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We present two novel approaches to improve the behaviour of DES in the region where an attached boundary layer (handled with RANS) flows into a separated shear layer (to be resolved using LES). The approaches aim to be generally-applicable and retain the non-zonal nature of DES. Furthermore, the formulations are local and can be readily implemented in general-purpose solvers. One approach introduces an adaptive grid scale definition, sensitised to the local vorticity orientation. The second approach, which can be combined with the first, involves the incorporation of alternative SGS model formulations that discern between quasi 2D and developed 3D flow states. Both modifications lead to a strong reduction of eddy viscosity in the early shear layer. Consequently, a significant acceleration of RANS to LES transition is demonstrated for a plane shear layer, a backwards-facing step and a round jet, with results from two different flow solvers shown. The greatest improvement is seen when the approaches are applied in combination.

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A detailed validation study is presented for the detached-eddy simulation (DES) of the flow around a circular cylinder at a high sub-critical Reynolds number. Direct comparability with unsteady experimental field data is ensured by the confined and clearly-defined geometry. The combination of DES with an appropriate low-dissipative hybrid numerical convection scheme and high temporal resolution delivers excellent agreement with the experiment for the time and phase-averaged fields and spectral content. A strong sensitivity of the solution to the numerical time step size has been identified, which is attributed to time-filtering effects damping the development of resolved turbulence in the early shear layer. Recommendations are made concerning a CFL-type criterion for the temporal resolution of DES, and the remaining small discrepancies are attributed to a still insufficient time resolution, giving a clear direction to future studies.

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The objective of this work is the comparison of three different DES-style hybrid RANS/LES implementations based on the Wilcox k-ω model. The three variants are designed to investigate alternative methods of substitution of the DES length scale within the background Reynolds Averaged Navier-Stokes (RANS) model. Basis for comparison is provided by both the idealised case of the decay of isotropic turbulence (DIT) as well as the practical case of the massively separated, turbulent flow around an airfoil at high angle of attack. The results of the investigations are discussed in detail, the outcome of which is an emphasis of the importance of DIT as a method for calibration, as well as of the relative freedom with which alternative DES-inspired approaches can be implemented for flows of practical relevance.

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