Abstract
A model turbofan was tested in the NASA Glenn 9- by 15-ft low speed wind tunnel to explore far-field acoustic effects of a variable area bypass nozzle using three fixed nozzle flows. The baseline nozzle was sized to produce maximum stage performance at cruise condition. However, wind tunnel testing is conducted near sea level condition. Therefore, to simulate and obtain performance at other operating conditions, two additional nozzles were tested-one with +5% increase in weight flow (+5.4% area increase), sized to simulate the performance at the stage design point (takeoff) condition, and the other with a +7.5% increase in weight flow (+10.9% area increase) sized for maximum weight flow at sea level condition. Measured acoustic benefits with increased nozzle area showed effective perceived noise level reductions of 2 or more dB (for a 1500 ft fan flyover with a 3.35 scale factor) while the stage thrust actually increased by 2 to 3%. Noise reductions, principally in the level of broadband noise, were observed everywhere in the far field. Laser Doppler velocimetry measurements downstream of the rotor showed that the total turbulent velocity decreased with increasing nozzle flow, which may explain the reduced rotor broadband noise levels.
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