Air Bubble Entrainment by Breaking Waves and Associated Energy Dissipation

Hoque, Ashabul (2002). Air Bubble Entrainment by Breaking Waves and Associated Energy Dissipation PhD Thesis, Architecture and Civil Engineering, Toyohashi University of Technology, Japan.

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Author Hoque, Ashabul
Thesis Title Air Bubble Entrainment by Breaking Waves and Associated Energy Dissipation
School, Centre or Institute Architecture and Civil Engineering
Institution Toyohashi University of Technology, Japan
Publication date 2002-01-01
Thesis type PhD Thesis
Open Access Status Other
Subjects 291205 Ocean Engineering
291100 Environmental Engineering
290000 Engineering and Technology
260403 Physical Oceanography
291200 Maritime Engineering
290802 Water and Sanitary Engineering
260400 Oceanography
290800 Civil Engineering
291199 Environmental Engineering not elsewhere classified
Abstract/Summary A simple mathematical model (air bubble model) that describes energy dissipation due to air bubble entrainment is proposed and applied to a series of laboratory experiments for plunging jet flows (steady) and surf zone waves (unsteady). This leads to a formulation wherein the rise velocity of bubbles is included. For the unsteady case, i.e. air entrainment by wave breaking, some parameters of the model have been estimated from the experiments and expressed in terms of local wave height and distance. Results obtained through the air bubble model are summarized in the following. Experiments in vertical circular plunging jets (steady) were performed for both in freshwater and seawater, which highlighted the distribution of void fraction, that follows closely analytical solutions derived by Chanson (1997). In addition, various properties of void fraction field were emphasized. Three scale models were used with freshwater for identical Froude numbers in the experiments, which highlighted significant scale effects when Weber number is less than 1000. Similar experiments were also performed with freshwater and seawater and the results showed lesser air entrainment in seawater plunging jets. The pseudo-bubble chord sizes obtained from the experimental data were in the range from less than 0.5 mm to more than 1.0 mm. The rate of energy dissipation due to entrained air was investigated by applying the air bubble model for three typical phenomena. The results demonstrated that the ratios of energy dissipation due to air bubble entrainment with respect to total energy loss were 25%, 1.4% and (2-4)% for hydraulic jump, 2-D vertical plunging jet and vertical circular jet, respectively. Experiments on unsteady air bubble entrainment by wave breaking were conducted in a wave channel. Maps of the evolution of the void fraction distribution in surf zone generated by various sizes breaking waves were presented. A significant fraction of the potential energy of entrained air was measured from the void fraction distributions provided by breaking waves. Measurements showed high void fraction up to 19% in plunging breakers at still water surface whereas 16% in spilling breakers. The ratio of energy dissipation due to entrained air to total energy loss was found (18-22)% and (17-19)% for spilling and plunging breaker, respectively. The characteristics of time averaged wave parameters (e.g. potential energy, kinetic energy, energy flux, radiation stress) for regular waves were discussed taking into account the air bubble effects. Analytical solutions were sought and explicit expressions were obtained for wave parameters under sinusoidal waves. Effects of the air entrainment on density, pressure and velocity fields were also discussed in detail. The conservation equations for energy and momentum were solved numerically using finite difference methods. Boundary conditions were used at the breaking point. Scale effects were discussed based on laboratory air entrainment in 2-D wave flume, which was believed to occur in small size models. The data were in good agreement with the basic assumption for vertical distribution of void fraction both in spilling and plunging breakers. The results of the air bubble model were compared with experimental data and found to give good agreement between them for the wave height and wave setup. Water level rise by entrained air was determined and found significant effects on surf zone hydrodynamics. In addition, wave run-up was measured and discussed.
Keyword ir Bubble Entrainment
Breaking Waves
Energy Dissipation
Physical Modelling
Plunging Jets
Mathematical Model.
Additional Notes The full bibliographic details are : HOQUE, A. (2002). "Air Bubble Entrainment by Breaking Waves and Associated Energy Dissipation." Ph.D. thesis, Dept of Architecture and Civil Eng., Toyohashi Univ. of Technology, Japan, 151 pages.

Document type: Thesis
Collection: School of Civil Engineering Publications
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Created: Thu, 05 Oct 2006, 10:00:00 EST by Hubert Chanson on behalf of Research Management Office