Table 4: Comparison between RANS and LES.

RANSLES

The velocity field equations’ quantities are averaged with respect to time.The equation of velocity field quantities is averaged with respect to space.

The eddy structures are not solved, and therefore the model has to be calibrated with respect to the type of turbulent current under consideration, wherefrom their behaviour depends. The eddy structures within the inertial range of the turbulent spectrum are solved by using a sufficiently fine grid, while the subgrid scales of motion are parameterized and modeled, as they can be considered isotropic.

The motion can also be solved in two dimensions, if it is averagely static.The motion can only be solved in three dimensions, because of the constant presence of the time derivatives due to the unstatic behaviour of the solved eddy structures.

The equations’ number is cut within a predetermined order, and the unknown variables belonging to a higher order are modeled with approximate relations.The equations’ number is cut with respect to the position within the inertial range, so the difference between what is computed and what is modeled is energetic rather than geometric.

The closure problem is introduced by the Reynolds’s stresses’ tensor and is generally solved by using the turbulent cinematic viscosity, which often causes blunders.The closure problem is introduced by the subgrid stresses’ tensor and is solved by subgrid models, which have no significant influence on the solution’s precision.

High efficiency, especially for the k-e based closure.Extra efficiency and computational precision.

Low computational effort and CPU memory required.High computational effort and CPU memory required.