Unsteady wind effects on natural ventilation

近年来，在非住宅建筑物中，自然通风作为机械通风的理想替代方案越来越被接受。这不仅源于降低成本（资本和维护）的潜力，还源于环境效益（减少二氧化碳排放;减少病态建筑综合症的发生率）。随着这一趋势的出现，设计程序也在逐步改进，但在某些领域，这些程序仍然有所欠缺。本建议所涉及的一个领域，即不稳定风的影响。风的存在导致开口周围的速度场和压力场的不稳定性。先前的研究表明，这可能对通风口的流动特性产生两个主要影响。一个影响是开口的流量系数的减小。流量系数是开口通过流量的能力的量度，并且是设计计算中的主要因素。目前的设计程序利用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使系数减小两倍或更多，而对于另一种类型，影响可以忽略不计。目前的理解是，该效果主要取决于开口的形状、通过开口的流动方向以及开口周围的外部速度场。如果这是正确的，那么就有可能在设计过程中考虑这种影响，基本上是通过应用量纲分析的基本概念。因此，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，以简洁和简单的形式表达结果，以便纳入设计程序。第二个重要的风效应是通风烟囱（烟囱）中的流动逆转。烟囱越来越多地用于非住宅建筑，因为原则上，它们允许设计师在所有天气条件下固定空气进入和离开的流动模式。当发生流动逆转时，基本设计受到损害，因此期望设计堆叠/建筑物配置，使得避免流动逆转。在这方面做的工作很少，但申请人以前的研究已经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。研究的另一个目的是看看这是否可以应用于更现实的配置。拟议研究的基础是（a）实验技术和（B）理论模型，两者都是申请人根据EPSRC以前的合同开发的。实验技术是特别有趣的，因为它允许在堆栈中的瞬时流量进行测量，这是不可能的。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）采用。理论模型很重要，因为它允许在风洞中进行不可能的测试。然而，风洞可以用来评估理论模型，这是研究的第三个目的。本提案将使与东京理工大学的合作得以继续。这将是一个整体的好处，因为虽然他们的目标是相似的，他们集中在夏季通风的大开口，这需要一个不同的理论方法。诺丁汉大学的工作是关注更小的开口，理论模型是专门为此开发的。

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.