This study presents the first set of experiments with the Controlled-Diffusion (CD) airfoil in the newly-built anechoic wind-tunnel at Université de Sherbrooke. Velocity measurements in the latter show very uniform mean flow and low turbulence level (0.4 %) up to 56 m/s in the 30 cm square nozzle exit section. Acoustic and velocity measurements have been carried out at several flow velocities and angles of attack. Three distinct flow regimes are observed. At high angle of attack and high velocity the usual broadband noise signature found in the Ecole Centrale de Lyon anechoic wind tunnels is recovered. At low angle of attack, the power spectral density of the microphone signal is dominated by a primary tone with secondary tones, typical of Tollmien-Schlichting noise radiation. This dominant tone is also recovered in the power spectral density of the flow velocity signal. Signal processing tools (spectrogram and bicoherence) are used to investigate the presence of the secondary tones. Two tonal regimes can then be distinguished: one stationnary and one intermittent where some tones disappear intermittently and are formed by a non-linear process. Finally, two parallel microphone arrays associated with high resolution imaging is used to localize the noise sources at the primary tone frequency, which are found at the airfoil trailing edge. At this low frequency, the L1-GIB algorithm is clearly seen more efficient than classical beamforming.
An experimental analysis of the noise radiated by a high pressure flow discharge through diaphragms and perforated plates is carried out for a large range of subsonic and supersonic operating conditions (nozzle pressure ratio (NPR) from 1 to 3.6). A parametric study of the geometrical parameters is also achieved to characterize their influence onto the acoustic radiation and ways to reduce it. This reaches from single diaphragms to multi-perforated plates with variable hole diameters and arrangements that are placed at the exit of a duct. Different acoustic behaviors are observed: in all cases the far-field acoustic radiation is dominated by a broadband contribution associated with the turbulent mixing in shear layers. In the diaphragm cases, this broadband noise has similar characteristics as the mixing noise of classical unheated jets while in the multi-perforated plates cases, it is composed of two distinct humps associated with different parts of the jets development. For supersonic regimes, in addition to this broadband radiation, shock associated noise (screech and broadband shock associated noise) appears for all diaphragm cases and for the perforated plate with the closest holes. Finally for the smallest NPR, a high frequency tonal noise has been observed in most of the multi-perforated cases and for the smallest diaphragm. Different regimes of this radiation have also been observed with a possible amplitude modulation of the dominant tone. This radiation may be attributed to vortex shedding due to the sharp section reduction that would trigger a flow resonance between the small ducts of the holes and their sharp edges.
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