The exchange of mass (gases, water & salts) between the oceans and the atmosphere is vital to the maintenance of life on earth. At high wind velocities most of this exchange is attributable to breaking wave entrained air bubbles. A vertical supported planar plunging jet experiment was used to model the entrainment process. The bubbles were detected with a dual tip conductivity probe and a video camera. At plunging jet velocities below 1.0m/s there is no bubble entrainment. This inception velocity appears to have a Froude and Weber number scaling for large rough turbulent jets. At jet velocities up to 5m/s air appeared to be entrained via intermittent air cavities at the jet - plunge pool intersection. The entrained air packets subsequently break in the two phase free shear layer under the entrainment point. At higher jet velocities there may be partial penetration of the aerated jet surface via pulsating induction cavities plus air entrainment via jet self aeration before impact. Plunging jet air flow data displays the different types of entrainment mechanisms. Mono-phase diffusion models can be successfully adapted to describe the shear layer developing zone. The diffusion of the air bubbles is approximately a Gaussian self similar process. The mean bubble velocity profiles can be modelled using the Goertler Error function or Hyperbolic Tangent models. The bubble spectra is approximately Lognormal with a geometric mean diameter of 1.0-2.0mm for a range of jet velocities. A bubble Weber number is found to model the maximum bubble size of approximately 10mm diameter. An original adaptation of the potential flow solution for the vortex sheet is shown to be a simple and reasonably accurate finite amplitude model for water surface gravity waves, especially in deep water. This model has some interesting features, such as both vertical and horizontal asymmetry and standing wave water profile modelling. A simple and possibly insightful model of wave growth due to the wind is introduced, using a constant sea surface Reynolds number U*.sqrt(L.F)/Gamma , where U* = wind friction velocity, L = wavelength, F = fetch, and Gamma = wave field vortex circulation per wavelength. The results may have application in the modelling of air - sea gas exchanges, predicting breaking wave forces on structures and the use of the planar plunging jet as an aeration device in industry.
| Identifer | oai:union.ndltd.org:ADTP/253633 |
| Creators | Cummings, Peter D. |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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