Sidebar: All about Eddy
Airfoil modeling is not a new science, although steady advancements in computing power enable the creation of simulations with an unprecedented degree of complexity. Several methods exist for expressing fluid flow in mathematical terms.

Large eddy simulation (LES) is a technique for solving the differential equations associated with turbulent flow. The system of airfoil and flowing fluid is represented by points on a grid, and calculations are carried out at these points. A finer-meshed grid produces a greater accuracy, but is more expensive in terms of computer resources and time requirements. In the LES method, simulations are generated for only the larger eddies, and the smaller eddies are handled using a sub-grid scale model. LES represents an intermediate level between direct numerical simulation (DNS), which models turbulent motion at all relevant scales, and the more approximate Reynolds-averaged Navier–Stokes (RANS) model.

In variational multiscale LES, calculations are separated into two or more groups that address different size scales. The filters used in the LES method are replaced by variational projections, an approach that produces greater efficiency and accuracy than the conventional LES method.

Detached eddy simulations (DES) uses a RANS method to solve regions that have less than a specified turbulent length scale and that are near solid boundaries. If the turbulent length scale exceeds the grid dimension, the regions are solved using LES. The DES method requires less grid resolution than the LES method, which reduces the cost of the computation.

Source: AHPCRC Bulletin, Vol. 1, Issue 3 (2009)