- The role of wind tunnels
7.1 Introduction
Most practising structural engineers will not themselves operate wind tunnels, but they may be clients of wind tunnel groups who will provide wind loading information for new or existing structures, usually by means of model tests. For this reason, this chapter will not attempt to describe wind tunnel techniques in detail. There are detailed references, guide books and manuals of practice available which perform this function (e.g. Cermak, 1977; Reinhold, 1982; Australasian Wind Engineering Society, 2001; American Society of Civil Engineers, 1999). However sufficient detail is given here to enable the educated client to be able to lsquo;ask the right questionsrsquo; of their wind tunnel contractors.
In the following sections, a brief description of wind tunnel layouts is given, and methods of simulation of natural wind flow and experimental measurement techniques are discussed.
7.2 Wind tunnel layouts
7.2.1 Historical
The first use of a wind tunnel to measure wind forces on buildings is believed to have been made by Kernot in Melbourne, Australia (1893). A sketch of the apparatus, which he called a lsquo;blowing machinersquo;, is given in Figure 7.1 (Aynsley et al., 1977). This would now be described as an lsquo;open-circuit, open test-sectionrsquo; arrangement. With this equipment, Kernot studied wind forces on a variety of bluff bodies – cubes, pyramids, cylinders, etc., and on roofs of various pitches.
Figure 7.1 Sketch of W. C. Kernotrsquo;s lsquo;blowing machinersquo; of 1893 (Aynsley et al., 1977).
140 The role of wind tunnels
Figure 7.2 Layout of an open-circuit wind tunnel.
At about the same time, Irminger (1894) in Copenhagen, Denmark used the flow in a flue of a chimney to study wind pressures on some basic shapes (Larose and Franck, 1997).
Wind tunnels for aeronautical applications developed rapidly during the first half of the twentieth century, especially during and between the two world wars. The two basic wind tunnel layouts: the open circuit, or lsquo;N.P.L. (National Physical Laboratory) typersquo;, and the closed circuit, or lsquo;Gouml;ttingen-typersquo; were developed during this period, named after the research establishments in the U.K. and Germany where they originated. These two types are outlined in the following sections.
7.2.2 Open-circuit type
The simplest type of wind tunnel layout is the open-circuit or N.P.L. type. The main components are shown in Figure 7.2. The contraction, usually with a flow straightener, and fine mesh screens, has the function of smoothing out mean flow variations, and reduc-ing turbulence in the test section. For modelling atmospheric boundary layer flows, which are themselves very turbulent, as described in Chapter 3, it is not essential to include a contraction, although it is better to start with a reasonably uniform and smooth flow before commencing to simulate atmospheric profiles and turbulence.
The function of the diffuser, shown in Figure 7.2, is to conserve power by reducing the amount of kinetic energy that is lost with the discharging air. Again this is not an essential item, but omission will be at the cost of higher electricity charges.
Figure 7.2 shows an arrangement with an axial-flow fan downstream of the test section. This arrangement is conducive to better flow, but since the function of the fan is to produce a pressure rise to overcome the losses in the wind tunnel, there will be a pressure drop across the walls and floor of the test section that can be a problem if leaks exist. An alternative is a lsquo;blowingrsquo; arrangement in which the test section is downstream of the fan (see Figure 7.5). Usually a centrifugal blower is used, and a contraction with screens is
Figure 7.3 The Counihan method for short test sections.
The role of wind tunnels 141
essential to eliminate the swirl downstream of the fan. However, in this arrangement the test section is at or near atmospheric pressure.
Both the arrangements described above have been used successfully in wind engineer-ing applications.
7.2.3 Closed-circuit type
In the closed circuit, or Gouml;ttingen-type, wind tunnel, the air is continually recirculated, instead of being expelled. The advantages of this arrangement are as follows:
It is generally less noisy than the open-circuit type
It is usually more efficient. Although the longer circuit gives higher frictional losses, there is no discharge of kinetic energy at exit
More than one test section with different characteristics can be incorporated.
However, this type of wind tunnel has a higher capital cost, and the air heats up over a long period of operation before reaching a steady-state temperature. This can be a prob-lem when operating temperature-sensitive instruments, such as hot-wire or other types of thermal anemometers, which use a cooling effect of the moving air for their operation.
7.3 Simulation of the natural wind flow
In this section, methods of simulation of strong wind characteristics in a wind tunnel are reviewed. Primarily, the simulation of the atmospheric boundary layer in gale, or large-scale synoptic conditions, is discussed. This type of large-scale storm is dominant in the temperate climates, for latitudes greater than about 40 degrees, as discussed in Chapter 1.
Even in large scale synoptic windstorms, flows over sufficiently long homogeneous fetch lengths, so that the boundary-layer is fully developed, are relatively uncommon. They will occur over open sea with consistent wave heights, and following large fetches of flat open country or desert terrain. Bu
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目 录
7.风洞的作用
7.1引言
大多数实践中的结构工程师不会自己操作风洞，但他们可能是风洞组的客户，他们通常会通过模型测试为新建或现有结构提供风荷载信息。 出于这个原因，本章不会详细描述风洞技术。 有详细的参考资料，指导手册和实践手册（例如Cermak，1977; Reinhold，1982; Australasian Wind Engineering Society，2001; American Society of Civil Engineers，1999）。 然而，为了让受过教育的客户能够“向他们的风洞承包商提出正确的问题”，这里给出了足够的细节。
在下面的章节中，给出了风洞布局的简要描述，并讨论了自然风流模拟方法和实验测量技术。
7.2风洞布局
7.2.1历史
Kernot在澳大利亚墨尔本（1893年）首次使用风洞测量建筑物的风力。 图7.1给出了他称之为“吹气机”的装置的草图（Aynsley等，1977）。 现在这将被描述为“开路，开放测试部分”安排。 有了这些设备，Kernot就研究了各种钝体 - 立方体，金字塔，圆柱体等以及各种沥青屋顶上的风力。
图7.1 1893年W. C.克诺的“吹制机”草图(Aynsley et al.， 1977)。
图7.2开路风洞布置图
大约在同一时间，丹麦哥本哈根的Irminger（1894）利用烟囱烟道中的流动来研究一些基本形状的风压（Larose and Franck，1997）。
在二十世纪上半叶期间，特别是在两次世界大战期间和之间，用于航空应用的风洞发展迅速。 两种基本的风洞布局：开路，或“N.P.L. （国家物理实验室）类型“，并在此期间建立了闭路或”戈廷根型“，以英国和德国的研究机构命名。 以下部分概述了这两种类型
7.2.2开路的类型
最简单的风洞布局是开路式或N.P.L型。主要部件如图7.2所示。收缩，通常具有流动矫直器和细筛网，具有平滑平均流量变化的功能，并减少试验段的紊流。为了模拟大气边界层流动，它们本身非常湍急，如第3章所述，不必包括收缩，尽管最好是在开始模拟大气廓线和湍流之前以合理均匀和平滑的流动开始。
扩散器的功能，如图7.2所示，是通过减少与排出空气失去的动能来节省功率的。再次，这不是一个必要的项目，但省略将以更高的电费为代价。
图7.2示出了在试验段下游的轴流风扇的布置。这种布置有利于更好的流动，但由于风扇的作用是产生压力上升，以克服风洞中的损失，在试验段的壁和地板上会有压降，如果存在泄漏，这可能是一个问题。另一种方案是“吹送”装置，其中测试部分位于风扇的下游（见图7.5）。通常使用离心式鼓风机，屏幕收缩是消除风机涡流的必要条件。然而，在这种布置中，测试部分处于或接近大气压力。上述两种布置都已成功地应用于风工程应用中。
图7.3简短测试部分的Counihan方法。
7.2.3闭路的类型
在封闭回路或Go-ttingen型风洞中，空气不断被再循环，而不是被排出。 这种安排的优点如下：
它通常比开路类型噪音小
它通常更高效。 虽然较长的回路会产生较高的摩擦损失，但出口处不会释放动能。
可以包含多个具有不同特性的测试部分。
然而，这种类型的风洞资本成本较高，并且在达到稳态温度之前，空气在长时间的运行中升温。 在使用温度敏感的仪器时，如热线或其他类型的热风速仪，这些仪器在运行时会使用移动空气的冷却效果，这可能是一个问题。
7.3模拟自然风流
在这一节中，回顾了风洞中强风特性的模拟方法。主要讨论大风边界层或大尺度天气条件的模拟。正如第1章所讨论的，这种大规模风暴在温带气候中占主导地位，纬度大于40度。
即使在大规模的天气风暴中，流过足够长的均匀提取长度，使边界层得到充分发展，也是相对不常见的。它们将在波浪高度一致的公海上发生，并且会在大片平坦的开阔乡村或沙漠地形之后出现。然而，暴露于这些条件下的建筑物或其他结构的数量很少。具有足够长度的均匀的均匀迎风粗糙度以产生边界层的充分发展的都市场地也相对少见。然而，在接近理想条件下进行了足够的测量，以产生用于工程目的的公认的强风大气边界层的半理论模型。这些模型已被有效地用作大气现象风洞模拟的基础，第3章讨论了这些要点。
对于建筑物，塔楼，桥梁等建筑物的风荷载和响应，这些模型充分描述了大型，成熟，超热带，洼地产生的风，它们形成风的基准，风隧道流量通常会被评估。然而，对于一些湍流特性，如长度尺度和光谱，在确定风力和动力响应方面有重要意义上的重大差异。在评估风洞试验的可靠性时，应将这些不确定性考虑为对真实结构的风效应的预测。
如第3章所述，这些模型对于局部热机制所产生的暴风来说也不是很好，即热带气旋（飓风，台风），雷暴（包括龙卷风）和季风。由这些风暴产生的风是距离赤道约40度范围内的纬度结构设计的主导风向。
下面的部分讨论了需要长测试段的自然生长方法，用于风洞的短测试段的方法，以及用于模拟大气边界层内部或表层的方法。最后讨论了热带气旋和雷暴条件下强风模拟的一些可能性。这些现象的实验室模拟还处于早期发展阶段，但关于这一课题的一些观点在
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