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Title: The interaction of gravity-capillary water waves with a laminar air flow
Author: Tsai, Yuan-Shiang
ISNI:       0000 0001 3536 9738
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2002
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The physical processes responsible for the growth of water waves under the action of a turbulent air flow are known to be extremely complex. To date theoretical models addressing this phenomenon have not been entirely successful because of the complexity of turbulent flow and the interaction physics. On the other hand, a laminar flow yields a relatively simple and fixed model for the interaction process. It has been proved for example that on a solid plate, the initial growth of artificial two-dimensional air waves inside the laminar boundary layer can be explained by linear instability theory. Similarly, on an air-water interface of two phase flows, it is interesting to investigate whether or not the growth process of a perturbation on the initially flat water surface under the shearing action of a laminar air flow will be dominated by linear instability theory. For this reason, an experimental and numerical study of the interaction of gravity-capillary water waves with a laminar air flow has been carried out. A filtered integration method was introduced to solve the coupled Orr-Sommerfeld equations, focusing on the spatial amplification in order to facilitate direct comparison with the experimental observations. A low turbulence wind tunnel, specially designed and built for the present study of wind-wave interaction, provided the information of the wave growth rates and phase velocities. A simple Pitot tube was used, with close attention to near boundary correction procedures, to obtain the laminar boundary layer velocity profile over the water surface. A highly sensitive optical technique of twin reflected laser beams was developed to measure the wave slopes and phase velocities of long-crested gravity-capillary water waves with small amplitude. These were generated by a pneumatically-activated diaphragm generator using different input frequencies. The results of the Pitot tube measurements confirmed for the first time both the continuation of a Blasius type laminar boundary layer over the water surface and the boundary conditions conventionally assumed in theoretical analysis. The numerical model predicted that the growth rate and the phase velocity are frequency and fetch dependent. The wave height distribution with respect to fetch, integrated from the local growth rate, shows good agreement with the experimental results. It is concluded that the initial stage of the growth of small amplitude gravity-capillary water waves under the action of a laminar air flow can be predicted by linear instability theory.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Fluid mechanics