An Investigation of the Mechanisms of Wind Generated Surface Waves
Janajrah, Ma'moun Ali Mohammad
The goal of wind-waves research is to predict the waves field and its effect on the environment. That environment could be natural or imposed by human endeavour. The mechanism of wind generated waves is described in the present work as a wind-bulk flow interaction rather than as a mechanical process which only transfers the wind energy to the wave. In the light of this description, the generation and growth of surface waves are functions of the physical properties of the interface, density of the bulk flow perturbations and wind shear stress. While the present models for the prediction of surface growth and evolution show some consistence - in some cases - with observations that were conducted in laboratories and in real fields, the work presented in this thesis justifies and explains the inconsistency or contradictions in other cases between the observations and the predictions. Also, physical interpretations for observations, for example wave growth with fetch, are suggested in the present work. To illustrate the physical mechanism responsible for wave generation and growth under the effect of wind action, two approaches are used. The first involves studying the effect of the physical properties of the water surface on atmospheric input into the bulk and thus the effect on the formation and growth of capillary waves. The second involves studying the correlation between the wave formation and growth and the density of the bulk perturbations. Wide ranges of previous data are used to analyse the effect of the physical properties of the water surface on wave generation and growth mechanism for the first approach. Also, a group of experiments using the PIV system (Particle Image Velocimetry) were conducted to study the correlation between the wind speed, bulk flow evolution and wind-waves‟ generation and growth for the second approach. The main physical parameters which are responsible for the generation and growth of capillary waves are determined. The Ohnesorge number is modified to predict the generation and growth of surface waves. In the second part, additional physical parameters of the bulk flow are introduced to illustrate the correlation between the wind generated waves and bulk flow evolution. A new parameter is used to scale the transition of the bulk flow from laminar flow to turbulent flow or the transition of the water surface from an undisturbed surface to a fully disturbed surface. The history of wind-wave research is relatively short. Although there were basic developments in the 18th century, a concentrated effort really began as a result of the military imperative of the Second World War. These developments were however, largely empirical. A theoretical frame work began to develop with the studies of wind-wave generation in the last century. The present work is conducted to fill some gaps in wind generated surface waves research and to introduce new approaches to simplify understanding wind-waves field and its effect on the environment.