Correlation in the Near and Far Field of Compressible Jet to Identify Noise Source Characteristics

A noise source characteristic of an unheated compressible jet is experimentally investigated for the jet with exit Mach number 0.8. Measurements of acoustic pressure fluctuations were performed in the near-field and far-field independently to identify noise source characteristics using the correlation technique. Near-field and far-field acoustic pressure fluctuations were also measured simultaneously to identify the acoustic component in near-field pressure fluctuations measured. Measurements were taken for the base and chevron nozzle with the nozzle exit diameter of 30 mm. From auto-correlation and cross-correlation in the near and far fields, it is observed that region surrounding to mixing layer is dominated by random small scale structures which radiate noise to sideline direction in the far-field. Correlation obtained at downstream of the potential core in the near-field represents the dominance of coherent large scale structure which radiates the noise to the far-field in the aft angle direction. Chevron changes the characteristics of noise sources. The size of the dominant large-scale noise source reduces due to chevron. Coherence of the noise source in the region surrounding to mixing layer reduces due to chevron which is responsible for reducing the energy of the dominant noise sources downstream of the potential core. Near-field far-field cross-correlation confirms that spatial near-field location for radiating acoustic pressure fluctuations more efficiently to the far-field at polar angle 30 degrees is dependent on axial as well as transverse position in the near-field. Chevron reduces the convective velocity of the flow. Chevron also reduces the spatial correlation length scale of the dominant noise source. Chevron changes the acoustic pressure in the near-field thereby shifting the dominant noise source towards the nozzle exit.