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  • 1
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    Active control of fan noise from a turbofan engine (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: A three-channel active control system is applied to an operational turbofan engine to reduce tonal noise produced by both the fan and the high-pressure compressor. The control approach is the feedforward filtered-x least-mean-square algorithm implemented on a digital signal processing board. Reference transducers mounted on the engine case provide blade passing and harmonics frequency information to the controller. Error information is provided by large area microphones placed in the acoustic far field. To minimize the error signal, the controller actuates loudspeakers mounted on the inlet to produce destructive interference. The sound pressure level of the fundamental tone of the fan was reduced using the three-channel controller by up to 16 dB over a +/- 30-deg angle about the engine axis. A single-channel controller could produce reduction over a +/- 15-deg angle. The experimental results show the control to be robust. Outside of the areas contolled, the levels of the tone actually increased due to the generation of radial modes by the control sources. Simultaneous control of two tones is achieved with parallel controllers. The fundamental and the first harmonic tones of the fan were controlled simultaneously with reductions of 12 and 5 dBA, respectively, measured on the engine axis. Simultaneous control was also demonstrated for the fan fundamental and the high-pressure compressor fundamental tones.
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 32; 1; p. 23-30
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    The identification of helicopter noise using a neural network (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Experiments were carried out to demonstrate the ability of an artificial neural network (ANN) system to distinguish between the noise of two helicopters. The ANN is taught to identify helicopters by using two types of features: one that is associated with the ratio of the main-rotor to tail-rotor blade passage frequency (BPF), and the ohter that describes the distribution of peaks in the main-rotor spectrum, which is independent of the tail-rotor. It is shown that the ability of the ANN to identify helicopters is comparable to that of a conventional recognition system using the ratio of the main-rotor BPF to the tail-rotor BPF (when both the main- and the tail-rotor noise are present), but the performoance of ANN exceeds the conventional-method performance when the tail-rotor noise is absent. In addition, the results of ANN can be obtained as a function of propagation distance.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 90-3973
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Rotational fluid flow experiment: WPI/MITRE advanced space design GASCAN 2 (1991)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The design and implementation is examined of an electro-mechanical system for studying vortex behavior in a microgravity environment. Most of the existing equipment was revised and redesigned as necessary. Emphasis was placed on the documentation and integration of the mechanical and electrical subsystems. Project results include the reconfiguration and thorough testing of all the hardware subsystems, the implementation of an infrared gas entrainment detector, new signal processing circuitry for the ultrasonic fluid circulation device, improved prototype interface circuits, and software for overall control of experiment design operation.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-189979 , NAS 1.26:189979
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 4
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    Identification of helicopter noise using a neural network (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 30; 624-630
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Active control of fan noise from a turbofan engine (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A three channel active control system is applied to an operational turbofan engine in order to reduce tonal noise produced by both the fan and high pressure compressor. The control approach is the feedforward filtered-x least-mean-square algorithm implemented on a digital signal processing board. Reference transducers mounted on the engine case provides blade passing and harmonics frequency information to the controller. Error information is provided by large area microphones placed in the acoustic far field. In order to minimize the error signal, the controller actuates loudspeakers mounted on the inlet to produce destructive interference. The sound pressure level of the fundamental tone of the fan was reduced using the three channel controller by up to 16 dB over a 60 deg angle about the engine axis. A single channel controller could produce reduction over a 30 deg angle. The experimental results show the control to be robust. Simultaneous control of two tones is done with parallel controllers. The fundamental and the first harmonic tones of the fan were controlled simultaneously with reductions of 12 dBA and 5 dBA, respectively, measured on the engine axis. Simultaneous control was also demonstrated for the fan fundamental and the high pressure compressor fundamental tones.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 93-0597 , ; 10 p.|AIAA, Aerospace Sciences Meeting and Exhibit; Jan 11, 1993 - Jan 14, 1993; Reno, NV; United States
    Format: text
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    NASA TECHNICAL REPORTS
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