Any of the following boundary conditions can be applied to any boundary, regardless of whether it is one of the mesh extremes or a geometry surface. The desired boundary conditions can be selected with the solver parameters surface_bc and extremal_bc00, extremal_bc01, extremal_bc10, extremal_bc11, extremal_bc20, extremal_bc21.

Freestream

{freestream}
Sets the boundary state equal to the freestream state. For viscous flows, none of the transport fluxes are modified. Mathematically, this is well-posed only for supersonic inflow. Numerically, you can usually get away with it as long as the boundary is far away from any wall boundaries or interesting flow features. However, if you try to apply it to an outflow boundary in a viscous flow, you may introduce spurious noise as the boundary layer or wake is artificially accelerated to the freestream speed.

Characteristic

{characteristic}
The most generally applicable farfield boundary condition. Sometimes referred to as "Riemann Invariants" boundary condition. In a nutshell, it sets the state as close to the freestream state as possible without overdefining the boundary condition. It decomposes both the flow state and the freestream into the characteristics of the locally linearized Euler equations, and then recombines into a single state vector using the outgoing characteristics from the flow state and the incoming characteristics from the freestream. For viscous flows, the transport fluxes of the outgoing characterstics are set to zero and those of the incoming characteristics are unmodified. This BC is well-posed regardless of the flow direction and Mach number. Thus it can be used for farfield boundaries that aren't actually very far away and outflow boundaries that ingest a wake or boundary layer. The drawback is that it is not very robust and will sometimes cause a crash in hypersonic flows. A more robust alternative is Pressure Outflow.

Pressure Outflow

{pressure_outflow}
An outflow boundary condition that balances robustness and generality. For supersonic outflow, the flow state is simply left alone. For subsonic outflow, the pressure is set equal to the freestream pressure. For inflow, this boundary condition is invalid (it is not numerically stable, let alone well-posed). For viscous flows, all of the transport fluxes are set to zero. This technically makes it ill-posed (specifically, overdefined) for subsonic outflow, and is an artifiact of pure laziness, since you can usually get away with it. In the future, I intend to set the appropriate number of transport fluxes to make this boundary condition well-posed.

Outflow

{outflow}
Does not constrain the state and sets the viscous flux to zero. This is valid for supersonic outflow boundaries. For genuinely inviscid flow, it is not valid for any other type of boundary. For viscous flow, the math becomes less clear (since you are technically adding a constraint on every flow variable by setting the flux to zero), but I have been able to get away with it for some outflow boundaries with limited subsonic regions. Indeed, for the hypersonic cylinder test case, it produces quite nice results.

Nonpenetration

{nonpenetration}
A slip wall boundary condition. The normal component of the momentum is set to zero, and otherwise the state is unmodified. All transport fluxes except that of normal momentum are set to zero. This boundary condition should be used for walls in inviscid flows and for symmetry planes in any flows.

No Slip

{no_slip}
A no-slip wall boundary condition. The entire momentum vector is set to zero, and the density is unmodified. The transport flux of the momentum is unmodified, and that of the density is set to zero (although it should be zero to begin with). Depending on the value of thermal_bc, either the energy or the transport flux of energy will be prescribed, and the other will be unmodified. This boundary condition is only valid for wall boundaries in viscous flows. Some users may be tempted to use this for walls in inviscid flows because they can get away with it in a finite volume solver, but this is not acceptable. Due to the Local Lax-Friedrichs interface flux and the dispersive nature of numerical errors in high-order DG schemes, attempting to use this in an inviscid flow will result in severe spurious oscillations.