Impinging high-speed planar jets are susceptible to self-excited aeroacoustic feedback mechanisms due to the coupling of the highly unstable shear layer and upstream travelling pressure waves created by the jet impingement. This aeroacoustic feedback mechanism results in intense narrowband acoustic tones and large amplitude oscillations of the jet column which are undesirable for its use as an actuator for coating weight control in the continuous gas-jet wiping line. This thesis experimentally investigates the use of auxiliary high-speed planar jets for the purpose of interrupting and reducing the amplitude of the negative effects of the aeroacoustic feedback mechanism.
Testing was performed using a planar multi-slot nozzle jet over a range of impingement distances, velocities and nozzle widths. The amplitudes of the acoustic tones were found to be a function of the ratio of velocities between the main and auxiliary jets with the tones found to be eliminated at sufficiently high-velocity ratios. Larger auxiliary jet widths were found to further reduce the amplitude of the tones in conjunction with the velocity ratio. The reduction of the amplitude of the tones was accompanied by a reduction in the maximum fluctuating pressure at the plate by 75% and an increase in the maximum static pressure by 30% indicating a reduction in the oscillations of the jet column.
A proper orthogonal decomposition of particle image velocimetry vector fields revealed that an increase in the auxiliary jet velocity increased the percentage of the kinetic energy of the mean flow field of the jet but decreased the percentage of the kinetic energy of the modes associated with the aeroacoustic feedback mechanism. The vorticity of the modes associated with the aeroacoustic feedback mechanism shows that the coherent structures inherent to the feedback mechanism reduce in size and strength with increasing auxiliary jet velocity. Time-averaged particle image velocimetry vector fields revealed that at jet conditions where the acoustic tones were reduced, the interaction of the auxiliary jets reduced the maximum vorticity of the shear layer by 35% at the jet exit. Smaller amplitude and thicker shear layers are known to result in smaller maximum growth rates of disturbances in shear layers. The reduced growth rate resulted in smaller coherent structures in the jet shear layer which resulted in the smaller jet column oscillations and the elimination of the acoustic tones. / Thesis / Candidate in Philosophy
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25107 |
| Date | January 2019 |
| Creators | Finnerty, Donal |
| Contributors | McDermid, Joseph, Ziada, Samir, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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