Effect of nozzle exit conditions on subsonic jet noise

Significant differences in subsonic jet noise databases are reported in earlier review papers. Specifically, „University-type‟ facilities involving higher contraction ratios and possibly cleaner flows 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 turbulence may not be the source. An observation in a previous study is confirmed that shows that two nozzles of same exit diameter but different internal geometry can produce significant differences in 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 actually marked by larger turbulence intensities consistent with the higher radiated noise. While the boundary layer characteristics have not been reported with the earlier databases, the present results suggest that differences therein might explain the anomaly.