Functional models are simpler than structural models, focusing only on dissipating energy at a rate that is physically correct.  These are based on an artificial eddy viscosity approach, where the effects of turbulence are lumped into a turbulent viscosity.  The approach treats dissipation of kinetic energy at sub-grid scales as analogous to molecular diffusion.  In this case, the deviatoric part of  is modeled as:
where  is the turbulent eddy viscosity and  is the rate-of-strain tensor.
Based on dimensional analysis, the eddy viscosity must have units of  .  Most eddy viscosity SGS models model the eddy viscosity as the product of a characteristic length scale and a characteristic velocity scale.