Forward Velocity Effects on Jet Noise with Dominant Internal Noise Source

Abstract

Flow-related internal noise generated by a control valve in an air jet-noise rig or, by analogy, combustors in an engine can become the dominant noise source at low jet velocities or provide a floor for jet noise suppressors. Acoustic data, with and without forward velocity, were obtained with a convergent circular nozzle using a quiet flow system and one dominated by a low-frequency internal noise source. Forward velocity effects were obtained by installing the test nozzle in a free jet. Farfield noise data, obtained at jet pressure ratios from 1.3 to 1.7 and forward velocities up to 260 ft/sec, are presented in terms of SPL and PWL spectra, directivity, and total power. With a quiet flow system, jet noise is reduced by forward velocity as a function of the product of the sixth power of the relative velocity and the second power of the jet velocity. However, with a dominant low-frequency core noise source, the portion of the noise spectra dominated by this source was not appreciably affected by forward velocity. The high-frequency spectral portion of the jet noise was reduced by forward velocity independently of the flow systems used.