Effects of Initial Conditions on Turbulence Length Scale and Energy Distributions in the Near to Intermediate Field of a Round Free Jet
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This thesis examines the effects of spatial location, Reynolds number and near exit flow modification on the development region of a round, free, turbulent jet. It is based on the publications generated by the author. The experiments were carried out over the range of Reynolds numbers between 10000 < ReD < 50000, where ReD is calculated based on the jet exit mean velocity and the nozzle exit diameter. The measurements were performed in the near- to intermediate-field region of a free jet defined between 0 ≤ x/D ≤ 30. In order to control the flow near the exit, two wire rings, with square cross-sections, of sides h = 1.5 mm, and outer diameter Dwire = 71.6 mm (positioned in the shear layer and called Rsl) and Dwire = 60 mm (positioned in the potential core and called Rpc) were placed at a stand-off distance downstream of the jet nozzle exit plane x/D = 0.03. Both stationary and flying hot wires were used to investigate the jet flow field. The results showed a considerable reduction in the jet spread rate and turbulence intensity using the passive rings. The reduction in the velocity decay rate was more obvious in the case of Rsl in lower Re; however, it was observed that as Re increases, the velocity decay rate became nearly the same for both cases of Rsl and Rpc. The axial velocity spectra showed the initial shear layer instability (shear layer mode) was suppressed while the jet preferred instability (preferred mode) remained active as the shear layer and potential core were modified. This shows the separation of these modes and is at variance with ideas that appeared in the literature that claimed the dependency of these two modes.