Large Eddy Simulation (LES) Technique
LES takes advantage of the fact that the smallest dissipative eddies are isotropic and can be efficiently modeled using simple subgrid scale models. Meanwhile, the dynamics of larger eddies, which are anisotropic, is simulated using a grid and time step sufficiently fine to resolve them accurately. The method, therefore, is applicable to flows at relatively high Re numbers but is still expensive for use as an engineering design tool.
14.6.2 Detached Eddy Simulation (DES) Technique
DES is considered halfway between the LES and the Reynolds Averaged Navier – Stokes (RANS) techniques. The method employs a RANS turbulence model for near-wall regions of the flow and a LES-like model away from the wall. The method was first proposed by Spalart et al. in 1997 and is still the subject of research. It may become a standard engineering tool, but it is currently unlikely to be an element of the conceptual and preliminary design toolkits.
14.6.3 RANS Equation Technique
The time-dependence of turbulent fluctuations is averaged to form the RANS equations. This results in the appearance of the so-called Reynolds stresses in the equations, and the modeling of these equations (i. e., turbulence modeling) becomes problematic. There are many turbulence models but each falls prey to the fact that turbulence is flow-dependent; consequently, no turbulence model can be generally applicable, and a CFD practitioner must be cognizant of the strengths and failings of the models employed. Nonetheless, RANS allows relatively inexpensive modeling of complex flows; when allied to a suitable optimization method, it can be a powerful tool for design synthesis.