Inertial Effects on the Vertical Transport of Suspended Particles in a Turbulent Boundary Layer
Inertial Effects on the Vertical Transport of Suspended Particles in a Turbulent Boundary Layer
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DOI：
10.1007/s10546-017-0325-3
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发表时间：
2018-05
影响因子：
4.3
通讯作者：
D. Richter;M. Chamecki
中科院分区：
文献类型：
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作者：
D. Richter;M. Chamecki
在许多大气流动中，一个分散相被湍流积极地悬浮，湍流与重力沉降的竞争最终决定了它的垂直分布。分散相的例子包括雪、浪花、灰尘或沙子，其中单个元素的密度比周围的空气大得多，从表面发射出来后被湍流运动携带。在假设粒子仅因其重力沉降而偏离局部流体运动的情况下（即，它们是惯性的），传统的通量平衡预测粒子浓度与高度的幂律依赖关系。然而，尚不清楚粒子惯性如何影响这种关系，而这个问题是这项工作的重点。直接数值模拟了载有特定惯量拉格朗日粒子的明渠湍流；通过这种方式，研究的重点是发生在行星边界层最低几米的湍流输送，这些区域对于将发射通量与全尺寸边界层所感受到的通量连接起来至关重要。在保持无量纲沉降速度不变的情况下，进行了大范围的颗粒斯托克斯数模拟，以了解颗粒惯性对垂直分散的作用。人们发现粒子偏离其惯性行为的方式不容易被传统理论捕获；浓度随斯托克斯数的增加而降低。此外，当颗粒经历惯性加速度时，基于相似性的颗粒浓度涡流扩散率失效，排除了无惯性颗粒情况下颗粒浓度的封闭形式解。这一结果的主要后果是，将地表排放模式（例如，跃变模式或浪花产生函数）与升高的边界条件联系起来的典型通量参数化可能会高估颗粒浓度，因为两者之间的惯性导致垂直输送减少；同样，如果从空中的浓度测量推断，粒子排放可能被低估。
In many atmospheric flows, a dispersed phase is actively suspended by turbulence, whose competition with gravitational settling ultimately dictates its vertical distribution. Examples of dispersed phases include snow, sea-spray droplets, dust, or sand, where individual elements of much larger density than the surrounding air are carried by turbulent motions after emission from the surface. In cases where the particle is assumed to deviate from local fluid motions only by its gravitational settling (i.e., they are inertialess), traditional flux balances predict a power-law dependence of particle concentration with height. It is unclear, however, how particle inertia influences this relationship, and this question is the focus of this work. Direct numerical simulations are conducted of turbulent open-channel flow, laden with Lagrangian particles of specified inertia; in this way the study focuses on the turbulent transport which occurs in the lowest few meters of the planetary boundary layer, in regions critical for connecting emission fluxes to the fluxes felt by the full-scale boundary layer. Simulations over a wide range of particle Stokes number, while holding the dimensionless settling velocity constant, are performed to understand the role of particle inertia on vertical dispersion. It is found that particles deviate from their inertialess behaviour in ways that are not easily captured by traditional theory; concentrations are reduced with increasing Stokes number. Furthermore, a similarity-based eddy diffusivity for particle concentration fails as particles experience inertial acceleration, precluding a closed-form solution for particle concentration as in the case of inertialess particles. The primary consequence of this result is that typical flux parametrizations connecting surface emission models (e.g., saltation models or sea-spray generation functions) to elevated boundary conditions may overestimate particle concentrations due to the reduced vertical transport caused by inertia in between; likewise particle emission may be underestimated if inferred from concentration measurements aloft.