Eddy-resolving approaches
For most industrial applications, large eddy simulation remains very expensive, even unaffordable, except for a few niche applications, and there is considerable interest today for intermediate methods between RANS and LES. Indeed, the knowledge of the unsteadiness of global quantities is crucial in many fields: peak forces, noise prediction and control, material fatigue due to force or temperature fluctuations, combustion, etc. A multitude of low-cost unsteady strategies, in particular hybrid RANS/LES, has been developed during the last two decades.
Some of these approaches are qualified as zonal, in the sense that they are based on a division of the domain into several sub-domains, in which a classical model, RANS or LES, is used. All difficulties are then left to the interfaces between these domains, in particular the generation of unsteady boundary (or interfaces) conditions for the LES.
Other methods are qualified as continuous (or non-zonal, or global), because in this case there is no partitioning of the domain, but a continuous transition of the model used from a RANS behavior to a LES behavior. In this last case, different difficulties arise, such as, first, the definition of an adequate formalism, and, above all, the modeling of the interaction between the unsteady resolved motion and the unsolved motion.
