Low-Frequency Rotational Noise In Closed-Test-Section Wind Tunnels

Wind-tunnel walls affect measurements of the sound field of an aircraft model, thereby complicating noise measurements of aircraft in a wind tunnel. This paper discusses the effect of a closed-test-section wind tunnel on sound fields from a helicopter rotor. A computational model of the sound and its propagation in the tunnel are used to examine changes in wind-tunnel geometry, wall absorption. measurement location. and source radiation on the sound in the wind tunnel. Throughout most of the wind tunnel, the first harmonic of rotational noise of a rotor in a hard-walled wind tunnel about 2 rotor diameters wide (typical size for a rotor in a wind tunnel), which reflects most of the incident acoustic energy is shown to be unrepresentative of the acoustic free-field. Furthermore, the difference between the sound pressure level in a wind tunnel and free-field depends on details of the wind tunnel, sound radiation or source and, location in the wind tunnel. In the wind tunnel, the acoustic far-field of the rotor is dominated by the reverberant field. and thus accurate low-frequency measurement. cannot be easily made there. Near-field measurements are dominated by the direct acoustic field. However, the near acoustic field of a rotor is complicated, and one measurement in the near-field cannot be related to the far-field sound. Realistic acoustic lining absorption coefficients do not increase the lowfrequency absorption at the first rotor harmonic enough to significantly expand the region in the wind tunnel where the acoustic field resembles the free-field.