Ask the Turbulence Expert: Dr. Florian Menter

Ask the Turbulence Expert: Dr. Florian Menter

What is the best turbulence model to use?

There is no single best turbulence model. Ansys believes in providing the widest range of turbulence modeling capabilities so the user can choose the right tools for the specific job at hand. While today’s CFD simulations are mainly based on Reynolds-averaged Navier-Stokes (RANS) turbulence models, it is becoming increasingly clear that certain classes of flows are better covered by models in which all or a part of the turbulence spectrum is resolved in at least a portion of the numerical domain. Such methods are termed scale-resolving simulation (SRS) models.

When should SRS models be used?

There are two main motivations for using SRS models in favor of RANS formulations. First, users need additional information that just cannot be obtained from RANS simulations. For example, acoustics simulations can include turbulence-generated noise sources, which cannot be extracted with accuracy from RANS simulations. Other examples include unsteady heat loading in unsteady mixing zones of flow streams at different temperatures — which can lead to material failure — or multi-physics effects like vortex cavitation, in which the unsteady turbulence pressure field is the cause of cavitation. In such situations, SRS can deliver more engineering insight even in cases where the RANS model would, in principle, be capable of computing the correct time-averaged flow field.

The second reason for using SRS models is related to accuracy. RANS models are strongest for wall-bounded flows, where the calibration according to the law of the wall provides a sound foundation for further refinement. But their performance is limited in other flow situations.

Where do RANS models show their limitations?

For free shear flows, the performance of RANS models is much less uniform. There are a wide variety of such flows, ranging from simple, self-similar flows such as jets, mixing layers, and wakes to impinge flows, flows with strong swirl, massively separated flows, etc. Considering that RANS models typically have limitations covering the most basic self-similar free shear flows with a single set of constants, there is little hope that even the most advanced Reynolds stress models (RSM) will eventually provide a reliable foundation for all such flows.

Typically, for free shear flows, it is much easier to resolve the largest turbulence scales, as they are of the order of the shear layer thickness. Within wall boundary layers, however, the turbulence length scale near the wall becomes very small relative to the boundary layer thickness (increasingly so — at higher Re numbers). This poses severe limitations for large-eddy simulations (LES), as the computational effort required far exceeds the computing power available to industry. For this reason, hybrid models are being developed to resolve large eddies away from walls, cover wall boundary layers by a RANS model. Examples of such global hybrid models have detached eddy simulation (DES) and scale-adaptive simulation (SAS). More recent developments are the shielded detached eddy simulation (SDES) and the stress-blended eddy simulation (SBES) proposed by the Ansys turbulence team.

Are there other hybrid modeling approaches?

An additional step is to apply a RANS model to only the innermost part of the wall boundary layer, and then to switch to an LES model for the main part of the boundary layer. Such models are termed wall-modeled LES (WMLES). Finally, for large domains, it is frequently necessary to cover only a small portion with SRS models, while the majority of the flow can be computed in RANS mode. In such situations, zonal or embedded LES methods are attractive as they allow the user to specify ahead of time the region where LES is required. Such methods are typically not new models in the strict sense, but they combine existing models/technologies in a flexible way in different portions of the flow field. Important elements of zonal models are interface conditions, which convert turbulence from RANS mode to resolved mode at pre-defined locations. In most cases, this is achieved by introducing synthetic turbulence based on the length and time scales from the RANS model.

There are many hybrid RANS-LES models — often with somewhat confusing naming conventions — that resolve eddies of different sizes. SRS models are very challenging in their proper application to industrial flows. The models typically require special attention to various details, including:

  • Model selection
  • Grid generation
  • Numerical settings
  • Solution interpretation
  • Post-processing
  • Quality assurance

How should engineers choose their turbulence model? How do you get the right model for the job at hand?

Unfortunately, no unique model covers all industrial flows, and each individual model poses its own set of challenges. The user of a CFD code must understand the intricacies of the SRS model formulation in order to select the optimal model and use it efficiently.

Ansys provides Theory and User documentation that describes in detail how to select and activate these models in Ansys CFD. We also provide a best practice application brief for a general understanding of the underlying principles and the associated limitations of each of the described modeling concepts. It also pairs flow types with suitable models and identifies which combinations should be avoided. The impact of numerical settings on model performance is also discussed.

What is new with Turbulence modeling?

A new, single-equation laminar transition model is making a buzz in the industry.

Modeling the transition from laminar to turbulent flow has been one of the most difficult challenges of computational fluid dynamics (CFD), even though many industrial flows have Reynolds numbers in the range of 10^4 to 10^6 — regimes in which significant portions of the boundary layers can be laminar. Our Ansys team succeeded in solving this problem about 10 years ago with the local-correlation-based transition modeling (LCTM) approach. LCTM successfully introduced transition effects into general CFD. The first model (named γ-ReΘ) solved two transport equations and incorporated experimental correlations to trigger the transition onset. The model formulation was strictly local and, therefore, fully compatible with modern general-purpose CFD codes.

We recently published a second-generation model that simplifies the original γ-ReΘ model of the LCTM concept, by reducing the number of equations to be solved from two to one. The new transition model (called the γ-model) is now available in Ansys CFD solutions.

By reducing the number of transport equations to be solved, the new γ-model substantially decreases the complexity and solution time of boundary layer simulations. The γ-model is also more robust because an even wider range of flows, both generic and industrial, was considered during model calibration, relative to the γ-ReΘ model.

About Dr. Florian Menter

A world-recognized expert in turbulence modeling, Dr. Menter developed the widely used shear-stress transport (SST) turbulence model, which has set a milestone in the accurate prediction of aerodynamic flows. He has also contributed to the formulation of one-equation turbulence models, and advanced near- wall treatment of turbulence equations, transition modelling and unsteady flow models. He has been in charge of the turbulence modeling program at Ansys for more than 17 years and has been involved in a wide range of industrial modeling challenges. He has published more than 50 papers and articles at international conferences and in international journals. Most recently, Dr. Menter has been involved in the implementation of new turbulence models for unsteady flow simulations, including scale-adaptive simulation (SAS) and embedded/zonal LES models. These models are particularly relevant for the many industrial applications where time-varying information is essential to the engineering outcomes (aerodynamics, acoustics, combustion, fluid-structure coupling, etc.).