This review presents the state of the art of hybrid RANS/LES modeling for the simulation of turbulent flows. After recalling the modeling used in RANS and LES methodologies, we propose in a first step a theoretical formalism developed in the spectral space that allows to unify the RANS and LES methods from a physical standpoint. In a second step, we discuss the principle of the hybrid RANS/LES methods capable of representing a RANS-type behavior in the vicinity of a solid boundary and an LES-type behavior far away from the wall boundary. Then, we analyze the principal hybrid RANS/LES methods usually used to perform numerical simulation of turbulent flows encountered in engineering applications. In particular, we investigate the very large eddy simulation (VLES), the detached eddy simulation (DES), the partially integrated transport modeling (PITM) method, the partially averaged Navier-Stokes (PANS) method, and the scale adaptive simulation (SAS) from a physical point of view. Finally, we establish the connection between these methods and more precisely, the link between PITM and PANS as well as DES and PITM showing that these methods that have been built by different ways, practical or theoretical manners have common points of comparison. It is the opinion of the author to consider that the most appropriate method for a particular application will depend on the expectations of the engineer and the computational resources the user is prepared to expend on the problem.

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The State of the Art of Hybrid RANS/LES Modeling for the Simulation of Turbulent Flows

This paper is an overview of the physical mechanisms and length scales governing the propagation of wildfires. One of the objectives is to identify the physical and mathematical constraints in the modelling of wildfires when using a “fully” physical approach. The literature highlights two regimes in the propagation of surface fires, i.e. wind-driven fires and plume-dominated fires, which are governed by radiation and convective heat transfer, respectively. This division leads to the identification of two governing length scales: the extinction length characterising the absorption of radiation by vegetation, and the integral turbulent length scale characterising the interaction between wind and canopy. Some numerical results published during the last decade using a fully physical approach are presented and discussed with a focus on the models FIRESTAR, FIRELES, FIRETEC and WFDS. Numerical simulations were compared with experimental data obtained at various scales, from laboratory to field fires in grassland and in Mediterranean shrubland. Some perspectives are presented concerning the potential coupling between physical fire models with mesoscale atmospheric models to study the impacts of wildfires at larger scale. Some of the topics on wildfire physical modelling that need further research are identified in the conclusions.

Physical Phenomena and Length Scales Governing the Behaviour of Wildfires: A Case for Physical Modelling

Pedestrian level wind environment assessment around group of high-rise cross-shaped buildings: Effect of building shape, separation and orientation.

In the recent years, fluid structure interaction (FSI) models of the aortic valve and root have become increasingly common for two main reasons. The medical reason is that millions of patients suffer from aortic valve disorders. The second reason is that this challenging problem combines several fields of computational mechanics. The key motive for these modeling attempts is their potential to shed light on phenomena that cannot be captured in experiments or in simplified models of solely hemodynamics or structural mechanics. The aim of this paper is to review the state-of-the-art FSI methods in general and their application to the aortic valve in particular. A brief overview of the medical background is provided. The numerical methods and appropriate assumptions are then presented with examples of previous aortic valve models, followed by a discussion of the limitation of current models and recommendations for overcoming them in future research. The methods presented in this paper could help readers to choose the modelling approach and assumptions that are most suitable for their goals.

Numerical Methods for Fluid–Structure Interaction Models of Aortic Valves

The aim of the present work is to develop a seamless hybrid Reynolds-averaged Navier–Stokes RANS large-eddy simulation LES model based on transport equations for the subgrid stresses, using the elliptic-blending method to account for the nonlocal kinematic blocking effect of the wall. It is shown that the elliptic relaxation strategy of Durbin is valid in a RANS steady as well as a LES context unsteady. In order to reproduce the complex production and redistribution mechanisms when the cutoff wavenumber is located in the productive zone of the turbulent energy spectrum, the model is based on transport equations for the subgrid-stress tensor. The partially integrated transport model PITM methodology offers a consistent theoretical framework for such a model, enabling to control the cutoff wavenumber c, and thus the transition from RANS to LES, by making the C 2 coefficient in the dissipation equation of a RANS model a function of c. The equivalence between the PITM and the Smagorinsky model is shown when c is in the inertial range of the energy spectrum. The extension of the underlying RANS model used in the present work, the elliptic-blending Reynolds-stress model, to the hybrid RANS-LES context, brings out some modeling issues. The different modeling possibilities are compared in a channel flow at Re = 395. Finally, a dynamic procedure is proposed in order to adjust during the computation the dissipation rate necessary to drive the model toward the expected amount of resolved energy. The final model gives very encouraging results in comparison to the direct numerical simulation data. In particular, the turbulence anisotropy in the near-wall region is satisfactorily reproduced. The contribution of the resolved and modeled fields to the Reynolds stresses behaves as expected: the modeled part is dominant in the near-wall zones RANS mode and decreases toward the center of the channel, where the relative contribution of the resolved part increases. Moreover, when the mesh is modified, the amount of resolved energy changes but the total Reynolds stresses remain nearly constant. © 2010 American Institute of Physics. doi:10.1063/1.3415254

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A seamless hybrid RANS-LES model based on transport equations for the subgrid stresses and elliptic blending

Modeling and Simulation of Turbulent Flows

Apres une presentation des concepts de base, des outils mathematiques et des methodes de fermeture, cet ouvrage analyse les modeles de fermeture au deuxieme ordre ainsi que les modeles plus simples, puis l'influence de parametres physiques supplementaires : effet du nombre de Reynolds de la turbulence, effet des parois, des forces de gravite, de la courbure et de la rotation, des variations de la masse volumique et du desequilibre spectral. Enfin, il se consacre aux methodes de simulation de grandes echelles turbulentes. Ce livre met l'accent sur la methodologie des fermetures statistiques en tant qu'outil d'analyse et de developpement des modeles de turbulence. La presentation des differents modeles est completee d'illustrations sur les techniques numeriques pour leur mise en oeuvre pratique et l'implementation informatique.

Roland Schiestel

Papers

Modeling and Simulation of Turbulent Flows

Méthodes de Modélisation et de Simulation des Ecoulements Turbulents

Simplified Closures: Two and Three Transport Equation Models