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Permanent link (DOI): https://doi.org/10.7939/R3C924

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Experiments investigating momentum transfer, turbulence and air-water gas transfer in a wind wave tank Open Access

Descriptions

Other title
Subject/Keyword
Boundary Layers
Air-Water Gas Transfer
Microscale Breaking Wave
Laboratory Experiments
Wind Wave
Surface Wave Profile
Momentum Transfer
Digital Particle Image Velocimetry (DPIV)
Turbulence
Type of item
Thesis
Degree grantor
University of Alberta
Author or creator
Mukto, Moniz
Supervisor and department
Loewen, Mark (Department of Civil and Environmental Engineering)
Examining committee member and department
Hicks, Faye (Department of Civil and Environmental Engineering)
Flynn, Morris (Department of Mechanical Engineering)
Guigard, Selma (Department of Civil and Environmental Engineering)
Steffler, Peter (Department of Civil and Environmental Engineering)
Graber, Hans (Rosenstiel School of Marine and Atmospheric Science)
Department
Department of Civil and Environmental Engineering
Specialization

Date accepted
2011-01-11T22:11:37Z
Graduation date
2011-06
Degree
Doctor of Philosophy
Degree level
Doctoral
Abstract
A series of laboratory experiments were conducted at three fetches of 4.8, 8.8 and 12.4 m, and at six wind speeds ranging from 4.1 to 9.6 m/s at each fetch in a wind-wave-current research facility. In addition, five surfactant-influenced experiments were conducted at concentrations ranging from 0.1 to 5.0 ppm at a wind speed of 7.9 m/s and a fetch of 4.8 m. The goals were to examine the momentum transfer and to characterize the turbulent flow structure beneath wind waves, and to investigate the relationship between wind waves and the gas transfer rate at the air-water interface. Digital particle image velocimetry (DPIV) was used to measure two-dimensional instantaneous velocity fields beneath the wind waves. The friction velocities and roughness lengths of the coupled boundary layers were used to characterize the flow regime and momentum transfer. The air-side flows were found to be aerodynamically rough and the water-side flows were found to be in transition and then become hydrodynamically smooth as wind speed increased. Airflow separation from the crests of breaking waves may be responsible for making the air-side boundary layer rougher and water-side boundary layer smoother. Momentum transfer was studied by examining the partitioning of the wind stress into the viscous tangential stress and wave-induced stress. It was found that the wave steepness was the most important wind-wave property that controls the momentum transfer in the coupled boundary layers. Two distinct layers were observed in the near-surface turbulence in the presence of a surfactant and three layers in clean water. In the surfactant-influenced experiments, the energy dissipation rate decayed as zeta^(-0.3) in the upper layer and in the lower layer energy dissipation rate decayed as zeta^(-1.0) similar to a wall-layer. For clean experiments, the energy dissipation rate could be scaled using the depth, friction velocity, wave height and phase speed as proposed by Terray et al. (1996) provided that layer based friction velocities were used. In the upper layer, the near-surface turbulence was dominated by wave-induced motions and the dissipation rates decayed as zeta^(-0.2) at all fetches. Below this in the transition layer turbulence was generated by both wave-induced motions and shear currents and the dissipation rate decayed as zeta^(-2.0) at a fetch of 4.8 m. However, at fetches of 8.8 and 12.4 m, the dissipation rate decayed at two different rates; as zeta^(-2.0) in the upper region and as zeta^(-4.0) in the lower region. In the third layer, the dissipation rate decayed as zeta^(-1.0) similar to a wall-layer at a fetch of 4.8 m. Four empirical relationships commonly used to predict the gas transfer rate were evaluated using laboratory measurements. The gas transfer rate was found to correlate most closely with the total mean square wave slope and varied linearly with this parameter. The three other parameterizations using wind speed, wind friction velocity and energy dissipation did not correlate as well.
Language
English
DOI
doi:10.7939/R3C924
Rights
License granted by Moniz Mukto (mmukto@ualberta.ca) on 2011-01-11T19:21:48Z (GMT): Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of the above terms. The author reserves all other publication and other rights in association with the copyright in the thesis, and except as herein provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
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