UNSTEADY VERTICAL WIND FORCES APPLIED ON A LARGE ROOF

通常结构设计主要考虑水平风荷载。然而，当结构位于丘陵地区或具有大跨度屋盖时，必须考虑瞬时垂直风力。这种垂直风力被认为是由屋顶上产生的涡流引起的。本文通过流场显示和风洞实验研究了在斜入射条件下，在立方体和正方形屋顶模型上，利用乙醇溶液的雾化雾进行了流场显示和实验研究，旨在揭示这些涡的形成机理。用激光片和闪光灯观察了屋顶上的流态。在方屋顶模型的屋顶上，观察到一对锥形涡，这是稳定的类似于三角翼上产生的。另一方面，在立方体模型的屋顶上， 关于我们 锥形涡存在，这改变了侧几乎交替。为了研究这种现象产生的原因，进行了风洞实验，测量了模型顶部和壁面上96个点的压力。通过本征正交分解（POD）分析所获得的压力的时间序列。POD是从随机波动的压力的多个时间记录中发现系统结构的有效方法。作为具有平坦水平屋顶的模型的情况的结果，最显著的模式如下。屋顶一侧的压力波动方向与另一侧相反。另一方面，一侧壁面上的压力波动方向与顶板同侧和另一侧壁面上的压力波动方向相反。流场显示和POD分析结果表明，锥涡产生于驻点所在壁面的对侧，从而阐明了顶面锥涡的切换与壁面驻点的位置密切相关。少

Generally the horizontal wind force is mainly considered for the design of structures. However, in the case that the structure is located at a hilly country or has a large-span roof, the instantaneous vertical wind force must be considered. This vertical wind force is considered to be induced by vortices generated on a roof. The present study is aimed to make clear the mechanism of these vortices by means of the flow visualizations and the wind tunnel experiments concerned with the pressure applied on the roof and the wall of the model.The flow visualizations with the cubic and the square roof models were carried out when the incident flow is for oblique angle, using the mist vaporized of the alcohol solution. The flow patterns on the roof was observed with laser sheets and a strobe light. On the roof of the square roof model, a pair of conical vortices is observed which are steady similar to those generated on the delta wing. On the other hand, on the roof of the cubic model only one … More conical vortex exists and this changed the side almost alternately. In the following this phenomenon is called the switching.In order to investigate the cause why switching is induced, the wind tunnel experiments were carried out, which pressures applied on 96 points on the roof and the wall of the model were measured. The obtained time series of pressures were analyzed by Proper Orthogonal Decomposition (POD). POD is the available technique to find the systematic structure from many time records of the pressure which fluctuates randomly. As a result of the case of the model with flat horizontal roof, the most distinguished mode was as follows. The pressures on one side of the roof fluctuate in the opposite direction to of the other side. On the other hand, the pressures on one side wall fluctuate in the opposite direction to the pressures of same side of the roof and the other side wall. These results of the flow visualization and POD analysis means that the conical vortex generated on the opposite side to the wall which the stagnation point exists on. Consequently it is clarified that the switching of conical vortex on the roof is closely related with the position of the stagnation point on the wall. Less