Experimental Investigation of the Influence of Hot-Humid Air on Jet Noise

This study experimentally investigates the effect of high humidity (near 100%) on air jet noise amplification. A small air jet was generated using a round nozzle, with acoustic Mach numbers ranging from approximately 0.4 to 1.1, covering subsonic and, to some extent, supersonic jet exit conditions. Experiments were conducted for both dry- and humid-air jets, with jet exit temperatures ranging from approximately the ambient temperature up to 2.75 times the ambient value, where visible condensation was observed. Far-field noise and near-field pressure measurements were carried out to gain deeper insight into the mechanisms driving jet noise amplification under humid conditions. It was demonstrated that humidity amplifies jet noise across the entire tested ranges of acoustic Mach number and jet exit temperature, with a more pronounced effect at lower acoustic Mach numbers and higher jet exit temperatures. A similar trend was consistently observed in the near-field pressure measurements. The findings suggested that, beyond density variations due to molecular composition, humidity also induces condensation, releasing latent heat and causing thermal fluctuations. As a result, in addition to the conventional dipole and quadrupole noise sources—associated with temperatureinduced density fluctuations and velocity fluctuations, respectively—a monopole source becomes significant. This monopole contribution arises from mass flow rate variations and thermal expansion effects, thereby enhancing the overall jet noise compared to the dry-air case.