**6. RANS and LES capacities for turbulent flows analysis**

Originally, RANS models were conceived to include the turbulence average effect in the Navier–Stokes equations solution. Therefore, they are widely recommended to simulate steady-state fully developed turbulent flows, being especially good reproducing off-wall phenomena. When the problem needed to solve is highly depended on the adequate prediction of pressures and forces on the wall, it is recommended to apply functions for "enhanced wall treatment" and to have at least one nodal element positioned within the viscous sublayer (y+ < 5), with at least ten nodal elements along the whole boundary layer to achieve acceptable results [23]. Another use of RANS models is to provide preliminary results that allow to shorten the simulation time for unsteady problems based on the same technique or the use of LES. Regarding the kind of flows that RANS models have been successful reproducing, it can be said that internal flows, flows over boundary layers, turbulence prediction in a fluid that floats due to density changes, combustion coupled problems, two-phase flows, agitated wall induced turbulence, etc. have had successful results. As for problems where RANS models perform poorly, these are periodic or oscillatory flows, boundary layers affected by large pressure gradients, flow regimes that depend on separation point, and jets.

On the other hand, the LES technique is capable of giving average and instantaneous information about the main structures of turbulent flows without the complete dependence on the average modeling of Reynolds stresses found in the traditional RANS approach. This is advantageous to capture velocities and flow patterns in three dimensional problems. Among the successes of the LES model, it can be mentioned the prediction of separation points and fluctuating forces in external flows for geometries such as circular cylinders, eolic turbines blades, as well as structures and frequencies in jets and atomization systems, etc. Unfortunately, due to the large number of temporal and spatial scales that requires modeling for an acceptable performance, computer time and requirements are still high. The walls produce high demands since it requires the use of grids with nodal elements positioned at y+ ≈ 1, very short temporal steps and the size of the eddy scales that the smallest grid elements are capable of capture. This has made impossible the massive application of this technique for flow problems that have many complex geometries and many scales. It results evident that for compressible flows, the Reynolds stresses are more complex than for incompressible flow, being that SGS models for complex flows those that involve multiple phases and reactive flows found in combustions are still in a stage of development and validation [24].
