### 12.1.7. Embedded/Zonal LES (ELES, ZLES)

The idea behind ELES is to predefine different zones with different treatments of turbulence in the preprocessing stage. The domain is split into a RANS and a LES portion ahead of the simulation. Between the different regions, the turbulence model is switched from RANS to LES/WMLES. In order to maintain consistency, synthetic turbulence is generally introduced at RANS-LES interfaces. ELES is actually not a new model, but an infrastructure that combines existing elements of technology in a zonal fashion. The recommendations for each zone are therefore the same as those applicable to the individual models.

In ANSYS Fluent, an Embedded LES formulation is available (Cokljat et al., 2009 [2]). It allows the combination of most RANS models with all non-dynamic LES models in the predefined RANS and LES regions respectively. The conversion from modeled turbulence to resolved turbulence is achieved at the RANS-LES interface using the Vortex Method (Mathey et al., 2003 [15]).

In CFX, a similar functionality is achieved using a method called Zonal Forced LES (ZFLES) (Menter et al., 2009 [17]). The simulation is based on a pre-selected RANS model. In a LES zone, specified via a CEL expression, forcing terms in the momentum and turbulence equations are activated. These terms push the RANS model into a WMLES formulation. In addition, synthetic turbulence is generated at the RANS-LES interface,

There is an additional option in ANSYS Fluent that involves using a global turbulence model (SAS, DDES, SDES, SBES), and activates the generation of synthetic turbulence at a pre-defined interface. The code takes care of balancing the resolved and modeled turbulence through the interface. This option can be used to force global hybrid models into unsteadiness for cases where the natural flow instability is not sufficient. Unlike ELES, the same turbulence model is used upstream and downstream of the interface. In ELES, different models are used in different zones on opposite sides of the interface.

Such forcing can also be achieved in ANSYS CFX by specifying a thin LES region and using the SAS or SBES model globally. The SBES model is most suitable for this scenario.

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