Advance in RANS-LES Coupling, a Review and an Insight on the NLDE Approach

The aim of this article is twofold. The first purpose is to propose a review of existing RANS-LES methods and is addressed in the first part in a comprehensive way, detailing the advantages and the drawbacks of the different techniques. In a second time, a hybrid RANS-LES approach is presented, which can be interpreted as the most general case of the NLDE approach as defined by Morris et al. A decomposition into three parts of the exact solution of the Navier-Stokes equations is considered: mean flow, resolved fluctuations and unresolved (subgrid) fluctuations. The mean flow is computed using a classical RANS method, while resolved fluctuations are derived from a LES method. Several features on this approach are at first discussed in this paper, that are: the development of a non-zero mean for the resolved fluctuations (also called hereafter details), the computational problems due to the use of different schemes and meshes for the RANS and LES calculations, and the use of a boundary condition suited to the fluctuating part of the field. This approach is then used to simulate the acoustic sources of the flow around the slat of a high-lift system in landing configuration. The mean instabilities of the flow are studied and the resulting acoustic near field is carefully investigated.