Study of Load Effects on Structures Induced by Gust-Fronts

建筑环境经历了一系列破坏性大气事件产生的自然力，包括与雷暴有关的下沉气流引起的阵风锋面。准确预测结构与这些极端事件的相互作用所产生的荷载效应是设计和建造建成环境以及保护其居住者的关键部分。这个项目的主要目标是更好地了解和量化风荷载及其在瞬时风事件(例如阵风锋面)下对建筑物的影响。为了实现这一目标，提出了一种系统的方法，重点是在瞬变流场模拟器中捕捉与阵风锋面相关的瞬变流动的显著特征以及伴随的荷载效应对几何尺度的一般建筑形状模型的影响。通过对美国和世界各地的雷暴数据库的分析，可以很容易地推测出瞬变风事件的重要性及其对负荷的影响，这些数据库表明，这些风代表了许多地点的设计风速。因此，这些风速成为在传统边界层风洞中验证的编码化设计程序的输入。然而，与对流阵风锋相关的阵风机制在运动学和动力学上都与传统湍流有很大的不同，例如它们的对比速度分布和瞬变性质。因此，人们应该质疑在规范和标准中使用基于传统分析框架的设计是否合适，因为它们通常以相同的方式处理这些根本不同的现象。此外，结构上的极端荷载可能对瞬变流的影响很敏感，即荷载系数可能会根据阵风形式和由此产生的周围流动的局部快速变化而增加。此外，这些流动结构的特征表明，与常规流动相比，所产生的负荷效应可能在更大的区域内相关。这些空气动力学结果清楚地表明，结构上的载荷可能比目前的规范和标准所预测的更高，因此需要对这些程序进行仔细的检查。以上强调的问题是研究本提案中阵风锋面的主要动机。这些调查将首先从雷暴外流期间的地面风数据中收集信息，以补充现有的知识库。然后，将在由单独控制的风扇组组成的瞬变流模拟器中模拟已识别的瞬变流场的显著特征，例如均匀加速流动。对暴露在这些气流中的按比例建筑模型获得的实验数据的分析将加强理解，并有助于量化由此产生的空气动力修改。通过将数值模拟的结构荷载效应与阵风锋时频分析和建模以及压力分布和荷载的测量相结合，这项研究将能够检验广泛使用的设计规范和标准的有效性，并强调可能需要进行修改。此外，与日本两个著名的空气动力学/风工程中心的合作将提供一系列令人印象深刻的经验，并提供独特的实验设施，以验证本研究产生的流动模拟和随之而来的负载。在美国，与下击暴发和龙卷风相关的阵风锋面每年造成约80人死亡，1500人受伤，财产损失约10亿美元。这一统计数据正在迅速升级。基于更好地了解阵风锋面对建筑物的影响而改进的规范肯定会减少这种危险，从而使整个社会受益。因此，这项工作的更广泛影响将通过减少这些损失来衡量，包括财产损失，更重要的是生命损失。同时，该项目的其他主要举措，即职业发展、教育举措等，有望在提高公众对全国各地美国人日常生活中常见现象的认识和了解方面产生持久的影响。流动模拟器将成为这方面的公共教育的一个有吸引力的示范工具。此外，参与拟议研究的学生将继续体现过去研究团队在种族和性别方面的多元化形象，热情地保持大学前外展计划的既定传统，以中小学生为目标，并让代表性不足的群体参与。最终，这项拟议的研究承诺为建筑和民用基础设施的防风设计提供坚实的智力基础，并得到受过良好培训的研究生、本科生和大学预科学生的支持。

The built environment experiences forces of nature generated by a host of damaging atmospheric events, including gust-fronts induced by downdrafts associated with thunderstorms. An accurate prediction of load effects that result from the interaction of structures with these extreme events is a critical part of designing and constructing the built environment and protecting its occupants. The primary goal of this project is to better understand and quantify wind loads and their effects on buildings under transient wind events, e.g., gust-fronts. To accomplish this goal, a systematic approach is proposed that focuses on capturing salient characteristics of transient flows associated with gust-fronts and attendant load effects on geometrically-scaled models of generic building shapes in a transient flow field simulator. The significance of transient wind events and their load effects can be readily surmised from an analysis of thunderstorm databases both in the U.S. and around the world, which suggest that these winds represent the design wind speed for many locations. As a result, these wind speeds become the input to codified design procedures validated in traditional boundary layer wind tunnels. However, the mechanics of gusts associated with convective gust-fronts differ significantly from conventional turbulence both in its kinematics and dynamics, e.g. their contrasting velocity profile and their transient nature. Accordingly, one should question the appropriateness of using design based on conventional analysis frameworks in codes and standards, which generically treat these fundamentally different phenomena in the same manner. Further, extreme loads on structures are potentially sensitive to the influence of transient flows, i.e., the load coefficients may be enhanced based on the gust form and the resulting localized, rapid changes in the surrounding flow. In addition, the characteristics of these flow structures suggest that the resultant load effects are likely to be correlated over larger areas than in conventional flows. These aerodynamic consequences clearly point at the prospect of higher loads on structures than would be predicted by the present codes and standards, calling for a careful examination of these procedures. The issues highlighted above serve as the primary motivation to study gust-fronts in this proposal. These investigations will initiate by gleaning information from surface wind data available during thunderstorm outflows to complement the existing knowledge base. The identified salient features of transient flow fields, e.g., uniformly accelerating flows, will then be simulated in a transient flow simulator comprised of individually controlled fan banks. The analysis of experimental data garnered from scaled building models exposed to these flows will enhance understanding and facilitate quantification of the resulting aerodynamic modifications. Upon combining numerically simulated structural load effects with the time-frequency analysis and modeling of gust-fronts and the measurements of pressure distributions and loads, the study will be capable of examining the efficacy of widely used design codes and standards, highlighting the possible need for modifications. Further, the collaboration with two distinguished aerodynamic/wind engineering centers in Japan will make available an impressive array of experience and provide access to unique experimental facilities to validate flow simulations and attendant loads resulting from this study. Gust-fronts associated with downbursts and tornadoes in the U.S. result in estimated 80 deaths, 1500 injuries, and property damage in the neighborhood of a billion dollars each year . a statistic that is rapidly escalating. Improved codes that are based on better understanding of the impact of gust-fronts on structures certainly promise to mitigate this hazard, thus benefiting society at large. Therefore, the broader impacts of this work will be measured in the reduction of these losses, both in property and, more importantly, in lives. Concurrently other major initiatives of this project, i.e., professional development, educational initiatives, etc., promise to have lasting impacts in raising public awareness and understanding of a phenomenon commonly experienced in the daily lives of Americans throughout this country. The flow simulator will be an attractive demonstration tool for public education in this regard. Further, the students involved in the proposed research will continue to embody the ethnically and gender diversified profile of the past research teams, enthusiastically maintaining the established traditions of pre-college outreach programs, targeted at primary and secondary level students, and involving underrepresented groups. Ultimately, the proposed study promises to provide a strong intellectual foundation, supported by well-trained students at graduate, undergraduate and pre-college levels for wind resistant design of buildings and civil infrastructure.