LES-RANS coupling for the simulation of complex flows

对于复杂的流动，大涡模拟方法比RANS方法有明显的优势，特别是当大尺度结构主导湍流输运时，在过渡情况下，当计算动态力和噪声产生时。然而，由于极高的分辨率和时间平均要求，大涡模拟的成本仍然太高，不能直接应用于实际感兴趣的流动。另一方面，RAN可以在许多情况下以更低的成本确定具有工程精度的平均流量，特别是在复杂流动的子区中经常存在的附着流。因此，对于复杂的高雷诺数流动，在流动特性较简单的分区使用RAN而在更关键的区域使用大涡模拟的混合方法似乎是理想的方法。本项目的目的是以一种所谓的分区办法开发和测试这种方法：RAN和LES将在计算领域的预定义分区中应用，并通过适当的耦合条件联系起来。考虑了三种耦合情况：LES区在RANS区的上游，RANS在LES区的上游，界面与流动相切。法国和德国的合作伙伴将联合开发所有三种类型的耦合程序，并首先测试它们的简单渠道流。然后，这些方法被应用于测试和进一步改进从该小组常见测试案例中获得的日益复杂的流动，例如由德国合作伙伴在山丘上、燃烧室中和汽车形状机身上方流动，以及由法国合作伙伴分离边界层，最后由法国合作伙伴在完整翼型上流动。

LES has clear superiority over RANS-methods for complex flows, especially when large-scale structures dominate the turbulent transport, in transitional situations and when dynamic forces and noise generation are calculated. However, due to the extremely high resolution and time-averaging requirements, LES still is too costly to be applied directly to the flows of practical interest. On the other hand, RANS can determine the mean flow with engineering accuracy at much lower cost in many cases, especially attached flows, which often prevail in subareas of complex flows. Hence, a hybrid method using RANS in subareas with simpler flow behaviour and LES only in the more critical areas appears to be the ideal approach for complex high Reynolds number flows. The present project aims to develop and test such methods in a so-called zonal approach: RANS and LES will be applied in pre-defined subareas of the calculation domain and linked by suitable coupling conditions. Three coupling situations are considered: the LES-region is upstream of the RANS-region, RANS is applied upstream of LES, and the interface is tangential to the flow. The French and German partner will jointly develop coupling procedures for all three types and test them first for simple channel flow. The methods are then applied for testing and further improvement to increasingly complex flows taken from the common test cases of the group, such as to flow over hills, in a combustor and over a car-shaped body by the German partner and to separating boundary layers and finally the flow over a complete airfoil by the French partner.