Experimental Evidence of Latent Heat Amplification of Turbulent Jet Noise Modulated by a Cylinder at Low Mach Number

Tornadoes produce infrasound (i.e., sound at frequencies below human hearing), which carries information about the tornado and its formation processes. The fluid mechanism responsible for this acoustic signature is unknown. However, it has been hypothesized that latent heat production due to condensation within the tornado core could cause the amplification at frequencies corresponding the tornado size. While there is an analytical solution that predicts this latent heat induced amplification, this had not previously been experimentally observed. Thus, the current work experimentally measured the lift and drag dipole noise sources generated by a cylinder positioned in a turbulent jet with and without condensation to quantify the effects of latent heat on noise sources. Four microphones were positioned around the cylinder axis at positions dominated by the lift and drag sources in the far-field. Tests were performed with a Mach 0.1 jet and varying jet exit temperatures for three conditions: dry air, humid air with low latent heat, and humid air with high latent heat. Using comparative analysis, evidence showed that if latent heat is present within the flow, no matter the amount, both the lift and drag dipole sources are significantly amplified. These results support the hypothesis that latent heat amplifies noise sources, but more testing is required.