A nearfield investigation into the sound pressure levels of high speed jet fumes

Sexton, Robert Christopher (1993) A nearfield investigation into the sound pressure levels of high speed jet fumes. University of Southampton, Doctoral Thesis.

Abstract

This thesis details a model used to simulate the nearfield Sound Pressure Levels of a high speed jet plume. Nearfield simulated contour diagrams were produced and a composite diagram was made by superimposing with the nearfield data. Many such examples are given in the appendix volume of this Thesis. The nearfield model successfully simulated much of the unshocked Shock Tube nearfield data. The model uses an extended line source distribution of sources as being acoustically equivalent to the Mixing Noise sources present in the shear layer of the jet plume. The source model uses a directivity function an integrated source strength and two adjustable parameters, namely the Centroid Position (X_c) and the Shape Parameter. The Strouhal (Str.) No. dependence of X_c is derived from experimental data. The experimental data used was acquired from a Manchester University Shock Tube experiment. The experiment involved exhausting a mixture of Helium and Argon gas from a 25mm convergent nozzle into an anechoic chamber in order to generate a range of `equivalent Air jet conditions'. Farfield Shock Tube data was used to derive the directivity function and the source strength and to generate simulations using the nearfield model. However some of the nearfield data could not be accurately simulated with the X_c's derived from the experimentally determined Strouhal number dependence of the X_c. It was found that for many of the large Strouhal number Manchester nearfield data sets that were difficult to simulate that the problem lay in the fact that the directivity derived from the farfield data was incompatible with the form of the nerfield data contours derived from the nearfield data. The reason for the incompatibility is not known but could be explained by the existence of a downstream source not taken into account by the nearfield source model.

The spectra of the Manchester pure Argon data was examined along with the remaining spectra for the shocked condition in the Manchester database. The spectra from the choked jet conditions showed evidence of Broadband Shock noise and Screech.

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