Modelling of the Unsteady Dynamics of Turbulent Disperse Bubbly Flows

泡状流存在于大量的工业过程中，包括石油威尔斯、发电以及食品、制药和化学工业。这样，全世界每年生产价值数十亿美元的化学产品。因此，可靠的数值模拟工具在过程安全和过程控制方面的优势是巨大的。通常，工业中的两相流模拟基于雷诺平均Navier-Stokes方程（RANS）。然而，即使对于单相流动，经过几十年的研究，仍然没有一个令人满意的湍流封闭模型。泡状流的特点是空间和时间变形的界面。气泡动力学、湍流和气泡本身的多种相互作用是如此复杂，以至于在不久的将来不太可能有一个通用的、准确的RANS模型来预测气泡流。大涡模拟（LES）提供了一个很好的折衷RANS基于封闭和直接数值模拟（DNS）的基本方程。与界面平流技术相结合，LES提供的优点是，许多物理过程可以在很大程度上解决，这是在RANS的整个范围内的长度和时间尺度的建模相反。在具有移动边界的两相流的上下文中，LES方程包含额外的未知项。封闭这些条款的发展才刚刚开始，并没有模型已被证明在文献中产生令人满意的结果，在一个真实的LES，但。本计画的目标是建立一个能以高精度预测实际感兴趣的分散泡状流的大涡模拟模式。该项目分为两个阶段。首先，将生成DNS数据库。为此，将在静止流体和湍流逆流配置中模拟垂直上升的单个气泡以及小气泡群。随后，数据将被过滤以获得伪LES数据（先验分析）。通过这种方式，可以将模型思想与DNS确定的精确未闭项进行比较。然而，模型的真实评估需要执行真实的LES。从第一阶段的模型，似乎最有前途的将在LES代码中实现，并与DNS和非常详细的实验数据从一个研究伙伴。这种所谓的后验分析将指导气泡两相流的LES模型的资格。在这个研究项目的延续，计划使用多尺度的方法，以便能够模拟大的气泡群。这将有助于实现全球目标，即模拟实际感兴趣的配置。

Bubbly flows are found in a large number of industrial processes including oil wells, power generation as well as in the food, pharmaceutical and chemical industry. In this way chemical goods worth billions of dollars are produced worldwide every year. Therefore the advantages of the ability of a reliable numerical simulation tool in regards of process safety and process control are enormous.Typically, two phase flow simulations in industry are based on the Reynolds averaged Navier-Stokes equations (RANS). However even for single phase flow, after decades of research, no satisfactory turbulence closure model is available. Bubbly flows are characterized by spatially and temporally deforming interfaces. The manifold interaction of bubble dynamics, turbulence and the bubbles itself are so complex, that the availability of a universal and accurate RANS model for the prediction of bubbly flows appears unlikely in near future. Large eddy simulation (LES) offers a good compromise between RANS based closures and the direct numerical simulation (DNS) of the underlying equations. Combined with an interface advection technique LES offers the advantage, that many physical processes can be resolved to a large extent which is in contrast to modelling the whole range of length and time scales in RANS. In the context of two phase flows with moving boundaries the LES equations contain additional unknown terms. The development of closures for these terms has just started and no model has been demonstrated in the literature to yield satisfactory results in a real LES, yet. The goal of this project is to establish an LES model which is able to predict disperse bubbly flows of practical interest with good accuracy. The project is subdivided into two phases. First a DNS data base will be generated. To this end vertically rising single bubbles as well as small bubble swarms will be simulated in a quiescent fluid as well as in a turbulent counterflow configuration. Subsequently the data will be filtered to obtain pseudo LES data (a-priori analysis). In this way model ideas can be compared with the exact unclosed terms determined from DNS. The true evaluation of a model requires however to perform a real LES. The models from phase one which appear most promising will be implemented in the LES code and compared with DNS and very detailed experimental data available from a research partner. This so called a-posteriori analysis will guide the qualification of an LES model for bubbly two phase flows. In a continuation of this research project it is planned to use a multiscale approach in order to enable the simulation of large bubble swarms. This will allow to reach the global goal, which is the simulation of configurations of practical interest.