Direct numerical simulation of gas transfer across the air–water interface driven by buoyant convection
Direct numerical simulation of gas transfer across the air–water interface driven by buoyant convection
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DOI：
10.1017/jfm.2015.696
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发表时间：
2015-12
影响因子：
3.7
通讯作者：
J. Wissink;H. Herlina
中科院分区：
文献类型：
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作者：
J. Wissink;H. Herlina
对浮力诱导对流驱动的空气-水界面传质进行了一系列直接的数值模拟，以阐明在热和大气气体传输中起作用的物理机制。浮力不稳定是由位于温水体顶部的一层薄薄的冷水引起的。随着时间的推移，热量和大气气体扩散到热边界层的最上面部分，随后通过下落的冷水薄片和羽流向下输送到大块。使用专门设计的标量对流和扩散离散数值程序，可以在实际的普朗特数($\mathit{Pr}=6$)和施密特数($\mathit{sc}=20$到$\mathit{sc}=500$)下精确地解析这种由浮力不稳定引起的大气气体传输。本文提供的模拟提供了对瞬时气体传输过程的详细了解。发现坠落的羽流在其核心含有高度气体饱和的流体，可以深入到主体内部。当水面和散体之间的初始温差略高于2$K时，观测到瞬时热通量超过$1600~\TEXT{W}~\TEXT{m}^-2}$，证明了浮力对流换热和气体传输的潜在有效性。此外，还证实了气体和换热速度比K_{L}/H_{L}\propto(\mathit{Pr}/\mathit{sc})^{0.5}$在整个施密特数范围内的标度律的有效性。利用速度起伏、对流单体大小或表面散度等表面信息，得到了较好的时间精度近似解。利用水平积分长度尺度和块体上部水平速度起伏的均方根，获得了合理的时间精度。
A series of direct numerical simulations of mass transfer across the air–water interface driven by buoyancy-induced convection have been carried out to elucidate the physical mechanisms that play a role in the transfer of heat and atmospheric gases. The buoyant instability is caused by the presence of a thin layer of cold water situated on top of a body of warm water. In time, heat and atmospheric gases diffuse into the uppermost part of the thermal boundary layer and are subsequently transported down into the bulk by falling sheets and plumes of cold water. Using a specifically designed numerical code for the discretization of scalar convection and diffusion, it was possible to accurately resolve this buoyant-instability-induced transport of atmospheric gases into the bulk at a realistic Prandtl number ( $\mathit{Pr}=6$ ) and Schmidt numbers ranging from $\mathit{Sc}=20$ to $\mathit{Sc}=500$ . The simulations presented here provided a detailed insight into instantaneous gas transfer processes. The falling plumes with highly gas-saturated fluid in their core were found to penetrate deep inside the bulk. With an initial temperature difference between the water surface and the bulk of slightly above $2$ K, peaks in the instantaneous heat flux in excess of $1600~\text{W}~\text{m}^{-2}$ were observed, proving the potential effectiveness of buoyant-convective heat and gas transfer. Furthermore, the validity of the scaling law for the ratio of gas and heat transfer velocities $K_{L}/H_{L}\propto (\mathit{Pr}/\mathit{Sc})^{0.5}$ for the entire range of Schmidt numbers considered was confirmed. A good time-accurate approximation of $K_{L}$ was found using surface information such as velocity fluctuations and convection cell size or surface divergence. A reasonable time accuracy for the $K_{L}$ estimation was obtained using the horizontal integral length scale and the root mean square of the horizontal velocity fluctuations in the upper part of the bulk.