Aeroacoustic Noise Measurements in Wind Tunnel,
MIDDLE EAST TECHNICAL UNIV ANKARA (TURKEY) DEPT OF MECHANICAL ENGINEERING
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The paper describes the general characteristics of the lowspeed Acoustic Research Wind Tunnel constructed in the Aerodynamics Laboratory of E.N.S.M.A PoitiersFrance and presents results of preliminary type. It has a test section of 30x30 sq cm and a mean velocity of 42 ms. Aerodynamic measurements revealed a maximum turbulence intensity of less than 1. The open test section is completely enclosed within an acoustically lined semi-anechoic chamber of dimensions 3.3x4x2.8 cu m. Acoustic calibration of the wind tunnel is done by using both white noise and pure tone noise sources and it was observed that the tunnel can be considered to be anechoic above 100 Hz for white noise tests. Preliminary measurements of aerodynamic noise are performed by using both smooth and artifically roughened circular cylinders placed in the open test section of the wind tunnel. An aeroacoustic coherence function is devised to classify the subsonic flow at moderate Reynolds numbers 50500 Re sub D 177,000 around a circular cylinder in its different flow regimes subcritical, transcritical and supercritical. The method uses simultaneous measurements of the power spectra of the far field acoustic pressure and of the turbulent longitudinal velocity fluctuations in the wake of the cylinder as well as their cross spectral density functions and especially the coherence function between the two signals. The values of the coherence function corresponding to the critical Strouhal frequencies are intense in the subcritical regime, decrease sharply in the transcritical domain and then increase and stabilize in the supercritical regime.