Experimental Investigation of Azimuthal Structure of Dipole Noise for Rigid Cylinder Inserted in Turbulent Jet

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The measurement procedure recently developed in the investigations of vortex ring noise and based on synchronous azimuthal decomposition of signal by hexagon array of microphones with subsequent ensemble averaging is used for the diagnostics of aeroacoustics sources in the situation of the cylinder rod (diameter 2, 3, and 10 mm) inserted in a turbulent flow. The turbulent jet with initial velocity 90 m/s issued from the cylindrical nozzle of diameter 20 mm. A microphone array for recording was placed in the far sound field of the sound sources. The frequency range 500-2000Hz is analyzed when the radiation region can be considered as a compact sound source. Two dipoles are recognized in the sound field – the first one is directed along the mean flow, the second one is orthogonal to the flow (drag and lift dipoles). The third dipole, which could be directed along the cylinder, is absent due to the absence of considerable pulsation force along the cylinder. The measurement procedure enables to recognize the rod angle of inclination in the plane normal to the mean flow axis. The quadrupole sources in the separation zone appear to be strongly connected with the dipole sources located on the cylinder. Their interference leads to surprising picture of the resulting sound field: equivalent dipole sources are located in the separation zone far downstream from the cylinder. This effect manifests a dual role of small quadrupole sources in the low frequency range: (i) quadrupoles located closely to the cylindrical surface create the dipole sources in the cylinder (which radiate the main noise) and (ii) quadrupoles displace the dipole source far downstream due to the interference.