Direct numerical simulation of buoyant-convectively driven gas transfer across gas-liquid interfaces

拟议的项目涉及由浮力-对流不稳定性驱动的气液界面上的气体物质转移。在浮力驱动的对流气体传输中发挥作用的物理机制还不是很清楚，尽管它们对全球热量收支和对环境重要的气体循环(包括温室气体循环)做出了重大贡献。虽然文献中已经报道了许多预测气体传递速度的经验公式，但浮力驱动流动中近地表湍流场与界面气体通量相互作用的动力学还没有得到充分的描述。由于这种低扩散(高施密特数)物质的界面传质的特点是在界面附近有非常薄的扩散层，而在其他区域则瞬间出现陡峭的浓度梯度，因此进行详细的实验室测量是非常困难的。同时，现有的直接数值模拟(DN)大多局限于低施密特数(通常小于10)和/或低雷诺数(Re)，受解析所有运动尺度所需的计算资源的限制。我们建议使用专门设计的用于离散标量对流和扩散的数值程序来对浮力驱动的气体传输进行直接数值模拟。据我们所知，本研究将首次直接数值模拟浮力-对流不稳定性在实际施密特数高达500时的传质过程。详细的数据将使我们能够首先确定波动的浓度场和速度场之间的相关性--今天在实验室实验中很难高保真地测量这个量。在那些能够测量这个量的为数不多的实验中，可靠的测量通常被限制在不太靠近界面和不太深入流体主体的区域。此外，对于这种流动，不存在前述场的三维时间和空间分辨率测量。拟议项目的主要影响将是在全球二氧化碳预算或水体氧气再曝气的框架内对气体转移率进行更可靠的预测。更具体地说，我们将确定界面传质的标度律作为施密特数和瑞利数的函数。在应用(如遥感)中，这一结果将允许仅通过重新标度关于热场的信息来重建给定气体的传质速率。

The proposed project deals with the transfer of gaseous substances across a gas-liquid interface driven by buoyant-convective instability. The physical mechanisms that play a role in buoyancy-driven convective gas transfer are not well understood, despite of their significant contributions to the global heat budget and environmentally important gas cycles (including green-house gas cycles). Though numerous empirical relations to predict the gas transfer velocity have been reported in the literature, the dynamics of the interaction between the near-surface turbulent field and the interfacial gas flux in buoyancy driven flow are yet to be fully described. As the interfacial mass transfer of such low-diffusive (high Schmidt number) substances is characterized by very thin diffusive layers near the interface and the instantaneous occurrence of steep concentration gradients in other regions, performing detailed laboratory measurements is extremely difficult. At the same time existing direct numerical simulations (DNS) - constrained by the high demand on computational resources needed to resolve all scales of motion - are mostly limited to low Schmidt numbers (typically less than 10) and/or low Reynolds numbers. We propose to perform direct numerical simulations of buoyancy-driven gas transfer using a specifically-designed numerical code for the discretization of scalar convection and diffusion. To our knowledge, the present study will be the first to perform direct numerical simulation of mass transfer driven by a buoyant-convective instability at realistically high Schmidt numbers up to 500. The detailed data will allow us to determine first and foremost the correlation between the fluctuating concentration and velocity fields -- a quantity which today is extremely difficult to measure with high fidelity in laboratory experiments. In those few experiments which are able to measure this quantity, the trustworthy measurement is typically restricted to a region not too close to the interface and not too deep into the bulk of the fluid. Furthermore, three-dimensional time- and space-resolved measurements of the aforementioned fields do not exist for this flow. The main impact of the proposed project will be a more reliable prediction of the gas transfer rates in the framework of e.g. the global CO2 budget or the oxygen re-aeration in water bodies. More specifically, we will determine the scaling law of the interfacial mass transfer as a function of the Schmidt and Rayleigh numbers. In an application (such as remote sensing) this result will allow to reconstruct the mass transfer rate of a given gas from rescaling of the information on the thermal field alone.