Understanding the Influence of Nonlinear Seas on Wind Generated Waves

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Understanding the Influence of Nonlinear Seas on Wind Generated Waves, Is A Well-Researched Topic, It Is To Be Used As A Guide Or Framework For Your Research.

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.
Using multiple data analysis techniques, this study reveals that 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.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ………………………………………………………………………………………………………….. ii
LIST OF TABLES …………………………………………………………………………………………………………………….. vii
LIST OF FIGURES ………………………………………………………………………………………………………………….. viii
CHAPTER
1. INTRODUCTION …………………………………………………………………………………………………………………… 1
1.1 Wind Waves ……………………………………………………………………………………………………………………… 1
1.1.1 Wind Wave Terminology …………………………………………………………………………………………….. 1
1.1.2 Classification of Wind Waves ………………………………………………………………………………………. 3
1.1.3 Wind Influence over the Ocean ……………………………………………………………………………………. 5
1.1.4 Wave Statistics and Wave Spectra ………………………………………………………………………………… 7
1.1.5 Fetch-Limited Wave Growth ……………………………………………………………………………………….. 9
1.1.6 Objective of the Wind Wave Study …………………………………………………………………………….. 10
1.2. Turbulence …………………………………………………………………………………………………………………….. 11
1.2.1 Measuring Turbulence ………………………………………………………………………………………………. 11
1.2.2 Turbulent Energy Spectrum ……………………………………………………………………………………….. 13
1.2.3 Modeling Wind and Wave Induced Turbulence ……………………………………………………………. 14
1.2.4 Objective of the Turbulence Study ……………………………………………………………………………… 15
1.3 Overview of Coming Sections …………………………………………………………………………………………… 16

2. PREDICTING WIND WAVE SUPPRESSION ON IRREGULAR LONG WAVES ……………………… 17
2.1 Chapter Abstract ……………………………………………………………………………………………………………… 17
2.2 Introduction …………………………………………………………………………………………………………………….. 17
2.2.1 Incorporation of Wind Wave Suppression into Sin…………………………………………………………. 19
2.3 Experimental Set-up and Data Collection ……………………………………………………………………………. 22
2.3.1 Test Facility …………………………………………………………………………………………………………….. 22
2.3.2 Instrumentation & Data Collection ……………………………………………………………………………… 22
2.3.3 Characterizing the Wind Field ……………………………………………………………………………………. 26
2.4 The Chen and Belcher (2000) Model ………………………………………………………………………………….. 27
2.4.1 Long Wave-Induced Stress ………………………………………………………………………………………… 28
2.4.2 Growth Rate of the Long Wave, …………………………………………………………………………….. 28
2.5 Methods: Data Analysis ……………………………………………………………………………………………………. 32
2.5.1 Experimental Energy Ratio, + ………………………………………………………………………………. 32
2.5.2 Quantifying the Growth Rate Coefficient and the Atmospheric Pressure Coefficient ………… 34
2.6 Results ……………………………………………………………………………………………………………………………. 34
2.6.1 CBM Results for the Monochromatic Waves ……………………………………………………………….. 34
2.6.2 CBM Results for Irregular Waves ………………………………………………………………………………. 35
2.6.3 The Growth Rate Coefficient for the Irregular Waves ……………………………………………………. 37
2.7 Discussion ………………………………………………………………………………………………………………………. 39
2.7.1 Modifying the CBM for Irregular Long Waves …………………………………………………………….. 40

2.7.2 Analysis of the Growth Rate Coefficient ……………………………………………………………………… 44
2.8 Conclusions …………………………………………………………………………………………………………………….. 49
3. TURBULENCE GENERATED BY IRREGULAR WAVES AND WIND …………………………………… 51
3.1 Chapter Abstract ……………………………………………………………………………………………………………… 51
3.2 Introduction …………………………………………………………………………………………………………………….. 52
3.3 Methods for Quantifying Turbulence from ADCPs and ADVs ………………………………………………. 56
3.3.1 Modified Structure Function for ADCPs ……………………………………………………………………… 56
3.3.1.2 Theoretical Assumptions of the Structure Function ………………………………………………… 57
3.3.1.2 Instrument and Data Collection Considerations for the Structure Function Method ……. 59
3.3.2 Inertial Dissipation Method for ADVs …………………………………………………………………………. 60
3.3.2.1 Theoretical Assumptions …………………………………………………………………………………….. 61
3.3.2.2 Removal of Surface Wave Bias …………………………………………………………………………… 62
3.3.2.2.1 Covariance Method …………………………………………………………………………………….. 63
3.3.2.2.2 Phase Method …………………………………………………………………………………………….. 64
3.3.2.2.3 Empirical Mode Decomposition (EMD) Method ……………………………………………. 65
3.4 Case Study ……………………………………………………………………………………………………………………… 67
3.4.1 Experimental Set-Up and Instrumentation ……………………………………………………………………. 67
3.4.2 Wave Environments ………………………………………………………………………………………………….. 69
3.4.3 Data Analysis …………………………………………………………………………………………………………… 70
3.4.3.1 Removal of Surface Waves …………………………………………………………………………………. 71
3.4.3.2 Quantifying TKE and u*……………………………………………………………………………………… 74

3.4.3.3 Wave Shape Analysis …………………………………………………………………………………………. 74
3.5 Results ……………………………………………………………………………………………………………………………. 75
3.5.1 TKE and u* ………………………………………………………………………………………………………………. 75
3.5.2 Variation in Long Wave Steepness along the Basin ………………………………………………………. 77
3.6 Discussion ………………………………………………………………………………………………………………………. 78
3.6.1 Observed Trend in TKE …………………………………………………………………………………………….. 79
3.6.2. Comparison of Observations with Theory …………………………………………………………………… 79
3.6.3 Improving the Experiment in the W2 …………………………………………………………………………… 81
3.6.4 Procedure for Planning an Experiment ………………………………………………………………………… 83
3.7 Conclusions …………………………………………………………………………………………………………………….. 85
4. CONCLUSIONS ……………………………………………………………………………………………………………………. 86
BIBLIOGRAPHY ……………………………………………………………………………………………………………………… 89
APPENDIX…………………………………………………………………………………….………….95
BIOGRAPHY OF THE AUTHOR ………………………………………………………………………………………………. 97

Additional information

Author

Taylor L. Bailey

No of Chapters

4

No of Pages

112

Reference

YES

Format

PDF

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