Effect of Initial Boundary-Layer State on Subsonic Jet Noise

Significant differences in subsonic jet noise databases have been reported in recent review papers. Specifically, university-type facilities involving higher contraction ratios andpossibly cleanerflows are noted to yield higher levels of noise relative to data from industrial-type facilities. An experimental investigation is carried out in an attempt to understand the sources of the anomaly. It is inferred that differences in jet core turbulencemay not be the source. An observation in a previous study is confirmed showing that two nozzles of the same diameter but different internal geometry can produce a difference in subsonic jet noise. The present measurements demonstrate that the noisier nozzle involves a highly disturbed laminar, or nominally laminar, boundary-layer state as opposed to a turbulent state with the other. The former boundary-layer state with the noisier nozzle is actuallymarked by larger turbulence intensities, consistent with the higher radiated noise. Although the boundary-layer characteristics were not reported with the earlier databases, the present results suggest that differences therein might be a source of the anomaly.