Flow Modeling
Direct Numerical Simulation (DNS) requires resolving all relevant length scales
Large Eddy Simulation (LES) assumes that an ordering of scales exists such that large scales can be resolved on the computational grid whereas the subgrid scales (SGS) can be modeled
Since in LES only the large scales need to be resolved, the resolution can be much less than the comparable DNS
The equations for the large scales (i.e. for LES) are derived by spatially filtering the DNS equations; the filtering introduces SGS terms which need to be modeled