Jet Noise Prediction

Mary Dunlop, California Institute of Technology

Pressure disturbances generated by a jet cause acoustic fluctuations, or jet noise. I work with a database (previously calculated) of pressure information for the region surrounding an unsteady, turbulent, subsonic jet. Using this data I am able to predict the frequency of pressure disturbances at a point far away from the jet using data gathered near (8 jet radii and closer) the jet.

Imagine an observer at a point A, far from the jet. If the observer moves at the same speed as the jet, he can measure pressure as a function of time, P(t), for point A. The pressure spectrum associated with this data provides a quantitative measure of the dominant frequencies contributing to jet noise. Given P(t), the pressure spectrum can easily be calculated for point A. These spectra are in excellent agreement with experimental data.

But what is of far greater interest is jet noise prediction. I am able to reconstruct the pressure frequency spectrum associated with a point far from the jet given data taken near the jet.

Again, consider the observer: if he is standing at point A it is clear that he can reconstruct the pressure frequency spectra because he is measuring P(t) directly. Now, if the observer moves slightly away from point A, and slightly closer to the jet, he is not measuring P(t) directly. However, the wave equation can be used to predict what will happen at point A given his data taken very close to A. My work explores the limits on how close the observer can move towards the jet and still accurately reconstruct the pressure spectrum. Beyond a certain point turbulence and other effects render this simple prediction method invalid and the pressure frequency spectrum at point A can no longer be predicted using data close to the jet.

Abstract Author(s): Mary J. Dunlop