Comparison of LES and RANS in bluff-body flows

The turbulent flow around bluff-bodies features a variety of complex phenomena, e.g. streamline curvature, separation and the formation of large unsteady vortical structures. In particular, an accurate representation of the interaction between mean transient motion and residual turbulence poses a challenge to numerical simulation procedures. In virtually all commercial simulation packages, the representation of turbulence relies on Reynolds-averaged Navier-Stokes (RANS) equations in conjunction with Boussinesq-viscosity models (BVM). Various studies have demonstrated the inability of established RANS methodologies to render the fundamental physics when applied to transient flows. In contrast to this, the computationally more demanding large eddy simulation (LES) is known to be a viable approach to simulate unsteady turbulent flows. The present study aims to assess the predictive prospects of advanced recent RANS practices, i.e. explicit algebraic stress models (EASM), in unsteady bluff-body flows. Results are reported in comparison to conventional BVM, LES and measurements. Examples included refer to three different cylinder flows, which indicate that the predictive accuracy obtained from an EASM is in close proximity to LES results, whereas the computational surplus remains moderate in comparison with a linear BVM.

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Comparison of LES and RANS in bluff-body flows