Modeling subgrid viscosity for advection-diffusion problems

We analyse the effect of the "subgrid viscosity" on a finite element discretisation, with piecewise linear elements, of a linear advection-diffusion scalar equation. We point out the importance of a proper tune-up of the viscosity coefficient, and we propose a heuristic method for obtaining reasonable values for it. The extension to more general problems is then hinted in the last section.