[Martin Hegedus]

Jinny,

Inviscid, incompressible, steady state flow is irrotational (due to vorticity transport equation), therefore inviscid incompressible steady state Euler is potential flow. Once the bound vortex reaches steady state, vorticity is not shed into the flow field, and then drag goes to zero. However, numerical error causes the bound vortex to decay and therefore vorticity needs to be constantly shed into the field to maintain it (and the implicit kutta condition). If there is no error, then the bound vortex does not decay and eventually all the vorticity in the field is transported away.

On this page is a grid refinement study of the NACA 0012 airfoil at M=0.5 and alpha=1.25. http://www.hegedusaero.com/examples/.../Vassberg.html. You can see that drag drops as the grid is refined. For the case of FLO82-HCUSP, drag basically approaches zero. AO_CFD has a more challenging time probably because of higher artificial dissipation. I assume the same is true for OVERFLOW.