“…However, this method is limited by a hypothesis of slow variation in the baseflow along the propagation direction, which includes also any variation in the transverse boundary conditions. A derivation of these One-Way equations has been presented in [20] and applied to the problem of a partially lined duct in [21]. This method is based on the full numerical construction of the projection operators that are needed to take into account the reflection and transmission of waves across any variation.…”