Latent Heat Induced Amplification of Turbulent Jet Noise

Tornado infrasound (i.e., sound at frequencies below 20 Hz) is known to carry information about the physical structure of the tornado. While the fluid mechanism for this signature is unknown, analytical and computational work has suggested that latent heat could play a critical role. However, the amplification of flow-induced noise due to latent heat production has not been experimentally observed. Thus, to quantify the impact of near-saturated humidity on air jet noise, experiments were conducted with a round nozzle generating jets at acoustic Mach numbers from ~0.4 to 1.1, spanning subsonic to slightly supersonic regimes. Both dry and humid jets were tested across a range of jet exit temperatures—from ambient up to 2 times ambient—where visible condensation occurred. Far-field acoustic and near-field pressure measurements were performed to characterize noise amplification mechanisms. Results show that humidity amplifies jet noise under all tested conditions, with more pronounced effects at lower acoustic Mach numbers, elevated jet temperatures, and higher frequencies. Near-field pressure measurements revealed similar trends, indicating increased acoustic activity in humid jets. The findings demonstrate that latent heat release during condensation, combined with humidity-induced density reduction, contributes significantly to noise amplification. These effects introduce monopole-like acoustic sources associated with unsteady mass addition and thermal expansion, in addition to traditional dipole and quadrupole sources linked to temperature and velocity fluctuations. The emergence of this monopole component under humid conditions results in noticeably higher noise levels compared to dry-air jets.