CAREER: Multiscale Firebrand Transport in the Turbulent Boundary Layer for Fire Science Advancement

在美国和世界各地，野火已经成为一种常见现象。当野火蔓延到社区时，其代价会被放大；通常称为荒地-城市接口（WUI）。在野火中，燃烧材料的碎片，被称为火种（或飞行余烬），可以产生，放飞，并被风带到主要火场前方几米到几公里的地方，并产生几个新的现场火灾。火灾后的研究表明，在WUIs中，火种是传播最快的，也是起火的主要原因。从余烬进入空中的那一刻起，火种的轨迹，也就是它的着陆点，就取决于湍流气流的特征。虽然湍流风的结构是由机械力和浮力决定的，但目前对这些因素如何影响发现风险还没有明确的基本认识。此外，湍流的小涡流在决定点火行为中起什么作用还不清楚，因为它们的影响在大多数基于物理的研究中被忽视了。该项目的主要目的是促进对湍流中火种传输现象的流体动力学理解，提高公众对消防安全的科学素养，并激励广大学生参与这一新兴领域。该研究将促进火灾在易发地区蔓延的预测和缓解，并提高火灾科学在国家景观中的重要性。该研究将使用大涡模拟、余烬的拉格朗日粒子跟踪和表面能量平衡分析来系统地研究湍流尺度相互作用对火种传输和发现风险的影响，以促进火灾科学的发展。该项目将首次开发并验证一个多功能的多尺度火种传输计算模型，该模型包含了小尺度湍流（即余烬大小的顺序）对火种演变的重要影响；B)研究由非均匀时空变化浮力引起的热驱动湍流风对火种输运的影响；c)研究由地表地形引起的机械驱动湍流结构对火种输运的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Wildfires have become a common occurrence in the U.S. and around the world. The costs of wildfires are magnified when they spread into communities; commonly referred to as wildland-urban interfaces (WUI). In wildfires, fragments of burning materials, known as firebrands (or flying embers) can be generated, lofted, and carried by the wind several meters to kilometers ahead of the main fire-front and create several new spot fires. Post-fire studies indicated that firebrands are responsible for the fastest spread and the major cause of the ignitions in WUIs. From the moment an ember becomes airborne, a firebrand’s trajectory, and thus its landing spot, depends on characteristics of the turbulent wind flow. While the structure of the turbulent winds is determined by mechanical and buoyant forces, there is currently no clear fundamental understanding of how these factors affect the spotting risks. In addition, it is not clear what role the small eddies of the turbulent flow play in determining firebrand behavior because their effects are neglected in the majority of physics-based studies. The principle aim of this project is to advance fluid-dynamic understanding of the firebrand transport phenomenon in turbulent winds, increase the general public’s scientific literacy about fire safety, and motivate a wide spectrum of students to engage in this emerging field. This research will advance the prediction and mitigation of fire spread into wildfire-prone areas and elevate the importance of fire science in the national landscape.The research will use Large-Eddy Simulations, Lagrangian particle tracking of the embers, and surface energy balance analyses to systematically investigate the effect of the turbulent scale interactions on firebrand transport and spotting risk for the advancement of fire science. The project will, for the first time, a) develop and validate a versatile multiscale computational model of firebrand transport that incorporates the important effects of small scale turbulence (i.e., the order of the size of the embers) on evolving firebrands; b) investigate the effect of thermally-driven turbulent winds due to heterogeneous spatiotemporally variable buoyant forces on firebrand transport, and c) explore the effect of mechanically-driven turbulence structures due to surface topography on firebrand transport.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.