Investigation on the velocity field near the ground during downburst impingement

作为下击暴流的实验室模型，研究了垂直重力流湍流速度场的统计特性。通过机械破碎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，形成垂直热流，撞击到水平地面上，径向向外扩散。为了保持较高的重复性，使用PIV对100多个样本进行了总体平均。得到了子午线面上速度矢量、方位向涡度和湍流应力的集合平均图。在此基础上，分析了下沉风段、撞击段和发散段的统计特性。说明了循环流动的本质和作用。结果表明，伴生环流不是高密度地块边界剪切层卷起的结果，而是纯无粘性的，符合Hill涡旋理论。计算结果与大气中观测到的真实下击暴流有较好的一致性。在此基础上，对飞机遭遇下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.