Date of Award

Summer 8-21-2020

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Lauren Ross

Second Committee Member

Kimberly Huguenard

Third Committee Member

Anthony Viselli

Additional Committee Members

Mary Bryant

Abstract

An understanding of the influence of wind of the surface on irregular waves is important for improving ocean forecasting models. While many studies have investigated the phenomenon of wind wave suppression on the surface of mechanically generated waves in the laboratory, few studies have investigated the occurrence of this phenomenon for irregular waves. Chen and Belcher (2000) developed the first model to predict the suppression of wind waves as a function of the steepness of the long wave on which they travel. The Chen and Belcher (2000) model however, was only validated using monochromatic waves, not irregular waves, which are more representative of real ocean sea states. Additionally, few studies have investigated turbulence under irregular waves in the presence of wind in a controlled environment.

This thesis aims to satisfy two research objectives. The first is to determine the applicability of the Chen and Belcher (2000) wind wave suppression model to irregular sea states. The second objective is to provide a procedure for selecting the appropriate method for indirect measurement of turbulence beneath waves in a laboratory. To meet these objectives, a comprehensive data set consisting of wind velocity, surface elevation, and water velocity data were collected in the Alfond W2 Ocean Engineering Lab at the University of Maine. The data set consisted of a variety of irregular and monochromatic wave environments and wind speeds.

Through the use of multiple data analysis techniques, this study reveals that in order for the Chen and Belcher (2000) model to be directly applicable to irregular seas, a modification must be made to the long wave-induced stress term. This modification accounts for the wave energy associated with each frequency in wave spectrum for irregular waves, whereas the original model only accounts for a single wave frequency. The modified model is able to accurately predict the trend in the suppression of wind waves on the surface of irregular, long waves as a function of the long wave steepness. Additionally, in this work a case study is presented that reveals several limitations associated with the existing methods for indirect measurement of turbulence in a laboratory.

The results of this work expand the implications of the Chen and Belcher (2000) model to be more applicable to ocean waves. This can aid in better prediction of model parameters, such as the drag coefficient and the sea surface roughness length, which are controlled by the high frequency waves on the ocean surface. This work also provides a guide for planning an experiment to measure TKE dissipation, ε, under waves in the presence of wind in a controlled, laboratory setting, which will aid in the planning of future experiments.

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