Hybrid LES-RANS-Coupling and Wall Modeling for Complex Flows with Separation

具有分离的复杂流动的混合 LES-RANS 耦合和壁面建模

• 批准号: 5415265
• 负责人: Professor Dr.-Ing. Michael Breuer
• 金额: --
• 依托单位: Division Modélisation Numérique - CNRS UMR 6598 (aufgelöst)
• 依托单位国家: 德国
• 项目类别: Research Units
• 财政年份: 2004
• 资助国家: 德国
• 起止时间: 2003-12-31 至 2009-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/5415265?language=en
• 关键词: Hybrid; LES; RANS; Coupling; Wall

The objektive of the present joint French-German research project is the development of urgently required techniques in order to apply large-eddy simulation (LES) and its derivatives to high Reynolds number turbulent flows of practical interest. Two main development strategies will be considered which are closely related and belong together. These are on the one hand appropriate wall models combined with suitable (auto-adaptive) subgrid scale models. The goal is to bridge the near-wall region and therefore allow high Re simulations without resolving the direct vicinity of the wall with extremely fine and CPU-time consuming grids. On the other hand, hybrid LES-RANS approaches will be developed, which try to split up the simulation (ideally not the domain) into a RANS part and an LES part. Typically, RANS is adequate for attached boundary layers requiring reasonable CPU-time and memory, where LES can also be applied but demands extremely large resources. Contrarily, RANS often fails in flows with massive separation and/or with large-scale vortical structures. Here, LES is without doubt the better choice. The basic concept is to combine the advantages of both methods yielding an optimal solution at least for a special class of flows. A real non-zonal hybrid technique is preferred since it avoids the predefinition of RANS and LES regions leading to an approach where the suitable simulation technique is chosen more or less atitomatically. Although the idea of combined LES-RANS methods is not new, a variety of open questions has to be answered. This includes in particular the demand for appropriate coupling techniques between LES and RANS, adaptive control mechanisms, and proper SGS/RANS models. Since approximate wall boundary conditions partially rely on RANS modeling and the classical wall function is a limiting case of a RANS model in the near-wall region, both topics of the present proposal, i.e., wall modeling and LES-RANS coupling, are strongly interconnected. In order to investigate the above mentioned open questions, in the first step it is definitely inevitable to consider some geometrically simple test cases including pressure-induced flow separation and subsequent reattachment, e.g. test configuration T2, the flow over periodic hills. The final objectives are, of course, flow configurations of practical interest including high-Re cases. Because all partners already studied the flow around airfoils at high angles of attack, such a configuration (test case T6) will serve as a second, more complex test case. As a long-term goal a geometrically three-dimensional configuration of practical interest will be investigated. One option is the flow around a simplified car model (Ahmed body, test case T4). Especially for the hybrid methods, the flow configurations chosen should include regions which can be reasonably resolved by RANS and additionally complex flow features such as large-scale flow separation which definitely require LES instead of RANS. These kinds of flows are also extremely challenging for the wall modeling techniques which allows a direct comparison and evaluation of both approaches based on the same test cases. The CFD groups at LIMSI Orsay and LSTM Erlangen have long lasting experience in the development and application of LES and DNS for complex flows. At Ecole Centrale de Nantes, the numerics group has many years of experience in RANS computations for complex geometries. Owing to these complementary fields of main expertise, the cooperation is expected to be particularly fruitful leading to strong synergy effects.

目前法德联合研究项目的目标是开发迫切需要的技术，以便将大涡模拟（LES）及其衍生物应用于实际感兴趣的高雷诺数湍流。将审议两个密切相关的主要发展战略。一方面，这些是适当的壁模型与适当的（自适应）亚网格尺度模型相结合。目标是桥接近壁区域，从而允许高Re模拟，而无需使用极其精细且耗时的网格来解决壁的直接附近区域。另一方面，将开发混合LES-RANS方法，该方法试图将模拟（理想情况下不是域）分成RANS部分和LES部分。通常情况下，RANS是足够的附加边界层需要合理的CPU时间和内存，LES也可以应用，但需要非常大的资源。首先，RANS在有大规模分离和/或大尺度涡结构的流动中经常失效。在这里，LES无疑是更好的选择。其基本概念是联合收割机的优点，这两种方法产生的最佳解决方案，至少为一个特殊类别的流动。一个真实的非分区混合技术是首选的，因为它避免了RANS和LES区域的预定义，导致一种方法，其中合适的模拟技术或多或少自动选择。虽然结合LES-RANS方法的想法并不是新的，各种开放的问题必须回答。这特别包括对LES和RANS之间的适当耦合技术、自适应控制机制和适当的SGS/RANS模型的需求。由于近似壁面边界条件部分依赖于RANS建模，并且经典壁面函数是近壁区域RANS模型的极限情况，因此本提案的两个主题，即，壁建模和LES-RANS耦合是紧密相连的。为了研究上述开放性问题，在第一步中，肯定不可避免地要考虑一些几何上简单的试验情况，包括压力引起的流动分离和随后的再附着，例如，试验配置T2，周期性山丘上的流动。当然，最后的目标是实际感兴趣的流动构型，包括高Re情况。因为所有的合作伙伴都已经研究过大迎角翼型的绕流，所以这种布局（试验例T6）将作为第二个更复杂的试验例。作为一个长期的目标，一个几何三维配置的实际利益将进行调查。一种选择是简化的汽车模型（Ahmed车身，测试用例T4）周围的流动。特别是对于混合方法，所选择的流动配置应该包括可以通过RANS合理解决的区域以及另外的复杂流动特征，例如大尺度流动分离，这肯定需要LES而不是RANS。这类流动对于壁面建模技术也是极具挑战性的，壁面建模技术允许基于相同的测试用例对两种方法进行直接比较和评估。LIMSI奥赛和LSTM埃尔兰根的CFD团队在复杂流动的LES和DNS的开发和应用方面拥有长期的经验。在南特中央理工学院，数值组在复杂几何的RANS计算方面有多年的经验。由于这些主要专业知识领域的互补性，预计合作将特别富有成效，产生强大的协同效应。