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In day-to-day life, acoustic emissions pose stressful and hazardous conditions for human race and structures in vicinity. Hence a large amount of research has been done and in progress to reduce such irksome noise. However to reduce noise effectively, noise source localization and characterization of highly emissive regions is of real importance. Once the source regions are identified many active and passive techniques can be effectively devised. Many effective source localization techniques have been devised in the past for this purpose. In this work, a technique called Near-field Acoustic Holography (NAH) is potentially used as an acoustic tool to visualize shock cells arising in orifice jet plume at screech frequencies. Acoustic source mapping is specifically done for screech tones since it the most dominant and major contributor among all other noise sources in jet. Experiments are carried in semi-anechoic facility at different Nozzle Pressure Ratios (NPR) for orifice jet. In order to effectively cover the jet domain, a improvisation is made to planar array setup and a linear array is used. The linear array is scanned in transverse direction (y-direction) and finally all scans are patched together to get a proper acoustic map. The disturbance in phase due to patching of linear array and non stationary associated with jet leads to source information loss which is taken care by placing set of microphones near the source which are used as a reference to get more detailed information about the source characteristics. If the source location is unknown as in case of jets, reference microphones are kept throughout the region of interest The acoustic field generated by dominant sources are captured by reference microphones which is cross correlated and processed with array microphones to generate a partial field of interest. With the help of reference microphones the whole acoustic field is systematically segregated in terms of partial fields, ranked on basis of source acoustic strength. The acoustic based holography measurement is done in tandem with density based shadowgraph technique to capture shock cells in tandem. The acoustic visualization of shock cells at screech frequencies done using NAH gives a clear image of highly emissive regions in jet plume and is also verified with shadowgraph image thus giving a different dimension altogether in understanding of jet characteristics.
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