Computational Investigation of the Effects of Surfactants on Bubble Dynamics, Bubble Swarm Interactions and Turbulent Flow

两相泡状流混合物中的表面活性材料或表面活性剂被用于许多具有重要科学和技术意义的领域，包括化学、生物分子、电力和石油工程应用。它们可以显著改变大尺度气泡聚集体的界面动力学、流体运动和结构，并导致气泡聚集和壁面附近的湍流抑制。然而，与这些观测相关的基本物理机制的基本细节尚不完全清楚，通过建模来阐明将对工程系统的设计和放大以及操作产生直接影响。这种复杂的流体系统既涉及体效应又涉及分子效应，由界面动力学和流体运动驱动的体效应是目前面临的主要挑战。他们对气泡群相互作用的理解，尤其是对惯性主导流区中的气泡群相互作用的理解，一般局限于实验可视化结果和经验数据。该项目旨在利用前沿的创新计算技术进行大规模模拟，以研究和阐明含表面活性剂的两相起泡系统和群体的原型流动中的潜在过程。这项研究的成功实施将产生新的工具、丰富的数据集和在前所未有的条件下的物理洞察，这将引起广泛的研究人员的兴趣。该项目将对教育产生多方面的影响，包括跨学科培训以及研究生和本科生的研究参与。这项拟议的研究处于机械/化学工程、物理和计算科学之间，其结果将在期刊论文和会议上广泛传播，在该项目下开发的新方法和代码将作为开放来源随时可用。这个项目有可能改变表面活性剂效应的建模方式，定性和定量地理解各种非线性流动状态中含有表面活性剂的泡状流。基于层叠格子Boltzmann(LB)公式的三维多相流模型的应用处于当前研究的前沿，并有望在建模和模拟能力方面取得根本性进展，以研究表面活性剂效应对对流主导的气泡动力学和群体相互作用的作用。它的动力学起源促进了介观模型的结合，而LB方法在具有自然并行化能力的复杂流体流动应用中非常成功。表面活性剂和体相流体分别被表示为偶极流体和范德华流体，它们在介观尺度上产生表面活性剂-流体、表面活性剂-表面活性剂平均场力相互作用，控制表面活性剂的排列、相分离和表面张力效应。级联LB方法中中心矩公式的使用提供了增强的物理和数值模拟能力。这一创新的计算方法将用于系统地研究特征参数的影响，以阐明对以下典型情况的物理理解：均匀湍流中的表面活性剂气泡破碎过程，剪切驱动的湍流槽流中表面活性剂对单个气泡运动的影响，以及单个和一对表面活性剂气泡的浮力驱动运动，以研究它们的路径不稳定性、上升速度、阻力和升力以及尾流结构。这将为研究人员提供对相关潜在机制的基本见解，包括表面活性剂对集群中团簇形成的作用，以及基于描述气泡路径和变形的各种区域的特征参数的相图，从而澄清表面活性剂在路径不稳定性(莱昂纳多悖论)中的作用。另一个主要成果将是混合模型中包含表面活性物质影响的预测闭合关系的发展。

Surface active materials or surfactants in two-phase, bubbly flow mixtures are used in a number of areas of important scientific and technological interest, including those in chemical, bio-molecular, power and petroleum engineering applications. They can significantly change the interfacial dynamics, fluid motion and structure of macroscale bubble aggregates, and are known to cause clustering of bubbles and turbulence suppression near walls. However, the fundamental details of the underlying physical mechanisms associated with such observations are not yet completely known, and elucidation through modeling will have direct impact on the design and scale-up and the operation of engineering systems. Such complex fluid systems involve both bulk and molecular effects, which are mediated by the interfacial dynamics and driven by the fluid motion present major challenges. Their understanding, especially for bubble swarm interactions in the inertia dominated flow regimes, is generally limited to experimental visualization results and empirical data. This project aims to perform large scale simulations using innovative computational techniques at the forefront in their development for the investigation and elucidation of the underlying processes in prototypical flows of surfactant-laden two-phase bubbly systems and swarms. Successful implementation of this research will yield new tools, rich sets of data and physical insights under unprecedented conditions that will be of interest to a wide community of researchers. The project will impact the education in a number of ways, including interdisciplinary training and research participation of graduate and undergraduate students. The results of this proposed research, which lies at the borderline between mechanical/chemical engineering, and physics and computational science, will be disseminated broadly in journal papers and conferences, and the new methods and codes developed under this project will be readily available as open sources. This project has the potential to transform the way the surfactant effects are modeled, qualitative and quantitative understanding of the surfactant-laden bubbly flows in various nonlinear flow regimes. The application of a three-dimensional multiphase flow model using a cascaded lattice Boltzmann (LB) formulation is at the cutting edge of current research and will potentially make fundamental advances in the modeling and simulation capabilities to study the role of surfactant effects on convection dominated bubble dynamics and swarm interactions. Its kinetic origins facilitate incorporation of mesoscopic models and the LB method is remarkably successful in complex fluid flow applications with natural parallelization capabilities. The surfactants and the bulk fluids are represented as dipoles and van der Waals fluids, respectively, which, in turn, generate surfactant-fluid, surfactant-surfactant mean-field force interactions at mesoscopic scales that govern the alignment of the surface agents, phase segregation and surface tension effects. The use of a central moment formulation in the cascaded LB method offers enhanced physical and numerical simulation capabilities. This innovative computational approach will be brought to bear on performing a systematic study of the effect of the characteristic parameters to elucidate physical understanding for the following prototypical cases: surfactant-laden bubble breakup processes in homogeneous turbulence, surfactant effects on the motion of a single bubble in a shear-driven turbulent channel flow, and buoyancy-driven motion of a single and a pair of surfactant-laden bubbles to study their path instabilities, rise velocities, drag and lift forces and wake structures. This will offer researchers fundamental insights into the underlying mechanisms involved, including the role of surfactants on cluster formation in swarms and phase diagrams based on characteristic parameters delineating the various regimes of bubble paths and deformations thereby clarifying the role of surfactants on path instabilities (Leonardo's paradox). Another major outcome will be in the development of predictive closure relations incorporating surfactant effects for mixture models.