Analysis of the acoustic flow at an abrupt change in section of an acoustic waveguide using particle image velocimetry and proper orthogonal decomposition

In the vicinity of an abrupt change in cross section, an acoustic wave generates a nonlinear flow. This is investigated experimentally using Particle Image Velocimetry ( PIV). The effect of both the acoustic level and the radius of curvature of the abrupt change in section on the flow is studied. At sufficiently high acoustic levels, and past a value of about 0.5 for the Strouhal number, the flow separates and a vortex is formed. Its evolution with the different parameters is studied. Proper Orthogonal Decomposition ( POD) is applied to the ensemble of phase-averaged flow fields in the vicinity of the abrupt change. It is used as a means to separate the global acoustic movement from the localized non linear movements induced by it. The quantity of energy. owing from the first ( acoustic) mode toward the higher ( nonlinear) modes is calculated and is shown to be largely governed by the Strouhal number.