3-Dimensional Fluid Flow Analysis for Aerodynamic Response Characteristics of Rectangular Cylinder

以平面箱梁桥为例，用数值流体流动分析来表达分离剪切层前缘分离并重新附着于构件后表面的矩形截面构件的精确气动特性和流动状态时，需要在一定雷诺数范围内进行三维分析。主要结果如下：(1)三维分析有助于定性地理解风洞试验结果所反映的矩形构件的气动特性和绕流状态。在2.5D ~ 3.0D范围内，在前缘分离的分离剪切层重新附着。这种现象不能被二维分析捕捉到。(2)除矩形截面构件前端面外，平均压力系数在二维分析和三维分析中表现出完全不同的分布。在二维分析中观察到较大的波动，这是造成二维分析中气动力估计过高或过低的因素之一。(3)在二维分析中，在前缘分离的分离剪切层在比风洞试验显示的再附着点更早的位置重新附着。此外，由于形成的涡流强度越大，表面压力波动也越大。这被认为是在波动升力中产生高频分量的原因之一。轴向波动流不能用二维分析来表示的原因是不能正确地计算能量耗散。

It has become clear that 3-dimensional analysis is required in a certain Reynolds number range, when expressing, by numerical fluid flow analysis, the accurate aerodynamic characteristics and the state of flow around a rectangular sectioned member in which the separation shear layer separates at the front edge and reattaches to the rear surface of the member, as represented by a flat surfaced box-girder bridge. The main results are the following:(1) The 3-dimensional analysis facilitates qualitative understanding of the aerodynamic characteristics and the state of flow around a rectangular member indicated by the results of wind tunnel tests. Moreover, the separation shear layer which separated at the front edge reattaches in the range between 2.5D and 3.0D. This phenomenon cannot be caught by 2-dimensional analysis.(2) Except for the front face of the rectangular sectioned member, the average pressure coefficient shows a totally different distribution between the 2-dimensional analysis and 3- dimensional analyses. Large fluctuations are observed in the 2-dimensional analysis, and this is one of the factors in the overestimation or underestimation of aerodynamic forces in 2-dimensional analysis.(3) In 2-dimensional analysis, the separation shear layer, which separated at the front edge, reattaches in a position much earlier than the point of reattachment indicated by the wind tunnel tests. Moreover, the pressure fluctuations on the surface are greater because of the greater strength of the vortex formed. This is believed to be one of the causes which produce high-frequency components in the fluctuating lift. The reason fluctuating flow in axial direction cannot be expressed by 2-dimensional analysis is the impossibility of correctly evaluating energy dissipation.