Propagation and damping of acoustical waves inside ducts with flow. Effect of section discontinuities

The present work deals with propagation of plane waves inside ducts with flow; waves are assumed to have their origin out of the studied portion; the junction between ducts is made by sudden (or continuous) section change. The purpose is to separate the effect of discontinuity from the other factors acting on propagation. In the light of a theoretical study it is shown how to separate damping owing to upstream and downstream boundaries and damping occurring during propagation (viscosity and yielding walls). The reflection coefficient due to the discontinuity is characterized. This analysis if generalized to the case with flow. An experimental arrangement has been modified in order to isolate in a better way the portion of circuit which is studied. It runs with aspiration from atmosphere, flow velocities extend between 0 and 83m/s. Frequency range is 100-500 cycles/second. A loudspeaker produces a sinusoidal excitation; the level is high enough to reject pressure fluctuations due to local generation. By way of frequency sweeping (slave filter method), total damping of the circuit is obtained. It does practically not depend on the measurement point. The loss factor varies between 1 and 8.10-2; its dependency on flow velocity and discontinuity geometrical characteristics is monotonic. A second method consists in producing a choosen frequency; a computation code plots the modal shape of fluctuating pressure and mass flow along each duct of constant cross section (amplitude and phase) from pressure fluctuations detected at several points. The quantities just before and after discontinuity are obtained; they allow to compute impedance. Values obtained for several contractions and frequencies near eigenfrequencies of the duct are consistent with theory