The propagation of time harmonic sound waves is usually described by the Helmholtz equation. When the pressure source is moving with respect to the fluid, or vice versa, the wave length is different in different directions - the phenomenon is referred to as the Doppler effect. In Elmer solver the Doppler effect can be taken into account by defining a constant velocity field over the computational domain (referred to as the convection velocity).

The figure below represents the sound waves produced by a simple vibrating pressure source in the middle of the computational domain. The fluid is moving with a constant velocity in the horizontal direction. Because of the movement, the wave length is shorter and the amplitude smaller in the upwind direction.

Here you can download an animation of the results. 2D-view: mpeg: [~110KB]. 3D-view: mpeg: [~90KB]