Features of turbulence during wildland fires in forested and grassland environments
Features of turbulence during wildland fires in forested and grassland environments
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森林和草原环境野火期间的湍流特征
DOI：
10.1016/j.agrformet.2023.109501
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发表时间：
2023
影响因子：
6.2
通讯作者：
Banerjee, Tirtha
中科院分区：
文献类型：
--
作者：
Desai, Ajinkya;Heilman, Warren E.;Skowronski, Nicholas S.;Clark, Kenneth L.;Gallagher, Michael R.;Clements, Craig B.;Banerjee, Tirtha
森林（树冠下）和草原环境中，随着环境变化，火灾引起的湍流和对火灾的反馈有所不同。在这里，我们综合了多个实验性地表火灾的观测结果：两起亚冠层背面火灾、一场亚冠层航向火灾和一场草原航向火灾。我们从湍流动量通量和湍流动能 (TKE) 预算项中识别并比较每种情况下最重要的相干结构和过程。在亚冠层燃烧中，湍流涡流在冠层顶部附近最强：高流向湍流通量伴随着低横流湍流通量，反之亦然。在草原火灾中，顺流涡流和横流涡流同时增强到一定高度，从而表明火灾影响的垂直长度尺度。此外，流向涡流的向前扫掠将热气体推向未燃烧的燃料，从而有助于火势蔓延。在亚冠层火灾中，对于更强烈的火灾，在冠层顶部附近产生的剪切力和浮力产生更大，而它们的强度随着火灾强度的降低而减小。在中冠层高度尺度，浮力产生主导剪切产生，成为TKE垂直运输的关键机制。在草原火灾中，剪切力产生在近地表附近的浮力产生中占主导地位，并且相对于火焰长度超过一定高度时，剪切力产生不显着，而浮力产生随着高度的增加而增加，远离地表则变得显着。湍流传输术语在这两种环境中也很活跃。对于强烈的冠层下火灾，由于 TKE 通过传输项被驱逐到高空边界层，因此会出现 TKE 损失，并通过逆转过程进行补偿：TKE 通过传输项流入。在草原火灾中，运输术语模仿这种行为，直到达到一定高度。了解每种环境中各自湍流通量和 TKE 预算项的相对重要性有助于简化控制火灾物理的复杂方程组。
Fire-induced turbulence and the feedback into the fire, following ambient changes, differ for forested (sub-canopy) and grassland environments. Here, we synthesize observations from multiple experimental surface fires: two sub-canopy backing fires, one sub-canopy heading fire, and a grassland heading fire. We identify and compare the most essential coherent structures and processes of each case from the turbulent momentum fluxes and turbulent kinetic energy (TKE) budget terms. In the sub-canopy burns, turbulent eddies are strongest near the canopy top: high streamwise turbulent flux accompanies low cross-stream turbulent flux and vice versa. In the grassland fire, both streamwise and cross-stream eddies strengthen simultaneously until a certain height, informing a vertical length scale for the fire-influence. Moreover, the forward sweep from streamwise eddies assists in the fire spread by pushing hot gases towards unburnt fuel. In the sub-canopy fires, shear production and buoyancy production are more substantial near the canopy top for more intense fires, while their magnitudes decrease with decreasing fire intensity. At mid-canopy-height scales, buoyancy production dominates shear production, becoming the key mechanism for vertical transport of TKE. In the grassland fire, shear production dominates buoyancy production near the surface and is insignificant beyond a certain height relative to the flame length, while buoyancy production increases with height, becoming substantial further away from the surface. Turbulent transport terms are also active in both environments. For intense sub-canopy fires, there is a loss in TKE due to its expulsion to the boundary layer aloft via the transport term, compensated by a reversal process: TKE influx via the transport term. In the grassland fire, the transport term mimics this behavior until a certain height. The insights into the relative significance of the respective turbulent fluxes and TKE budget terms in each environment can help simplify the complex system of equations governing fire physics.
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