The near field within the potential cone of subsonic cold jets

Abstract

This investigation describes a detailed study of the near pressure field within the potential cone of a subsonic circular turbulent jet. The components of the near pressure field in the potential cone in which the potential flow condition exists within the fist four and a half diameter downstream appear to be moving with a phase velocity equal to the local speed of sound. The direction of propagation is roughly normal to the shear layer surrounding the cone. Some components of the hot-wire signal can be associated with the jet structure as a simple complex source, while others are related to the local characteristics of turbulence. Differences in the characteristics of the pressure field within the potential cone exist between the vortex generated noise at very low jet velocity and the eddy generated noise at higher velocity. The power spectra obtained in the potential cone show the peak which is due to the pressure fluctuations and the flat portion due to the turbulence. The frequencies of the dominant components, in terms of the Strouhal number, are functions of both the axial and radial positions. Microphone measurements were made in the near field outside for detailed comparison with the potential cone results.