

3.7 Artificial wind
method
The fact that classical hydrodynamic equations are Galilean
invariant (Lorentz invariant in the relativistic case) is exploited
in the artificial wind (AW)
method [264
]. One chooses a
reference frame where the flow through zone interfaces is always
supersonic. This reduces the problem of upwinding to a trivial task
(avoiding the need of any spectral decomposition of the flux
Jacobians). In case of the global AW
method, the choice of the reference frame is global, whereas in
case of the local AW method an
appropriate choice is made at every numerical interface which
reduces the numerical diffusion. Explicit expressions for the
velocities of the reference frames (AW velocities) are given to
ensure stability and to reduce diffusion. The resulting expressions
for the numerical flux coincide formally with those of the HLL
method. In the differential AW method,
AW velocities are chosen as low as possible for each of the
intermediate states between contiguous numerical zones obtained
using weighted linear interpolations.

