随着全球气候变暖，以下击暴流等为代表的强对流天气逐渐增多，使得对其的研究成为目前国际风工程领域的热点问题之一。下击暴流是由雷暴天气中形成的强下沉气流冲击地面后，在地面加速扩展开的一种气流过程，由于下击暴流风剖面与大气边界层风剖面差异巨大，并且在距离地面很低处会产生强风荷载，往往会导致结构破坏。我国荷载规范只给出了大气边界层风荷载，对下击暴流的这类极端风并没有考虑在内。所以对下击暴流的研究就显得至关重要。
本文基于冲击射流模型建立下击暴流三维模型，通过CFD数值方法对下击暴流的风场进行了定常数值模拟，对下击暴流的风场特征进行了重点分析。下击暴流径向风速在近地面附近达到最大值，之后随着高度增大而迅速减小，径向风速剖面与大气边界层剖面有着显著的差异。在下击暴流射流管下方距离喷射中心半径为1D的圆内都存在着正压，并且中心处压力系数接近1。不同射流速度对无量纲化的径向风速剖面影响不大，不同的射流高度H对无量纲化的径向风速剖面有着一定的影响。下击暴流的边界层的发展是呈非线性变化的。
不同径向位置r处，下击暴流风剖面不同，建筑承受风荷载作用亦不同。利用CFD方法对不同径向位置处的高层建筑模型进行了风荷载特征分析。分别将模型置于距离下击暴流核心r=0D，r=1D，r=1.5D，r=2D四个不同位置，利用SST k-ω湍流模型进行定常数值模拟。不同径向位置处模型周围流场分析：在r=0D位置处，可以看到下沉的气流垂直撞击在高层建筑模型顶面，气流撞击顶面后向四周散开，并且在模型四个侧面周围气流形成一个封闭的汽缸。在r=1D，r=1.5D，r=2D位置处，在模型迎风面下部都存在一个气流驻点，在模型的侧面中上部，流动在侧面前端发生分离，在后端又发生了再附现象，而在模型下部没有发生再附现象。
不同径向位置处模型表面风压系数分布分析：模型位于r=0D位置处时，模型各个表面均承受较大的压力，此位置处，建筑结构设计时应有着足够的抗压强度。在r=1D，r=1.5D，r=2D位置处，模型迎风面风压系数随着径向位置r的增大，峰值风压系数变小，并且在峰值风压系数附近区域风压系数变得不再饱满。在侧面与背面主要产生吸力，侧面上部，前端吸力大于后端。总体来看，径向位置对侧面、背面压力系数分布影响较小。

With the aggravation of the global warming，the downburst as the representative of the strong convective weather gradually increased, making its research become one of the hottest issues in the field of international wind engineering. The downburst is a kind of airflow process that is developed by the strong sinking airflow in the thunderstorm that impacts the ground and accelerates. As the profiles of downburst and atmospheric boundary layer are different, and the downburst will produce strong wind load at a very low distance from the ground, it often brings damages to the structures. In Chinese Load Code for the design of building structures, the design wind load is based on ordinary atmospheric boundary layer wind field, downburst is not taken into ac. So the study on the downburst is extremely important.
In this paper, a three-dimensional model of the downburst is established by using the impinging jet model. The steady numerical simulation of the wind field of the downburst is carried out by CFD numerical method. The focus of downburst wind field was analyzed. The downburst wind speed reaches the maximum near the ground, and then decreases rapidly as the height increases, leading to huge difference from atmospheric boundary layer. There is a positive pressure within the circle with a radius of 1D on the wall below the torrent jet tube, and the pressure coefficient at the center is close to 1. The different jet velocities have little effect on the dimensionless wind speed profile, and the different jet height H has a certain influence on the dimensionless wind speed profile. The development of the boundary layer of the downburst is a non-linear change.
The downburst wind profiles are different in different radial positions, bringing different wind loads on the high-rise building model. The CFD method was used to analyze the wind loads in different radial locations. The SST k-ω turbulence model was used to simulate the steady-state numerical simulations, the model was placed in different radial positions, respectively, r = 0D, r = 1D, r = 1.5D, r = 2D. Analysis of flow field around model in different radial locations: In the radial position r = 0D, we can see that the sinking airflow collides vertically on the top of the structure, after striking the top surface the air flow spreads around it, and the air flow around the four sides of the model forms a closed cylinder. In the radial positions r = 1D, r = 1.5D, r = 2D , there is an airflow stagnation point in the lower part of the windward surface of the model. In the upper part of the side surface of the model, the flow separates at the front edge then attaches at the rear end. In the lower part of the side surface of the model, the flow separates at the front edge but does not attach at the rear end.
Analysis of wind pressure coefficient distribution on model surfaces in different radial locations: When the model is located at r = 0D, all surfaces of the model are subjected to greater positive pressure. In this position, the building structure should be designed with sufficient compressive strength. In the radial positions r = 1D, r = 1.5D, r = 2D, with the increase of radial position r, the peak wind pressure coefficient becomes smaller, and the wind pressure coefficient around this area becomes no longer full. The flow mainly produce suction at the side and back surfaces of the model, and in the upper part of the side surface, the front end suction is greater than the back end. In general, the radial positions have little effect on the pressure coefficient distribution at side surface and back surface.

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