Hybrid LES/RANS for improved heat transfer predictions in combustion engines

Large Eddy Simulation (LES) is very expensive in wall-bounded flow. To be able to extend LES to high Reynolds number many proposals have been made in the literature to combine LES with unsteady RANS (URANS) near the walls. The first and most common method is Detached Eddy Simulation
(DES). Later, other researchers have proposed hybrid LES/RANS and Scale-Adapted Simulations (SAS); for a review. DES and hybrid LES/RANS use the cell size as the SGS length scale. SAS does not use the cell size (except as a
limiter) but uses the karman lengthscale instead.

The main object of the methods mentioned above is that they reduce the
turbulent viscosity in the LES region. There are three options to achieve this.

* The turbulent viscosity, nu_t, is reduced by modifying its formulation;

* The dissipation term in the equation for the modeled, turbulent kinetic energy, k, is increased by decreasing the turbulent lengthscale, or

* the destruction term in the lengthscale equation (epsilon or omega) is decreased. This increases the dissipation term in the k equation as well as decreases the turbulent viscosity directly since epsilon (or omega) appears in the denominator of the expression for nu_t.