aBasics of wave motion
BASICS OF WAVE MOTION Prof.V. Sundar, Department of Ocean Engineering, Indian Institute Of Technology Madras, INDIA
2.1 GENERAL: Waves are periodic undulations of the sea surface, the complexity of which is most challenging to those working in the oceans. They impose highly variable and fatigue type loading on offshore and exposed coastal structures, and they may adversely affect coastlines and harbor facilities and induce violent motions in moored ships and floating structures. The designer of offshore structures must have a good understanding of the basics of wave generation, its characteristics and behavior before evaluating forces and other potential effects. Turbulent pressures fluctuations and variations in wind velocity cause an initial disturbance of the sea surface, thus precipitating wave growth. The initial movement of the sea surface is perpendicular to that of the wind, but it quickly aligns itself approximately with the wind direction or some oblique angle to the wind. A light air movement of only two knots or less will cause small ripples on the sea surface, which disappear immediately when the generating wind stops. These ripples are associated with capillary force and are in conflict with surface tension. As the wind speed increases, and if, it is of any duration, larger ‘Gravity waves’ begin to develop as the wave height builds up, and a more complex pressure distribution forms at the surface. Normal stresses as well as tangential stresses now act on the surface profile to further wave development. The exact way this growth begins is still not completely understood. These are, however, many semiemperical relationships that describe the growth of wind waves reasonably well. The ultimate state of wave growth depends primarily on three parameters basic to wave forecasting: the fetch (F) or the distance over which the wind blows, the wind velocity (V) and the duration (t) of time for which the wind blows. Thus, for a given...