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

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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.