UNSTEADY VERTICAL WIND FORCES APPLIED ON A LARGE ROOF

一般情况下，结构设计主要考虑水平风荷载。然而，如果结构位于丘陵地带或屋面跨度较大，则必须考虑瞬时竖向风荷载。这种垂直风力被认为是由屋顶上产生的涡流引起的。本研究的目的是通过流动显示和模型顶部和壁面压力的风洞实验，对入射气流为斜角时的立方形和方形屋顶模型进行流动可视化，并使用酒精溶液汽化的雾化。用激光薄片和频闪灯观察了屋顶上的流态。在方形屋顶模型的屋顶上，观察到一对与三角翼上产生的锥形涡旋类似的稳定的锥形涡旋。另一方面，在立方模型的屋顶上只有一个…更多的锥形涡旋存在，这几乎交替地改变了侧面。为了研究这种现象产生的原因，进行了风洞实验，测量了96个点在模型顶板和墙体上的压力。对得到的压力时间序列进行了本征正交分解(POD)分析。Pod是从大量随机波动的压力时间记录中发现系统结构的有效技术。对于平顶平顶的情况，最明显的振型如下。屋顶一侧的压力与另一侧的压力波动方向相反。另一方面，侧墙压力的波动方向与屋面同侧和另一侧墙的压力方向相反。这些流动显示和POD分析的结果表明，锥形涡产生于驻点所在壁面的对面。从而阐明了屋面锥形涡的切换与墙面上驻点的位置密切相关。较少

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