Eddy viscosity
From CFD-Wiki
The transfer of momentum caused by turbulent eddies is often modeled with an effective eddy viscosity in a similar way as the momentum transfer caused by molecular diffusion (i.e. friction) is modeled with a molecular viscosity. The hyptheses that the effect of turbulent eddies on the flow can be modeled in this was is called Boussinesq eddy viscosity assumption and was first formulated by Boussinesq in 1877.
