Jet-Surface Interaction Test: Phased Array Noise Source Localization Results

Abstract

An experiment was conducted to investigate the effect that a planar surface located near a jet flow has on the noise radiated to the far-field. Two different configurations were tested: 1) a shielding configuration in which the surface was located between the jet and the far-field microphones, and 2) a reflecting configuration in which the surface was mounted on the opposite side of the jet, and thus the jet noise was free to reflect off the surface toward the microphones. Both conventional far-field microphone and phased array noise source localization measurements were obtained. This paper discusses phased array results, while a companion paper discusses far-field results. The phased array data show that the axial distribution of noise sources in a jet can vary greatly depending on the jet operating condition and suggests that it would first be necessary to know or be able to predict this distribution in order to be able to predict the amount of noise reduction to expect from a given shielding configuration. The data obtained on both subsonic and supersonic jets show that the noise sources associated with a given frequency of noise tend to move downstream, and therefore, would become more difficult to shield, as jet Mach number increases. The noise source localization data obtained on cold, shock-containing jets suggests that the constructive interference of sound waves that produces noise at a given frequency within a broadband shock noise hump comes primarily from a small number of shocks, rather than from all the shocks at the same time. The reflecting configuration data illustrates that the law of reflection must be satisfied in order for jet noise to reflect off of a surface to an observer, and depending on the relative locations of the jet, the surface, and the observer, only some of the jet noise sources may satisfy this requirement.

Jet-Surface Interaction Test: Far-Field Noise Results

“…In this case, the low frequency augmentation associated with the jet-surface interaction noise is very similar on the sides of the surface indicating that the trailing edge noise is the dominant jet-surface interaction source below St Dj ¼ 0.2. This conclusion is supported by phased array data showing a low frequency noise source located at the trailing edge of the surface from both the shielded and reflected observed position [18]. Note that the increase at high frequencies on the reflected side is due to the reflection of jet mixing noise.…”

Testing Installed Propulsion For Shielded Exhaust Configurations

“…By defining the axial coordinate to start at the upstream-most nozzle exit x 0 , and normalizing by the calculated potential core length, the single-stream data collapses to a smooth curve that runs from the end of the potential core at St De = 0.3 to effectively the nozzle exit by St De = 5. The data shows considerable scatter at St De above 1 in this dataset, possibly because of the impact of the external plug; earlier data without a plug did not show this scatter 8 .…”

“…The axial source distribution of dual-stream jet plumes effectively have two major source regions, one well downstream of the potential core producing most frequencies, and one at the nozzle exit producing only the highest frequencies. This is in contrast with simple (one stream and no plug) single-stream jet flows that have a peak source location that continuously shifts downstream with decrease in frequency 8 . Figure 19a presents normalized distributions of peak source location as a function of frequency for the single-stream (obtained with either only the core flow on, or both the core and bypass set the same to mimic a single stream jet) and dual-stream jets tested in this test.…”

“…These were determined by acquiring data, especially phased array measurements of source location as a function of frequency, from separate flow nozzles with the various streams being run at the same temperatures and velocities, which should equate to the simple jet flows tested earlier. 8 When a match is found between the acoustic noise generated in the multi-and single-stream jets, the offset in axial and standoff positions will give the equivalence in nozzle exit and lipline. Matching acoustic results at the same equivalent geometric parameters but with the multi-stream nozzles running different flow conditions will establish a relationship between the single-jet flow parameters and the multi-stream flows.…”

Noise Reduction with Fluidic Inserts in Supersonic Jets Exhausting Over a Simulated Aircraft Carrier Deck

“…However, at jet to ground plane distances greater than 6 jet diameters these effects were shown to be less than 1 percent. Podboy [18] performed phased array measurements of a jet near a surface to localize the sound source and determine the jet surface interaction effects. Brown [19] also performed measurements with a near field phased array and additional far-field microphones.…”