Acoustic scattering experiments on spheres for studying helicopter noise scattering

This paper addresses first wind tunnel activities conducted in the GARTEUR Action Group HC/AG-24 dealing with acoustic scattering of spheres. GARTEUR Action Group HC/AG-24 is established to address noise scattering of helicopter rotors in presence of the fuselage. This test is a necessary step to help establishing an appropriate test setup for acoustic scattering of a generic GARTEUR helicopter model. The tests are conducted in the DLR Acoustic Wind Tunnel in Braunschweig (AWB). Two source systems, including both DLR Laser generated sound and ONERA SPARC (Source imPulsionnelle AeRoaCoustique), are used to perform shielding experiments. The tests include three spheres, two support systems and three wind speeds of 0m/s, 30m/s and 45m/s. The sizes of the spheres, a small one with 12cm in diameter and a big one with 34cm in diameter, are derived according to the maximum dimension of the BO105 fuselage with 12.5 scales down in both lateral- and stream-wise directions to consider that the down scaled rotor noise frequencies fit inside the effective frequency band of the noise sources. In the current paper, the analysis of the data post-processing steps required for obtaining correct spectral and time domain data for laser sound source is emphasized. The influence of the support systems on the acoustic scattering field is analyzed. The comparison of the test results with an analytical solution for sphere sound scattering is used to verify the accuracy of the experiment results. In addition, the test results can also be used as database to validate numerical tools.