Novel hybrid LES-RANS schemes for simulating physically and geometrically complex turbulent flows

大涡模拟(LES)正逐渐取代传统的雷诺平均N-S(RANS)模型，成为研究和工业实践中预测复杂湍流流动的首选方法。当需要解决不稳定现象(振动、声学、热剥落、压力峰值)时，情况尤其如此。然而，利用大涡模拟来预测实际的壁面受限流动，特别是那些涉及从曲面分离的流动，在高雷诺数时实际上是站不住脚的资源需求严重制约了这一发展。混合LEAN-RAN方案在近壁区域采用某种形式的RANS解决方案，通常被认为是一种绕过资源障碍的折衷策略。现有方案基于在非稳定模式下运行的RANS模型的使用，因为它们受到外部LES解决方案施加在层上的高幅度、高频波动的影响。因此，这些模型的运作远远超出了它们预期的适用范围。此外，在大多数方法中，没有被LES显式解析的小尺度运动由次网格尺度和RAN湍流模型的模糊混合表示-即在LES和RAN分量之间没有明确的分界线。不足为奇的是，这些模型显示了一系列令人不安的缺陷。这份意见书提出了两个组织之间的合作，这两个组织一直走在英国发展RAN-LES计划的前沿。事实上，这两个小组是唯一参与了为期四年的欧盟FP6项目DESider的英国学术合作伙伴，该项目专门致力于工业应用中的RANS-LES建模，以及后续的22个合作伙伴的FP7项目ATAAC(用于空气动力学应用的高级湍流模拟)。法国电力公司(EDF)将支持该方案，相当于PDRA的一人年水平。拟议的项目专门针对LEAN-RAN混合动力车，它们仔细区分LES和RAN元素，每种元素的应用都受到适当的、既定的限制，并合理地结合。该项目涉及两个主要方面：(I)发展一种新的纬向(两层)方案，它需要求解定常的抛物化RANS方程，受LES解得到的动态时间平均约束的约束，并使用叠加在LES区域上的薄近壁层上的各向异性分辨湍流模式；(Ii)将(I)以及新开发的RANS-LES混合体(Uribe et al[2007])的扩展版本(它与(I)中提议的模式有一些相同的基本概念)整合和验证为最先进的数值框架(Saturne)，该框架是EPSRC的CCP12作为计算HPCx和Hector上非常复杂几何形状的湍流的通用预测工具而推广的。Uribe等人的模型的一个关键特征是它考虑了将RANS导出的雷诺应力从固有的非定常大涡模拟中分离出来的需要，以及对RANS模型中已解决的摄动和亚网格尺度的应力不敏感的需要。在这方面，虽然两个模式在设计上截然不同，但该模式所依据的理念与将会发展的分区计划的理念相同。

Large Eddy Simulation (LES) is gradually replacing traditional Reynolds-averaged-Navier-Stokes (RANS) modelling as the method of choice for predicting complex turbulent flows in research as well as industrial practice. This is especially so when unsteady phenomena are to be resolved (vibrations, acoustics, thermal striping, pressure peaks). However, the exploitation of LES for predicting practical wall-confined flows, particularly those involving separation from curved surfaces, is seriously inhibited by practically untenable resource requirements at high Reynolds numbers. Hybrid LES-RANS schemes, employing some form of RANS-like solution in the near-wall region, are generally regarded as a compromise strategy circumventing the resource obstacle. Existing schemes are based on the use of RANS models that operate in unsteady mode, as they are subjected to high amplitude, high-frequency fluctuations imposed on the layer by the outer LES solution. These models thus operate far outside their intended range of applicability. Moreover, in most approaches, the small-scale motions not resolved explicitly by the LES are represented by an ill-defined blend of subgrid-scale and RANS turbulence models - i.e. there is no clear dividing line between the LES and RANS components. Not surprisingly, such models display a whole range of disconcerting defects.This submission proposes a collaboration between two groups who have been at the forefront of developing RANS-LES schemes in the UK. Indeed, the two groups are the only UK academic partners who have participated in the four-year EU FP6 project DESider, specifically devoted to RANS-LES modelling for industrial applications, and in the follow-up 22-partner FP7 project ATAAC (Advanced Turbulence Simulation for Aerodynamic Application Challenges). Electricite de France (EDF) will support the programme to the level of one man-year of PDRA.The proposed project aims specifically at LES-RANS hybrids that distinguish carefully between the LES and RANS elements, each applied subject to appropriate, well-established constrains and coupled rationally. The project involves two major strands: (i) the development of a novel zonal (two-layer) scheme, which entails the solution of steady, parabolized RANS equations, subject to on-the-fly time-averaged constraints derived from the LES solution, and the use of an anisotropy-resolving turbulence model over a thin near-wall layer superimposed onto the LES domain; (ii) the integration and validation of (i), as well as an extended version of a newly-developed RANS-LES hybrid (Uribe et al [2007]), which shares some basic concepts with proposed model under (i), into a state-of-the-art numerical framework (Saturne), which is promoted by EPSRC's CCP12 as a general prediction tool for computing turbulent flows in very complex geometries on HPCx and HECTOR. A key characteristic of Uribe et al's model is that it respects the need to separate the RANS-derived Reynolds stresses from the inherently unsteady LES, and to desensitize the resolved perturbations and the subgrids-scale stresses from the RANS model. To that extent, the model is based on the same philosophy underpinning the zonal scheme to be developed, although the two models differ radically in respect of their design.