Symmetry
The Symmetry boundary condition prescribes no heat flux, no penetration and vanishing shear stresses. The boundary condition is a combination of Dirichlet conditions and a Neumann condition.
 
These conditions correspond to the ones for the Single-Phase Flow, Laminar Flow interface. See Inlet and Outlet.
For a background on the derivation and implementation of the conditions, see Theory for the High Mach Number Flow Interfaces.
Selecting appropriate outlet conditions for the Navier–Stokes equations is not a trivial task. Generally, if there is something interesting happening at an outflow boundary, extend the computational domain to include this phenomenon.