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Large Eddy Simulation (LES) calculations are highly resource (CPU and memory) intensive and thus generally impractical for complex engineering flows, while RANS calculations are not strictly applicable to unsteady flows (note that the Navier-Stokes equations are temporally averaged to obtain mean flow equations plus unsteady fluctuating components that are modeled). Evolving techniques of combining traditional RANS with LES (hybrid RANS/LES) or with Detached Eddy Simulation (DES) now offer affordable alternatives.

The idea is to simulate only the largest turbulent eddy structures that can be adequately resolved on a given mesh. The smallest remaining structures and the turbulence energy contained within them are then modeled using a subgrid scale model. This approach allows a more appropriate representation of the unsteady turbulent fluctuations than RANS alone, and it is computationally feasible for transient flows over quite complex flows.

The example above shows the flow over a backward facing step using isocontours of the Q criterion (Q = 200 1/sec) colored by streamwise velocity to illustrate the turbulent flow structures. When the quantity Q is positive, the local fluid rotation dominates over shear stress which indicates that turbulent, energy-containing eddies are being resolved.

Read More About Hybrid RANS/LES

Professor Lars Davidson at the Chalmers University of Technology (Göteborg, Sweden) maintains a nice site that, among other CFD topics, includes a nice description of and more detailed information about the hybrid RANS/LES approach.