Unsteady wind effects on natural ventilation

近年来，在非住宅建筑中，自然通风作为机械通风的理想替代方案已越来越为人们所接受。这不仅源于降低成本(资本和维护)的可能性，也源于环境效益(减少二氧化碳排放；减少病态建筑综合症的发生率)。这一趋势伴随着设计程序的逐步改进，但在某些领域，程序仍有些欠缺。这项提议涉及到一个这样的领域，即不稳定风的影响。风的存在导致洞口周围的速度场和压力场不稳定。先前的研究表明，这会对通风口的流动特性产生两个主要影响。其中一个影响是降低了开口的流量系数。流量系数是孔口通过水流的能力的量度，也是设计计算中的主要因素。目前的设计程序使用的是在静止空气条件下获得的系数。对于至少一种类型的开口，已经观察到风的存在将系数减小两倍或更多，而对于另一种类型，这种影响可以忽略不计。目前的理解是，这种影响主要取决于洞口的形状、通过洞口的流动方向以及洞口周围的外部速度场。如果这是正确的，那么基本上通过应用量纲分析的基本概念，就可以在设计过程中考虑到这种影响。因此，这项研究的目的之一是在广泛的变量范围内进行风洞试验，并应用量纲分析将结果以简洁简单的形式表达出来，以便包括在设计程序中。第二个重要的风效应是通风烟囱(烟囱)中的气流逆转。烟囱越来越多地被用于非住宅建筑，因为原则上，它们允许设计者在任何天气条件下固定空气进出的流动模式。当发生流量反转时，基本设计会受到影响，因此需要设计堆叠/建筑结构，以避免流量反转。在这方面做的工作很少，但申请人之前的研究确定了一个相对简单的参数(基于风压)，可以用来确定是否会发生气流逆转。这项研究的另一个目的是看看这是否可以应用到更现实的配置中。拟议研究的基础是(A)实验技术和(B)理论模型，这两个基础都是申请人根据以前的EPSRC合同开发的。这项实验技术特别有趣，因为它允许测量堆栈中的瞬时流量，这在以前是不可能的。这项技术引起了国际上的兴趣，并已被三所海外大学(日本、瑞典和巴西)使用。理论模型很重要，因为它允许进行在风洞中不可能进行的测试。然而，风洞可以用来评估理论模型，这是研究的第三个目标。目前的提议将使与东京理工大学的合作得以继续。这将是总体上的好处，因为尽管它们的目标相似，但它们集中在夏季通风的大开口上，这需要不同的理论方法。诺丁汉大学的这项工作关注的是较小的开口，为此专门开发了理论模型。

In recent years natural ventilation has become increasingly accepted as a desirable alternative to mechanical ventilation in non-domestic buildings. This stems not only from the potential for reduced costs (capital and maintenance), but also from environmental benefits (reduced CO2 emissions; reduced incidence of sick building syndrome). This trend has been accompanied by gradual improvements in design procedures, yet in some areas the procedures are still somewhat lacking. One such area is addressed by this proposal, namely the effects of the unsteady wind. The presence of wind leads to unsteadiness in the velocity and pressure fields around openings. Previous research has shown that this can have two major effects on the flow characteristics of ventilation openings. One effect is the reduction in the discharge coefficient of an opening. The discharge coefficient is a measure of the ability of the opening to pass flow and is a major factor in design calculations. Current design procedures make use of coefficients obtained under still-air conditions. For at least one type of opening the presence of wind has been observed to reduce the coefficient by a factor of two or more, whereas for another type the effect is negligible. The current understanding is that the effect depends primarily on the shape of the opening, the direction of flow through the opening and on the external velocity field around the opening. If this is correct it should be possible to take account of the effect in design procedures, basically by applying the basic concept of dimensional analysis. One aim of the research therefore is to carry out wind tunnel tests over a wide range of variables and to apply dimensional analysis to express the results in a concise and simple form for inclusion in design procedures. The second important wind effect is flow reversal in ventilation stacks (chimneys). Stacks are increasingly being used in non-domestic buildings, because in principle they allow the designer to fix the flow pattern of air entry and exit under all weather conditions. When flow reversal occurs the basic design is compromised, so it is desirable to design the stack/building configuration such that flow reversal is avoided. There has been very little work done in this area, but previous research by the applicant has identified a relatively simple parameter (based on wind pressures) that can be used to determine whether or not flow reversal will occur. Another aim of the research is to see whether this can be applied to more realistic configurations. The bases for the proposed research are (a) an experimental technique and (b) a theoretical model, both of which were developed by the applicant under previous EPSRC contracts. The experimental technique is particularly interesting, since it allows the instantaneous flow in a stack to be measured and this has not been possible before. The technique has attracted international interest and has been used by three overseas Universities (Japan, Sweden and Brazil). The theoretical model is important since it allows tests to be made that are not possible in a wind tunnel. The wind tunnel can however be used to assess the theoretical model and this is the third aim of the research. The present proposal will allow the collaboration with Tokyo Polytechnic University to continue. This will be of overall benefit, because although their objectives are similar, they have concentrated on large openings for summer ventilation, which requires a different theoretical approach. The work at Nottingham University is concerned with smaller openings, for which the theoretical model was specifically developed.