Near field pressure fluctuations of a circular jet
The relationship between the evolution of vortices in a circular jet and the generation of fluctuating pressure was studied, using an apparatus consisting of a jet nozzle connected to a stagnation chamber with a fifth-order polynomial contraction. The data were digitized and computer-processed. The constant pressure contours near the jet shoow that the evolution of coherent structures is the source of pressure perturbations. The data obtained in the vicinity of the jet indicate that two length scales govern the decay of the fluctuating pressure. A thin hydrodynamic near-field extends about 1 instability wavelength from the jet edge, and pressure decays exponentially. A thick acoustic near-field takes about ten acoustic wavelengths to reach the far field. The pressure disturbances decay algebraically in this region.