Ph.D. Thesis Proposal
Nick P. Breen
(Advisor: Dr. Krish K. Ahuja)
COMPARISON BETWEEN BEAMFORMING and Nearfield Contours for source location in subsonic and supersonic jets of various geometries
2:00 PM, Thursday, September 21, 2017
In the context of aeroacoustics, source location refers to a methodology that allows identification of locations of noise sources of a given frequency in the noise-producing region of the flow. The need to understand and reduce aircraft noise emissions has led numerous researchers to apply various source location techniques to jet noise. There are a number of applications where the knowledge of the location of sources of various frequencies along the length of the jet is required. Prior to 1985, several methods for determining jet-noise source locations were explored: acoustic mirrors, microphone arrays, two microphone methods, causality correlation and coherence techniques, nearfield contour surveys, and automated source breakdown. More recently there have been developments in the microphone array, notably acoustic beamforming, and two microphone method techniques. Most of the older techniques, while they would successfully produce source location results, require significant amount of time to acquire data at each jet condition; this requirement is often caused by the necessity to move microphones, while the jet is running on condition, to obtain source locations at all frequencies. The acoustic beamformer, on the other hand, is not required to be moved during the acquisition of data, resulting in very rapid tests compared to most other source location methods.
In this work, the acoustic beamformer is analyzed as a jet noise source location tool. This method is compared against nearfield noise contours, an older well-developed method for jet noise source location, to see how consistent the two methods are for a variety of subsonic and supersonic jet conditions. Preliminary results show that these two methods agree fairly well for subsonic jet conditions. This work also examines how the noise source distributions of subsonic jets change due to low Reynolds number effects and due to nozzle geometry. The effects of pressure ratio, both on and off design condition, on the supersonic jet noise sources are analyzed. The noise source distributions of round twin jets are examined as a function of Mach number and separation distance. Attempts are made to empirically model the above results. Schlieren flow visualization videos are used in tandem with source location, to better explain how the noise source distributions change.