Adelaide Research and Scholarship
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|Title: ||Study of large-scale coherent structures in the near field and transition regions of a mechanically oscillated planar jet.|
|Author: ||Riese, Michael|
|Issue Date: ||2009|
|School/Discipline: ||School of Mechanical Engineering|
|Abstract: ||Enhancing the performance of mixing and fluid entrainment by excitation of quasi-steady jets
has been a subject of research for more than three decades. During the 1980s a special emphasis
was placed on mechanically oscillating planar jets and the possibility to augment thrust of V/STOL
aircraft. However, during this time, little attention was paid to the classification of flow regimes,
the development of coherent structures or the existence of different regions in the flow within the
jet near field.
For the present study, a large aspect ratio nozzle was oscillated in the direction transverse
to the width of the nozzle in simple harmonic motion. For a constant nozzle height, the stroke
length, oscillation frequency and jet velocity were systematically varied. Over 240 flow cases
were examined using a novel method of phase-locked flow visualisation. Following an initial
analysis of the acquired data, a small subset of flow conditions was selected for further quantitative
investigation using Particle Image Velocimetry (PIV).
The phase-locked flow visualisation led to the identification and classification of three separate
flow regimes, the Base Flow, the Resonant Flow and the Bifurcation Flow Regimes. Each regime
is linked to the other regimes by the presence of a small number of repetitive coherent structures
in the form of starting and stopping vortices.
The analysis revealed a relationship between the stroke-to-nozzle height ratio and the ratio of
the forcing frequency to the natural vortex shedding frequency in the planar jet. This directly contradicts
the relationship between the Strouhal and Reynolds numbers of the jet that was proposed
by previous investigators.
Comparison of phase-locked PIV and flow visualisation data confirms both, the validity of the
new regime classification and the identification of relevant large-scale structures. Time-averaged
vorticity data are also used to further illustrate the differences between the three flow regimes.
Investigation of the time-averaged qualitative data for the Base and Resonant Flow Regimes
show that three distinct flow regions exist within both regimes. Adjacent to the nozzle is the initial
formation region, where all large-scale structures form. This is followed by a coherent near-field
region in which the jet exhibits very little spread for both the Base and Resonant Flow Regimes.
Within this region no pairing of the large-scale vortices from the opposing sides of the flow can
be found. This region is followed by a transition region that is marked by the sudden breakup and
dissipation of all visible large-scale coherent structures.
The vortex formation distance is then investigated using the available PIV data and compared
with the results of previous investigations. The data show that the formation distance depends on
the jet velocity, oscillation frequency and the stroke length. The agreement with previous data is
poor due to differences in the method of measurement.
Quantitative data are also used to investigate the centreline velocity decay in relation to changes
of the jet Reynolds number and stroke-to-nozzle height ratio. The results show that the velocity
decay rate increases with increasing stroke length as is expected from findings of earlier studies. In
addition the centreline velocity decay rates in the mean jet transition region appear to be constant
for each stroke length in the cases examined.
Finally, conclusions are drawn and recommendations for future work are presented.|
|Advisor: ||Nathan, Graham|
Kelso, Richard Malcolm
|Dissertation Note: ||Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2009|
|Keywords: ||jet flow; planar; PIV; flow visualisation; oscillating|
|Provenance: ||Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.|
|Call number: ||09PH R561|
|Description (link): ||http://proxy.library.adelaide.edu.au/login?url=http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349701|
|Appears in Collections:||Research Theses|
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