The energy transport by the propagation of sound waves in wave guides with a moving medium

The problem of the propagation of sound waves radiated by a source in a fluid moving with subsonic velocity between two parallel walls or inside a cylindrical tube is considered in [2], The most interesting thing of this problem is that waves may occur with constant amplitude coming from infinity. This article gives the calculation of the energy transport in the wave guides. It is shown that it is not possible to gain energy from infinity.