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Noise Data from experiments on nozzles and fans are used to develop new computer programs that engineers can use to design quieter engines. When sound, pressure, temperature, and velocity measurements are com- pared against the calculations made to predict them, engineers learn many things, such as whether a new nozzle or fan design is quieter or whether calculations are correct. Then they know if adjustments need to be made in the designs or if the computer models should be improved. The teeth cut into the edge of the nozzle in the picture below are often called chevrons and are used to reduce jet exhaust noise. Tests at NASA Glenn con- firmed that chevrons are a practical way to reduce noise. Data show that the chevrons change the way that engine exhaust mixes with the surrounding air and can help reduce noise. Developing new computer codes that model the complex mixing of the exhaust streams will help design engineers optimize their noz- zles by deciding how many chevron teeth should be cut, what shape they should be, and so forth. Chevrons on Lear jet. Scale models of aircraft exhaust nozzles are test- ed in NASA Glenn’s Aero-Acoustic Propulsion Labora- tory (AAPL). It is a geodesic dome that houses sev- eral experiments. The dome helps researchers collect high-quality data in two ways: first by shielding the experiments from outside noise sources (such as cars driving by and airplanes flying overhead) and second by preventing sound reflections within the dome from contaminating the data. To prevent sound reflections from contaminating the data, fiberglass wedges are permanently installed on the inside of the dome and movable wedges are used to cover the floor during a test. All of these wedges absorb sound creating what is called an anechoic chamber. The walls of the dome also keep the noise of tests from bothering neighbors living near the lab. Test nozzle in AAPL. Have you ever been near an airport when an air- plane is landing and heard a “whine” above the roar of the engine? The large bypass fan of an engine can produce two types of noise, tone noise (the whine) and broadband noise (the roar). As the fan accelerates air down the fan duct, the flow is swirling because of the spinning fan. This swirl causes loss of momentum before the air exits the nozzle so it is straightened out with a set of nonmoving vanes called stators. Broad- band noise, which is sound at many different frequen- cies, is created by the turbulence in the airflow. Tone noise, which is sound at specific frequencies, is cre- ated when the fan wakes (disturbances in the airflow downstream of the fan blade) hit the stator vanes like waves hitting the beach. Tone noise from a bypass fan of an engine occurs at the blade passing frequency (BPF) and its harmonics (whole-number multiples of the BPF). The BPF is related to the number of fan blades B and the rotational speed N (in revolutions per sec- ond, or rps) by the equation BPF = BN, which equals the frequency of the tone produced in cycles per sec- ond (Hertz, Hz). Acoustic engineers are also interested in the wavelengths of tones generated, as there is a complex relationship between the length of the wave, AAPL. 64PDF Image | NASA Guide to Engines
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