Investigation of the Noise Radiation from Heated Supersonic Jets

This work focuses in the investigation of crackle and Mach wave radiation in heated supersonic jets. The skewness and kurtosis of the acoustic pressure signal and its time derivative were adopted as metrics for identifying crackling jets and quantifying levels of crackle. Cold and heated jets from supersonic nozzles with different geometric parameters and scales are analyzed to draw conclusions on noise sources and propagation. In order to complement the investigation, results are also presented for the mixing noise, broadband shock-associated noise and screech.;Chapter 4 focuses on the impact of jet operating condition on the skewness and kurtosis levels of a jet issuing from a converging-diverging conical nozzle, with a 1.5 design Mach number. An increase in convective Mach number, achieved by increasing jet temperature, proved to be related to elevated values of OASPL, skewness, and kurtosis, in both the near and far fields. Intense levels of the dP/dt high-order statistics appear to be generated at different locations in the shear layer of the jet and strengthen away from the jet by non-linear propagation effects.;Chapter 5 studies how adding chevrons to a converging-diverging nozzle impacts Mach wave radiation and crackle. The chevrons decreased OASPL in the downstream angles but increased the broadband shock-associated noise. Pressure skewness, dP/dt skewness and kurtosis were all reduced by the chevrons in the near field and far field, and thus they effectively mitigated crackle and Mach wave radiation; however, chevrons showed no evidence of changing the convective Mach number. The evolution of noise signals was analyzed in the near-field to the far-field to identify the strengthening of skewness through nonlinear propagation effects.;Chapter 6 investigates a jet exhausting over a plate at different stand-off distances, to simulate jets exhausting over airframe surfaces and jet-ground interaction during take-off and landing operations. Far-field acoustics were measured at the reflected direction, sideline, and shielded azimuthal directions. At the sideline, the plate attached to the nozzle lip diminished broadband shock-associated noise, and mitigated screech for the cold case. When the plate was moved away from the nozzle, screech tones were intensified at the under-expanded condition. Crackle levels were significantly intensified in the sideline, within a range of stand-off positions.;Chapter 7 analyzes the impact of nozzle scale and nozzle internal contours on the levels of crackle. Three scaled converging-diverging nozzles, with jet exit diameters of 0.542 in, 0.813 in, and 1.085 in were investigated. Far-field arrays were setup at a constant non-dimensionalized radial distance of 40 nozzle exit diameters. The pressure skewness and kurtosis plots collapsed for all three scaled nozzles when the pressure signals were not filtered. The dP/dt statistics collapsed when the signals were downsampled proportional to the nozzle exit diameters. Baseline nozzle results were also compared to a smooth contoured nozzle designed by the Method of Characteristics. This nozzle almost had no broadband shock-associated noise, but contained the same skewness and kurtosis levels, concluding that crackle is not linked to the shock-cell structures in the jet.