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the diameter of the duct in the engine, and the noise produced. Tests in the NASA Glenn 9- by 15-Foot Low-Speed Wind Tunnel were conducted to investigate theories that fan tone noise could be reduced by changing the number and orientation of the fan blades and stator vanes. One-fifth-scale fan models were created: The baseline fan stage with radial stators is shown in the figure below, and the low-noise fan stage with the duct- ing removed is shown in the inset. Tests confirmed that tone noise could be reduced by carefully selecting the number of rotor blades and stator vanes, sweeping the stator vanes downstream (increasing the distance be- tween the fan blades and the stator vanes), and lean- ing the stator vanes (preventing the wake from hitting just one stator vane at a time). These types of changes One-fifth-scale model in 9- by 15-Foot Low-Speed Wind Tunnel. reduce noise by altering the way the fan wakes interact with the stator without adding a lot of weight to the engine or increasing engine complexity. Noise reduction is an ongoing quest for propulsion engineers. Tests conducted at NASA Langley led to the design of a seamless “sound-absorbing” inlet liner by BFGoodrich, which reduced fan tones in front of the aircraft by up to 15 decibels (dB). Other research at Langley demonstrated the noise reduction capability of “wavy” rotor blades for helicopters as shown at top right along with a computer visualization of the data compared to the usual straight rotors. One interesting way to reduce noise is called “active noise reduction.” In this method, an inverted form of the unwanted sound wave is generated. Since these two waves are exactly out of phase with each other, when they meet they effectively cancel each other out and eliminate the sound. NASA Glenn has begun testing this concept with the Advanced Noise Control Fan, a 4-ft-diameter fan specifically designed to test this concept. Wavy-planform rotor on model helicopter in the wind tunnel. Equivalent flight angle, degrees 12 Descent 8 4 Level 0 cruise –4 Climb –8 Straight-planform rotor 0.20 0.25 0.30 Advance ratio Wavy-planform rotor 0.20 0.25 0.30 0.35 Advance ratio Noise 0.15 0.35 0.15 Pushing the Envelope: A NASA Guide to Engines 94 95 96 97 98 99 100101102103104105106107 108 Sound pressure level, dB Computer visualization of rotor blade noise. Reducing noise generated by complicated engine parts is challenging and interesting work. It takes the efforts of many people to design, build, and run the experiments and analyze the data. These include peo- ple skilled in the trades, engineers, mathematicians, scientists, and computer programmers, to name a few. Working as a team they continue to develop new theo- Advanced Noise Control Fan. 65 PHYSICS AND MATHPDF Image | NASA Guide to Engines
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