循环流动的性质和作用。结果表明，伴随的循环流动不是高密度包裹边界切变层卷起的结果，而是纯粹的无粘性流动，并被希尔涡理论很好地描述。结果与大气中观测到的实际降暴有较好的一致性。基于这些结果，对飞机遭遇下爆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.