UNSTEADY VERTICAL WIND FORCES APPLIED ON A LARGE ROOF

一般在结构设计中主要考虑水平风力。然而，当结构位于丘陵地区或屋顶跨度较大时，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。本研究旨在通过流动可视化和风洞实验来阐明这些涡的形成机制，这些实验涉及到模型顶部和壁面施加的压力。利用酒精溶液汽化后的薄雾，对斜角度入射流进行了立方顶模型和方顶模型的流动可视化。用激光片和频闪灯观察屋顶上的流动模式。在方形顶板模型的顶板上，观察到一对稳定的锥形涡，与三角翼上产生的涡相似。另一方面，在立方模型的顶部只有一个…更多的锥形涡存在，这几乎交替地改变了侧面。在下文中，这种现象称为切换。为了研究引起开关的原因，进行了风洞试验，测量了模型顶板和壁面上96个点的压力。对得到的压力时间序列进行正交分解（POD）分析。POD是一种从随机波动的压力的大量时间记录中找出系统结构的有效方法。以水平平顶模型为例，最显著的模型为：屋顶一侧的压力波动方向与另一侧相反。另一方面，一侧墙壁上的压力与屋顶同一侧和另一侧墙壁上的压力呈相反方向波动。这些流动显示和POD分析的结果表明，锥形涡在驻点存在的壁面对面产生。结果表明，顶部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