Characterization of the near-field flow structure of an acoustically self-excited jet in a large enclosure using particle image velocimetry (PIV)

Graduation date: 2008An experimental study of the near-field flow structure produced by an incompressible turbulent Acoustically Self-Excited Jet (ASEJ) in a large enclosure using Particle Image Velocimetry (PIV) is presented. The salient feature of this jet is that it provides an increase in near-field mixing and turbulence without additional external power input. In the present experiments, the jet exits into an enclosure whose nearest wall is 26 jet diameters from the nozzle center. Three nozzle exit Reynolds numbers (Re) of 27,000, 49,000 and 71,000 were studied for two frequencies of excitation of the jet, and compared with the free pipe jet without excitation. Results indicate that the peak turbulence intensity based on nozzle exit velocity in the nearfield is enhanced by as much as 50 percent for the ASEJ compared to the unexcited jet at downstream locations less than 2 nozzle diameters away from the nozzle exit. The second stage of excitation is found to increase the near-field turbulence to a greater extent than the first stage of excitation. Entrainment of surrounding air in the nearfield of the jet is enhanced due to the acoustic self-excitation. Streamwise variations of streamwise mean velocity along the centerline of the jet compare well with data reported by Hasan and Hussain (1982) for both the unexcited jet and first stage ASEJ. Peak turbulent intensity along the jet centerline is lower than that reported by Hasan and Hussain (1982) for first stage ASEJ but higher for second stage ASEJ. This\ud enhancement in turbulent intensity for second stage ASEJ is thought to be due to\ud acoustic feedback from the enclosure walls