Modelling and Simulation of Intermittent Flows

近年来，基于CFD的雷诺平均纳维尔-斯托克斯（RANS）在工业中的使用导致了工程各个方面的设计和开发方法的巨大变化，允许更有效的设计，降低开发成本，并使产品比以前更快更经济地推向市场。由于计算机运行时间的限制，在可预见的将来，RANS方法仍将是工程应用中代表湍流影响的主要方法。然而，在这种方法中体现的湍流模型仍然依赖于被检查的流动配置，需要在其规范中使用实验数据，并根据具体情况进行调整，以准确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，速度脉动从有旋到无旋迅速变化，反之亦然。间歇性在许多实际流动中是重要的，例如，在边界层转捩中，由于其与现代压缩机和涡轮机以及翼型的设计相关，因此边界层转捩仍然是一个主要挑战。间歇性对于许多燃烧装置的安全和有效操作也很重要，这取决于在具有显著间歇性的流动中发生的点燃，以及在关于化学和加工厂上的易燃释放物的潜在点燃的危害和风险评估中。在所有这些应用中，这些装置的增强性能和改进的安全性依赖于对湍流和点火过程的详细理解，从而依赖于湍流度。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推导的，因此不能期望精确地预测外部区域被无旋流动污染的自由剪切流。因此，发展适应湍流效应的湍流模型也具有根本的重要性。此外，包括这样的影响有助于推广现有的湍流模型，并提供更准确的预测的速度和标量场的上述应用程序的兴趣。EROFTAC转捩模型特别兴趣小组和艾萨克·牛顿数学科学研究所湍流计划也确认了开展这类工作的必要性，特别是需要提高湍流工程模型的通用性，以及改进非线性影响的模型，特别是对标量场的影响。该建议涉及使用分层方法，通过直接数值模拟和大涡模拟的协调应用，开发更精确的间歇性湍流工程模型，以告知二阶矩湍流闭合的制定和验证，这些方法比现有方法更基本。所提出的工作也将提高我们对这种流动的整体理解，湍流工程模型的普遍性，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.