Understanding Particle Scale Motion Initiation Physics for Loose-laid Building Rooftop Aggregates in Severe Windstorms

松散集料通常用于建筑屋顶，既可以作为道碴，也可以作为越来越受欢迎的绿色屋顶系统的一部分。然而，在飓风、龙卷风和雷雨等严重风暴期间，这些单独的石头可能会飞到空中。一旦悬浮在空气中，这些颗粒物会在顺风向损坏建筑物，并破坏拆除它们的屋顶的稳定性。人们对风是如何将单个颗粒吹起的确切机制知之甚少，特别是在建筑物周围和上方复杂的风流中。这项研究将把最先进的、大规模的风洞测试与详细的计算模型结合起来，以确定作用于单个颗粒的气动力。这项研究产生的知识将使工程师能够设计出极不容易聚合剥落的建筑屋顶系统，从而在严重风暴期间创造更安全的建筑和环境。基于研究结果的学习模块将被开发用于本科生和研究生的流体力学课程。这项研究还将被用作克莱姆森大学EMAG！NE项目的平台，让高中科学和数学专业的学生参与与风暴和其他自然灾害有关的外联活动。研究团队将包括一群不同的研究生和本科生，他们与南卡罗来纳州和佛罗里达州的教职员工一起工作。该项目的数据将被存档，并在国家科学基金会支持的自然灾害工程研究基础设施(NHERI)数据仓库(https://www.DesignSafe-ci.org/).)中公开提供这项研究将首次阐明在非均匀、高度分离的湍流中松散的粒子运动起始的物理学。大型和全尺寸风洞实验、大涡模拟(LES)计算建模和使用物理试验的模型验证相结合，将产生对粒子运动起始机理的全面理解。该项目将使用美国国家科学基金会支持的位于佛罗里达国际大学的NHERI风墙设施进行实验。实验和模拟将得到屋顶表面上的表面剪应力分布及其与压力场的关联数据。检验颗粒尺度载荷和开发基于物理的运动起始标准的研究方法可以改变对其他冲刷问题的理解，并标志着与假定均匀流动的模型的重大偏离。这项研究将从假设剪切力和水平气动阻力为运动起始过程的主导因素的冲刷经验方法中转移出来。通过模拟单独的物理过程，当耦合时，导致颗粒运动，这项研究可以扩展到非定常分离流中的其他侵蚀问题。关于女儿墙的缓解研究将产生一个关于将女儿墙放置在平顶上所引起的负荷变化的综合数据集。结合LES模型，这项研究将提高对护墙在建筑空气动力学中的作用的理解，并可以导致更安全的建筑设计。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Loose-laid aggregate is commonly used on building rooftops either as ballast or as part of increasingly popular green roof systems. However, the individual stones can become airborne during severe storms such as hurricanes, tornadoes, and thunderstorms. Once airborne, the particles can damage buildings downwind and destabilize the roof from which they are removed. The mechanics of exactly how the wind lifts up individual particles is poorly understood, particularly in the complex wind flow around and over a building. This research will combine state-of-the-art, large-scale wind tunnel testing with detailed computational modeling to establish the aerodynamic forces acting on individual particles as they become airborne. The knowledge generated from this research will enable engineers to design building roof systems that are significantly less prone to aggregate blow-off, thus creating safer buildings and surroundings during severe storms. Learning modules based on the research findings will be developed for undergraduate and graduate fluid mechanics courses. The research also will be used as a platform in Clemson University's EMAG!NE program to engage high school science and math students in outreach activities related to windstorms and other natural hazards. The research team will include a diverse group of graduate and undergraduate students working with faculty in South Carolina and Florida. Data from this project will be archived and made publicly available in the NSF-supported Natural Hazards Engineering Research Infrastructure (NHERI) Data Depot (https://www.DesignSafe-ci.org/). This research will, for the first time, elucidate the physics of loose-laid particle motion initiation in a non-uniform, highly separated turbulent flow. The combination of large- and full-scale wind tunnel experimentation, Large Eddy Simulation (LES) computational modeling, and model validation using physical tests will produce a comprehensive understanding of the mechanics of particle motion initiation. This project will use the NSF-supported NHERI Wall of Wind facility at Florida International University for experimentation. The experiments and simulations will produce data on the surface shear stress distribution over a roof surface and its correlation with the pressure field. The research approach of examining particle scale loading and developing physics-based criteria for motion initiation can transform the understanding of other scour problems and mark a significant departure from models that assume uniform flow. This research will move away from empirical approaches to scour that assume shear stress and horizontal aerodynamic drag as the dominant factors for the motion initiation process. By modeling the individual physical processes that, when coupled, result in particle motion, the research can be extended to other erosion problems in unsteady separated flows. The mitigation study on parapets will result in a comprehensive data set on the load changes that result from placing parapets above flat roofs. Combined with the LES modeling, this research will improve the understanding of the role of parapets in building aerodynamics and can lead to safer building designs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.