Vortex dynamics and radial outflow velocity evolution in downburst-like winds

Downbursts can cause severe winds near ground level, potentially damaging buildings and structures. A particular problem is that downburst-induced wind action is not considered in the design stage as it is not included in the building codes. This paper provides an in-depth characterization of a downburst flow field including its vortical structures in both space and time. The analysis is based on Large Eddy Simulations (LES) to reproduce dedicated experiments of a vertical downburst carried out in the test chamber of the WindEEE Dome laboratory. The trajectory of the radial velocity maxima is evaluated, which indicates that the height of the maximum velocity increases with the traveled distance after having produced the strongest wind gusts. The spatial evolution of the convective velocity of the primary vortex across the test chamber is evaluated and three regions are distinguished: the speed-up (up to r/D = 1.25), the slow-down (1.25 < r/D < 2.29) and the deflection region (r/D > 2.29). The analysis indicates that trailing ring vortices produce higher outflow velocities than the primary vortex after a sufficient time span, causing the radial locations between 0.8 and 1.8 r/D to be continuously exposed to strong gusts.