For CFD users, it is important to understand that turbulence modeling is an advancing technology and that Best Practice advice given some years ago might not be the Best Practice advice of today. When reading the current document, one will find that there is a consolidation in one of the most important subjects in RANS modeling – namely the choice of a scale-equation. While in Ansys CFX, there has historically been a strong emphasis on the -equation, this is not the case in the Ansys Fluent community, which still has a significant use of models. To simplify turbulence model selection on the user side, as well as turbulence model development on the Ansys side, it is desirable to unite most users behind a single model family. The model family of choice in Ansys-CFD is based on the -equation. This does not mean that -equation based models will be discontinued from the codes, but it does mean that the majority of Ansys turbulence modeling efforts will focus on the -equation family. Making such a choice requires good arguments, and one of the intentions of this document is to provide that rationale and to convince the Ansys CFD user base of its merits.

One of the historic reasons for having a multitude of turbulence models in a CFD code is the ability to select the most accurate model for a given application. When focusing on a single model family, it is necessary to address this issue and to provide the same flexibility within that given formulation. This is achieved by a concept called Generalized (GEKO) model [Generalized k-omega Two-Equation Turbulence Model (GEKO) in Ansys CFD] and [10]. GEKO is an -equation-based turbulence model framework, which introduces free parameters into the equations, which can be selected and tuned by the user within given ranges, without negative impact on the basic model calibration. Instead of switching between different turbulence models to optimize accuracy, it is anticipated that the user stays within the GEKO model and optimizes its free parameters. The parameter range of the GEKO model is designed to cover a wider solution space than the one historically available through different model families. Furthermore, specific parameter choices for GEKO constitute an exact transformation of the standard model (albeit with a superior wall treatment), whereas another parameter setting mimics e.g. the SST model. The need to use historic models is thereby drastically reduced. There is also a detailed Best Practice Report on how to apply the GEKO model [Generalized k-omega Two-Equation Turbulence Model (GEKO) in Ansys CFD].

Consistent with the above reasoning, the current document will not go through all historic model choices available in both CFD codes, but will provide a guide as to the most optimal selection and usage of the preferred models. This will make for a much easier read, which is already a significant benefit of the proposed strategy. However, the rationale of selecting the -equation will be provided, and the pros-and-cons of different modeling concepts will be discussed.

One-equation models are also part of the current Best Practice discussion but will be treated only briefly. The reason is that these models are not suitable for a building-block approach, where different elements of physics need to be combined seamlessly to form a complete modeling framework. This does not mean that the use of such models cannot be suitable in the specific application area for which they have been developed, but they do not form the main track of turbulence models in Ansys CFD codes.