FIG. 2.

Schematic of aeroacoustics problem: find the sound pressure at observer location, x, in an infinitely long duct of uniform cross section (radius R_p) containing a narrow constriction (radius R_min) of axial length H. The air in the pipe far away from the constriction is a combination of steady motion (velocity U) and the sound produced by an unsteady aerodynamic force on the constriction walls at source coordinate y. Forces are due to vorticity ω convected at local velocity v through the constriction.