Transport, evolution, and induced mixing of bubble swarms across pycnoclines

湖泊和海洋中的水密度并不均匀；从几米到一公里的长度范围内，水的密度各不相同。密度的变化(或密度分层)可由两个原因引起：i)热，其中密度因局部温度的变化而变化；或ii)盐，其中密度因盐浓度的变化而变化。观测到这种密度变化的次表层称为跃层。研究人员往往忽略了密度分层对上升气泡和液滴运动以及背景流体诱导混合的影响。然而，PI显示，这种密度梯度可以通过增强小水滴所经历的阻力来显著影响小水滴的上升。量化密度分层对气泡和液滴的影响是流体动力学中一个基本但未被充分探索的领域，在与湖泊和海洋有关的不同多相现象中普遍需要更好地了解这一领域，例如，气泡羽流系统使水库分层，海洋沉积物中释放甲烷，石油泄漏期间油羽扩散，以及利用人工产生的气泡羽流降低表面温度，以阻止湖泊的过度蒸发。这项研究中将获得的新的物理见解，以及已经和将要开发的数值技术，将对致力于流体动力学、水文学和海洋学之间的广泛研究的其他研究人员大有裨益。这项研究将使用最先进的、经过实验验证的计算工具来量化密度分层对(1)密度分层中层流泡状上行流对背景流体的混合的影响，(2)气泡/液滴群的迁移速度和微观结构的形成，以及(3)在存在背景湍流的情况下气泡/液滴的混合和运动。这项研究将为系统地研究跃层多相流的运动和由此产生的混合奠定基础。混合的程度将通过计算混合效率(混合引起的背景势能相对于过程中动能的耗散率的增加率)、昼夜涡旋扩散率(由于流体垂直输送引起的混合的测量)和温度微结构来量化。通过计算分散相和连续相的漂移速度和脉动速度，以及分散相的成对概率分布函数，可以量化气泡/液滴群的性质。

The density of water in lakes and oceans is far from uniform; it varies over length scales from meters to a kilometer. This variation in density (or density stratification) can be due to two reasons: i) thermal, wherein the density varies due to variations in local temperature or, ii) saline, wherein the density varies due to variations in the salt concentration. The subsurface layers across which this change in density is observed are called pycnoclines. Researchers often neglect the effects of density stratification on the motion of rising bubbles and droplets and the induced mixing of the background fluid. However, the PIs showed that such density gradients can significantly affect the ascent of small drops by enhancing the drag experienced by them. Quantifying the impact of density stratification on bubbles and drops is an essential, yet under-explored area of fluid dynamics, and the need to better understand this area is prevalent in different multiphase phenomena pertaining to lakes and oceans, for example, destratification of water reservoirs by air-bubble plume systems, release of methane in marine sediments, dispersion of an oil plume during an oil spill, and the use of artificially generated bubble plumes to lower surface temperatures in order to arrest excessive evaporation from lakes. The new physical insights to be gained in this study, along with the numerical techniques that have been and will be developed, will be significantly beneficial for a broad range of other researchers who are devoted at the interface between fluid dynamics, hydrology, and oceanography. This research will use state-of-the-art, experimentally validated computational tools to quantify the effects of density stratification on (1) the mixing of the background fluid by laminar bubbly upflows in density stratification, (2) the migration velocity and microstructure formation of bubble/droplet swarms, and (3) the mixing and the motions of bubbles/drops in the presence of background turbulence. The research will pave the way to systematically investigate the motion of multiphase flows at pycnoclines and the resultant mixing. The extent of mixing will be quantified by computing the mixing efficiency (a measure of the rate of increase in background potential energy due to mixing against the rate of dissipation of kinetic energy in the process), the diapycnal eddy diffusivity (a measure of mixing due to vertical transport of the fluid) and the temperature micro-structure. The properties of the bubble/droplet swarms will be quantified by calculating the drift velocity and the fluctuation velocities of the dispersed and continuous phases, and pair probability distribution functions for the dispersed phase.