NASA’s Aeronautics program is involved in engine test programs to identify dominate noise sources. Core noise is of interest since it becomes a significant contributor to the overall turbofan engine noise during takeoff or approach. A simple new procedure was created at the NASA Glenn Research Center that uses an aligned and unaligned coherence function to separate a mixture of “incoherent” noise, “coherent” noise, and tones so that noise components can be related to engine sources. The procedure provides a way to judge the significance of an aligned coherence-based function. Details are provided in references 1 to 3.
Data from a Pratt & Whitney PW4098 turbofan engine test are used to illustrate this technique. Instrumentation consisted of two pressure transducers in the combustor and four far-field microphones at 150 ft. Data were analyzed by using a periodogram averaging method to determine the amount of energy in a frequency band. The reciprocal of this band width is the sample segment length. If one calculates the coherence between a pressure transducer in the combustor and a far-field microphone, the two signals are completely out of synchronization since the analyzer only has the contents corresponding to the sample length, which is smaller than the propagation time for this test. The signals averaged do not include the combustor signal that was occurring when the noise left the engine.
This coherence is called the unaligned coherence. It measures the coherence of tones. Without tones, it has a value dependent on the number of segments averaged. An aligned far-field microphone signal must be created by the well-known procedure of time shifting the far-field time history forward by the time lag. The new procedure involves comparing functions calculated with the aligned and unaligned coherence so that one may judge their significance.
Three-signal coherence technique using one far-field microphone at 100° and two internal combustor pressure sensors. Here D is the number of time steps that the signals are shifted to obtain alignment or misalignment. The time delay created by shifting the signal an amount D is TAU = D/r, where r is the sample rate in samples per second (r = 48,000).
Long description of figure 1.
Three-signal coherence technique using three far-field microphones.
Long description of figure 2.
The figures illustrate the technique. Three-signal coherent power aligned and unaligned coherence functions calculated using a far-field microphone and two combustor transducers are shown in the first figure above. The small blip near 100 Hz in the aligned coherence rising above the unaligned coherence represents the correlated part of the total noise due to combustion. This blip is 9 dB below the total noise. Also note the presence of tones buried in the total noise. The separation of tones and broadband noise is also shown in the three-signal coherent power aligned and unaligned coherence functions created using three far-field microphones (see the preceding figure). The following figure illustrates the two-signal aligned and unaligned coherent output power calculated using a combustor sensor and a far-field microphone. The small peak near 100 Hz is attributed to the presence of an m = 0 circumferential mode (i.e., the plane wave mode). This method lets one observe phenomena as low as 18 dB below the ordinary measured autospectrum.
Total sound power, aligned and unaligned coherent output power calculation for a test condition of 1622 rpm (corrected rotor speed, N1 corr)—using signal 1 (from the combustor pressure sensor 1 at 127° clockwise from top dead center viewed from the rear) and 5 (at 120°).
Long description of figure 3.
The method confirms the presence of a coherent mode propagating from the combustor to the far field. It is being applied to turbofan engine acoustic data from Honeywell.
Find out more about this research at Glenn’s Acoustics Branch: http://www.grc.nasa.gov/WWW/Acoustics/
Glenn contact: Dr. Jeffrey Hilton Miles, 216-433-5909, Jeffrey.H.Miles@nasa.govLast updated: December 14, 2007
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