3.5 Wind-Generated Waves
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3.5 Wind-Generated Waves
3.5.1 How Does Wind Generate Waves?
The nature of external forces acting on ocean water dictates what types of waves
can be induced in the ocean. The most obvious cause of surface waves is the
action of wind. The ancient Greeks were well aware of the interaction between
the atmosphere and the sea surface. Aristotle (384-322 B.C.) realized that
wind acting on the sea surface played a very important role in the development
of waves. Pliny (AD 23-79) observed that oil poured upon waves calmed them.
However, from the time of Aristotle to the Renaissance of science in the 'Golden
Age of Discovery' in the late fifteenth century, very little progress was made
towards understanding the generation and growth of waves. It was not until
the nineteenth and twentieth centuries that more fundamental knowledge of
what causes waves, and how they behave, was accumulated.
Historically, many approaches have been utilized to describe the basic mechanism for the transfer of wind energy to surface waves. Some of these approaches
today possess only historical value, while others are still utilized in oceanographic practice. Present understanding of the wave generation mechanism is
based on two models proposed by Phillips (1957) and Miles (1957, 1962).
The Phillips model is applicable in the early stage of wave generation while
the Miles' predicts further wave growth. To explain these models, consider
the sequence of events when, after a period of calm weather, wind starts to
blow, increasing to a gale, and continues to blow at constant gale force for
some considerable time. At the beginning, no significant wave growth occurs
until the wind speed exceeds 1 m/s. Then, small steep waves form as the wind
speed increases. At this initial stage, wind energy is transmitted to the water by
pressure fluctuations. The natural flow of wind is always turbulent. The water
pressure fluctuates randomly and the air particles start to move in random
eddies throughout the air above the water surface, although the net direction
of movement remains in the direction of the main flow. Random pulses of air
pressure induce a resonant response of the water surface. Waves continue to
grow in size and their speed increases. The 'resonance model' of Phillips (1957)
assumes that there are no feedback reactions of the growing waves on the air
layer above water surface.
However, growing waves start to disturb the shear airflow and to induce extra pressure. The shear flow of fluid, gas or water, is a flow where successive
layers of fluid move with slightly different speeds. Each successive layer therefore shears over the layer beneath. The shearing air motion, modulated by the
motion of the water surface, leads to a resonant coupling between the surface
motion and the horizontal variation in the air pressure field. This coupling is a
key element in the Miles' generation model, sometimes called a 'shear model'.
These two wave generation models are compatible in the process of wave formation. At the initial stage, wave energy grows linearly in time, however, this
growth becomes exponential for the later stage of wave generation (Fig. 3.7).
89
3.5 Wind-Generated Waves
3.5.1 How Does Wind Generate Waves?
The nature of external forces acting on ocean water dictates what types of waves
can be induced in the ocean. The most obvious cause of surface waves is the
action of wind. The ancient Greeks were well aware of the interaction between
the atmosphere and the sea surface. Aristotle (384-322 B.C.) realized that
wind acting on the sea surface played a very important role in the development
of waves. Pliny (AD 23-79) observed that oil poured upon waves calmed them.
However, from the time of Aristotle to the Renaissance of science in the 'Golden
Age of Discovery' in the late fifteenth century, very little progress was made
towards understanding the generation and growth of waves. It was not until
the nineteenth and twentieth centuries that more fundamental knowledge of
what causes waves, and how they behave, was accumulated.
Historically, many approaches have been utilized to describe the basic mechanism for the transfer of wind energy to surface waves. Some of these approaches
today possess only historical value, while others are still utilized in oceanographic practice. Present understanding of the wave generation mechanism is
based on two models proposed by Phillips (1957) and Miles (1957, 1962).
The Phillips model is applicable in the early stage of wave generation while
the Miles' predicts further wave growth. To explain these models, consider
the sequence of events when, after a period of calm weather, wind starts to
blow, increasing to a gale, and continues to blow at constant gale force for
some considerable time. At the beginning, no significant wave growth occurs
until the wind speed exceeds 1 m/s. Then, small steep waves form as the wind
speed increases. At this initial stage, wind energy is transmitted to the water by
pressure fluctuations. The natural flow of wind is always turbulent. The water
pressure fluctuates randomly and the air particles start to move in random
eddies throughout the air above the water surface, although the net direction
of movement remains in the direction of the main flow. Random pulses of air
pressure induce a resonant response of the water surface. Waves continue to
grow in size and their speed increases. The 'resonance model' of Phillips (1957)
assumes that there are no feedback reactions of the growing waves on the air
layer above water surface.
However, growing waves start to disturb the shear airflow and to induce extra pressure. The shear flow of fluid, gas or water, is a flow where successive
layers of fluid move with slightly different speeds. Each successive layer therefore shears over the layer beneath. The shearing air motion, modulated by the
motion of the water surface, leads to a resonant coupling between the surface
motion and the horizontal variation in the air pressure field. This coupling is a
key element in the Miles' generation model, sometimes called a 'shear model'.
These two wave generation models are compatible in the process of wave formation. At the initial stage, wave energy grows linearly in time, however, this
growth becomes exponential for the later stage of wave generation (Fig. 3.7).