Author: Tang, Lian
Title: Numerical and experimental studies of air entrainment and turbulent flow field under breaking waves in the surf zone
Advisors: Wai, Wing Hong Onyx (CEE)
Degree: Ph.D.
Year: 2019
Subject: Hong Kong Polytechnic University -- Dissertations
Water waves -- Mathematical models
Ocean waves -- Mathematical models
Water -- Air entrainment
Fluid dynamics
Department: Department of Civil and Environmental Engineering
Pages: xxvi, 171 pages : color illustrations
Language: English
Abstract: The study described in this dissertation presents two key contributions: 1) it proposes a numerical model to simulate the air entrainment processes and void fraction distributions, so as to investigate the turbulent aerated flow under breaking waves in the surf zone and the turbulence behavior under the effect of entrained air bubbles; 2) it provides an experimental investigation of bubble plume evolution and void fraction distribution under a tsunami-like breaking solitary wave on a sloping beach. Wave breaking is an important physical process in the surf zone, which generates a high intensity of turbulence and traps a large amount of air into the water. The entrained air quickly bursts into small air bubbles to have strong interactions with the surrounding flow. Many studies have reported that the entrained air bubbles can change the process of turbulence generation, transport and enhance the turbulence dissipation, and affect the local turbulence field. Therefore, it is necessary to reasonably describe the air entrainment process and consider the entrained air bubbles' effect on the turbulent flow field when simulating the turbulent aerated flows under breaking waves in the surf zone. In this study, a mixture model is proposed to simulate the air entrainment and void fraction distribution as well as the turbulent aerated flows under breaking waves in the surf zone. A volume fraction equation is incorporated into a 2D numerical model NEWFLUME to simulate air entrainment processes and obtain the distribution of void fraction. The free surface is reconstructed based on the calculated void fraction. The model solves the Reynolds-averaged Navier-Stokes (RANS) equations for flows. The turbulence closure is accomplished by the k-ε two-equations model, in which the bubble-induced turbulence production and dissipation are considered by buoyancy production terms. The model is first validated against laboratory measured mean velocity and void fraction for self-aerated flow in an open channel. Then the predictive capability of the model in turbulent flows under breaking waves is investigated by simulating two laboratory experiment cases for spilling breaking waves on a sloping beach. The predicted free surface displacement, turbulent kinetic energy, and the mean velocities are compared with the available experimental data and other numerical results. The comparative analysis shows that the present model provides results that more closely correspond to the experimental data, especially for the aeration region. Based on the numerical results, the aeration effect on turbulence field under breaking waves are discussed. It is found that the exclusion of aeration-induced turbulence dissipation is a key reason for the overestimation of turbulent intensity under breaking waves. When the aeration effect is considered, the depth-averaged turbulent kinetic energy can be reduced more than 50% for a specfic vertical cross section, this is because the presence of bubbles suppress turbulence production and enhance the turbulence dissipation rate, resulting the decrease of the turbulence intensity of the flow. The proposed model demonstrates improved predictive capability for mean flow field and turbulence intensities under breaking waves, an improvement with helpful, practical applications in coastal engineering.
New laboratory measurements are conducted in a wave flume for a plunging breaker on a 1:20 sloping beach to investigate the air entrainment process and bubble plume evolution as well as the void fraction distribution. A high-speed camera (1280*800 pixels with a framing rate of 1000 fps) is used to capture the bubble plume evolutions after wave breaks on the slope. A fiber optic reflectometer (FOR) is used to measure the instantaneous void fraction in the aeration region. The plunging wave breaking process and bubble plume evolutions on a slope are described with the assistance of images by high-speed camera. The void fraction distributions in the bubble plume are presented and discussed. Then, the proposed numerical model is applied to simulate the plunging breaker. Comparisons of the free surface displacement, bubble plume evolutions and void fraction distributions between the predicted results and experiment measurements are made. The simulated maximum ensemble-averaged void fraction inside the collapsed cavity bubble cloud is 72%, which is slightly larger than the measured maximum value 62.6%. The distribution of void fraction in the collapsed cavity bubble cloud and splash ups are well captured by the proposed model, though a phase difference exists between the measured data and simulated results. Lastly, the entrained air bubbles' effect on turbulent intensities and stress tensor of the plunging breaker on a slope are examined for which laboratory methods are hard to achieve. It is found that in the high aerated region (FOR station 1), the time-averaged and depth-averaged turbulent kinetic energy is reduced by 49.1% and 47.9% when the air bubbles' effect is considered. In addition, the generalized shear stress is damped significantly when the aeration effect is considered. The diffusion shear stress is about 1/5 magnitude of the generalized shear stress tensor at each comparison positions, indicating its important role in the bed stress behavior. The analysis of stress tensor behaviors under turbulent aerated flows provides a basic foundation for investigation of sediment suspension and transport in the surf zone.
Rights: All rights reserved
Access: open access

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