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 often refered to as the Boussinesq eddy viscosity assumption and it was first formulated by Boussinesq in 1877.

The eddy viscosity is also commonly called the turbulent viscosity and it is normally written as .