The differentially heated rotating annulus is a widely studied tabletop-size laboratory model of the general mid-latitude atmospheric circulation. The two most relevant factors of cyclogenesis, namely rotation and meridional temperature gradient are quite well captured in this simple arrangement. The radial temperature difference in the cylindrical tank and its rotation rate can be set so that the isothermal surfaces in the bulk tilt, leading to the formation of baroclinic waves. The signatures of these waves at the free water surface have been analyzed via infrared thermography in a wide range of rotation rates (keeping the radial temperature difference constant) and under different initial conditions. In parallel to the laboratory experiments, five groups of the MetStröm collaboration have conducted numerical simulations in the same parameter regime using different approaches and solvers, and applying different initial conditions and perturbations. The experimentally and numerically obtained baroclinic wave patterns have been evaluated and compared in terms of their dominant wave modes, spatio-temporal variance properties and drift rates. Thus certain "benchmarks" have been created that can later be used as test cases for atmospheric numerical model validation.

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A technique is proposed for zonal coupling of Large Eddy Simulation (LES) with a downstream Reynolds-Averaged Navier-Stokes (RANS) calculation. At a pre-defined interface, mean velocities are coupled and velocity fluctuations of the LES zone are removed by employing a convective boundary condition. For incompressible flow, the handling of pressure at the interface is crucial to the success of the method. Global coupling as well as decoupling of the pressure are investigated. The latter is more robust and therefore more generally applicable, but it requires an additional mass flux correction at the interface. The resulting approach is used to explain observed short-comings of the so-called enrichment strategy when applied to downstream LES-RANS coupling and it represents an improved and more generally valid method without the need for calibration constants. The performance of the proposed method is scrutinised for turbulent flow in a channel and over periodic hills. The results corroborate the predicted increase in accuracy and robustness.@en

The turbulent flow in two asymmetric diffusers with complex three-dimensional separation was computed employing Large-Eddy Simulations (LES) and Reynolds-Averaged Navier-Stokes (RANS) calculations. The computational setup matches existing experiments in the literature. The objective of the present study is to obtain reference data to be used for assessing the performance of newly developed hybrid LES/RANS techniques.@en

The paper presents developments for a segregated approach to the coupling of Reynolds-Averaged Navier-Stokes (RANS) calculations with a zone computed as Large-Eddy Simulation (LES). The mean velocity fields are matched at predefined interfaces and velocity fluctuations of the LES zone are treated according to the type of interface. If the RANS zone is downstream of the LES, fluctuations are allowed to leave the domain by employing a convective boundary condition. If the RANS zone is placed between the LES and a wall, the fluctuations at the interface are scaled to match the statistics predicted by the RANS computation. The proposed method was applied to turbulent channel flow and the flow over periodic hills. It was found that the method delivered an improvement over alternative techniques in the literature while removing the need for calibration constants. For incompressible flows, it is also necessary to prescribe conditions for the pressure or an equivalent variable. Several alternatives were tested. Decoupling of the pressure combined with explicitly enforced mass conservation at the interface yielded the best and, for the hill flow with aggressively placed interface, the only acceptable results.@en

The paper presents hybrid LES/RANS computations of turbulent flows with a segregated approach. This approach employs strict steady RANS and strict LES in pre-defined regions of the computational domain coupling the solution between them by specifically designed interfaces. The latter can be obtained by enhancement of the interfaces needed for domain decomposition in any block structured code. The paper covers inflow and outflow conditions of the LES subdomain when linked to a RANS domain as well as tangential coupling. Compressible as well as incompressible simulations are reported.@en

The coupling of large eddy simulation (LES) with statistical turbulence models, i.e. Reynolds-Averaged Navier-Stokes (RANS) models, is arguably the main strategy to drastically reduce computational cost for making LES affordable in a wide range of complex industrial applications. The present paper presents a coherent review of the various approaches proposed in the recent literature on this topic. First, basic concepts and principal strategies highlighting the underlying ideas are introduced. This culminates in a general scheme to classify hybrid LES/RANS approaches. Following the structure of this novel classification, a larger number of individual methods are then described and assessed. Key methods are discussed in greater detail and illustrated with examples from the literature or by own results. The aim of the review is to provide information on how to distinguish different methods and their ingredients and to further the understanding of inherent limitations and difficulties. On the other hand, successful simulation results demonstrate the high potential of the hybrid approach.

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A RANS calculation is connected to an upstream LES via explicit coupling conditions at a pre-defined interface. The role of the interface is to allow for mean flow information to propagate upstream and for fluctuations to leave the LES domain without reflections. To this end, the mean velocity is directly coupled across the domain boundaries whereas the fluctuations at the downstream end of the LES zone are treated using either the so-called enrichment strategy (Quéméré & Sagaut, 2002) or a parameter-free generalization of this method based on a convective condition (von Terzi, Fröhlich & Mary, 2006). For incompressible flows, both techniques require a complementary coupling condition for the pressure or an equivalent variable enforcing continuity. Two distinct techniques are investigated: (i) The instantaneous pressure is computed in a coupled fashion for the union of the LES and RANS domains and (ii) the pressure is completely decoupled and mass conservation across the interface is ensured by an adjustment of the velocities on both sides. The performance of the different methods is scrutinized for turbulent channel flow and the flow over periodic hills. It was found that the convective condition with decoupled pressure fields and an explicit mass flux correction was the most robust technique delivering results of equal or increased quality in comparison to other combinations considered.@en