Modelling and Simulation of Intermittent Flows

近年来，基于雷诺平均纳维斯托克斯(RANS)的CFD在工业中的使用已经导致工程各个方面所使用的设计和开发方法发生了翻天覆地的变化，从而实现了更高效的设计，降低了开发成本，并比以前更快、更经济地将产品推向市场。由于计算机运行时间的限制，在可预见的未来，RANS方法仍将是表示湍流效应的主要方法。然而，这些方法中包含的湍流模型仍然依赖于所检查的流动结构，需要在其规范中使用实验数据，并逐个进行调整以准确地预测特定类型的流动。具有自由边界的湍流剪切流具有间歇性，速度的起伏由旋转迅速变为无旋转，反之亦然。间歇性在许多实际流动中是重要的，例如，在边界层过渡中，这仍然是一个重大挑战，因为它与现代压缩机、涡轮机和翼型的设计相关。间歇性对于许多燃烧设备的安全和有效运行也很重要，这些设备取决于在具有明显间歇性的气流中发生的点火，以及在关于化学和加工厂易燃排放可能着火的危险和风险评估中。在所有这些应用中，这些设备的性能和安全性的提高依赖于对湍流和点火过程的详细了解，因此也就是间歇性。目前使用的大多数工程湍流模型都是针对充分发展的流动而建立的，因此不能准确地预测外部区域被无旋流污染的自由剪切流。因此，发展适应间歇效应的湍流模型也是至关重要的。此外，这种效应的加入有助于推广现有的湍流模型，并为上述应用提供更准确的速度场和标量场预测。欧空委过渡模型特别兴趣小组和艾萨克·牛顿数学科学研究所关于湍流的方案也确认了这项工作的必要性，特别是在湍流工程模型中需要更大的普遍性，并改进了对间歇性影响的模拟，特别是对标量场的影响。这项建议涉及使用分层方法来开发更准确的间歇性湍流工程模型，通过协调应用直接数值模拟和大涡模拟来为二阶矩湍流闭合的制定和验证提供信息，这些闭合的基础比现有方法更基本。建议的工作还将提高我们对这种流动的整体理解，提高湍流工程模型的普适性，并将验证为应用于广泛的实际流动而开发的模型。

In recent years, the use of Reynolds-Averged Navier-Stokes (RANS) based CFD within industry has led to a sea change in the design and development approaches used across all aspects of engineering, allowing more efficient design, reducing development costs, and bringing products to market more rapidly and economically than was previously possible. Because of computer run time constraints, RANS approaches will remain the principal method for representing the effects of turbulence for engineering application for the foreseeable future. However, the turbulence models embodied within such approaches are still dependent on the flow configuration being examined, requiring the use of experimental data in their specification, and case-by-case adjustment to accurately predict particular types of flow. Turbulent shear flows with free boundaries display an intermittent character, such that the fluctuations of velocity rapidly change from rotational to irrotational, and vice versa. Intermittency is important in many practical flows, for example, in boundary layer transition which remains a major challenge because of its relevance to the design of modern compressors and turbines, and airfoils. Intermittency is also important in the safe and efficient operation of many combustion devices, which are dependent on ignition occurring in flows with significant intermittency, as well as in hazard and risk assessments concerning the potential ignition of flammable releases on chemical and process plant. In all these applications, the enhanced performance of these devices, and improved safety, relies on a detailed understanding of turbulent flow and ignition processes, and hence intermittency. The majority of engineering turbulence models in use today were derived for fully developed flows, and hence cannot be expected to accurately predict free shear flows where the outer regions are contaminated with irrotational flow. The development of turbulence models that accommodate intermittency effects is therefore also of fundamental importance. Additionally, the inclusion of such effects helps to generalize existing turbulence models, and provide more accurate predictions of the velocity and scalar fields of interest to the applications noted above. The necessity for such work has also been identified by both the EROFTAC Special Interest Group on Transition Modelling, and the Isaac Newton Institute for Mathematical Sciences Programme on Turbulence, particularly in relation to the need for greater universality in engineering models of turbulence, and the improved modelling of the influence of intermittency, particularly on scalar fields. This proposal concerns the use of an hierarchical approach to the development of more accurate engineering models of intermittent turbulent flows through the co-ordinated application of direct numerical simulation and large eddy simulation to inform the formulation and validation of second-moment turbulence closures that are more fundamentally based that existing approaches. The work proposed will also improve our overall understanding of such flows, the universality of engineering models of turbulence, and will validate the models developed for application to a wide range of practical flows.