Definition and realization of a spinning mode simulator

The measurement of acoustic properties of materials in conditions close to those of their utilization (compressor inlet and outlet ducts) required the definition and realization of a device of cylindrical geometry allowing the study of these materials as a function of spatio-temporal characteristics of the acoustic field (frequency, angle of incidence) on the one hand, and on the other of the flow parameters (velocity, boundary layer). A numerical modelling of this device provides the excitation conditions (amplitude and phase) of the sources (compression chambers) whose axi-azimuthal distribution imposes simultaneously the axial and azimuthal wave number thence, consequently, the radial number. This way, a single mode chosen among all the propagative modes at the given frequency is engendered. The comparison of the spectra of axial and azimuthal wave numbers, measured and calculated for various values of the flow velocity, including zero velocity, as well as for various frequencies and incidence angles, made it possible to verify the working of this device and to know its limits. At this occasion, it has been possible to measure the convection effect of the flow on the axial wavelengths. A particular application of this device to the measure of the reflection coefficient of a material as a function of incidence angle is presented as an example.