ocean waves
TRANSCRIPT
OceanWaves
Kinematic Viscosity of Sea
Water
Density of Fresh and Sea
Water
Distribution of Wind Speed
Wind Speed – Beaufort
Number
Wave Generation
Of the momentum transformed from wind to
water 97% go to the surface layer producing
surface currents. The remaining energy is
transferred to water surface creating waves
Gentle breeze creates ripples which provide
rough surface allowing wind to move water
If the wind dies ripples disappear quickly
If the wind continues, these are gradually
transformed to larger waves
Wave generation (contd.)
Newly formed waves are generally short and
choppy providing a roughened water surface for more energy to be transferred to water. These
waves are steep which slowly change to smooth
waves of larger length depending on steady wind
blowing over a long period over large body of
water.
Wave generation (contd.)
The wave size depends on
Wind speed
wind direction
time that the wind blows
fetch (distance over which wind
blows in a constant direction)
Sea State
Wave generation (contd.)
Waves break when height to length of wave ratio
reaches 1:7
Ocean waves reach maximum size when these can not accept more energy from wind. The sea
is then said to be fully developed (FDS) when it
can accept no more energy and thus waves
break. The energy supplied by the wind is equal to
the energy lost by breaking waves.
Wave generation (contd.)
As sea develops wave length and speed increase
and slope reduces. Eventually waves travel out of the wind area and these are called swells. Long
period swells travel long distances without loosing
energy (amplitude)
Waves in Shallow waterAs deep water waves move towards the coast they come to shallow water zone, become shorter and slower and steeper and ultimately break. Deep water waves move faster towards the coast compared to the waves in shallower regions. This interaction leads to the breaking waves being parallel to coastline irrespective of wave direction in deep water
Isobar (Surface Wave) elevation
(i) General
(ii) In deep water, h>>0 and z>= 0
(iii) In deep water on surface, z=0
Regular sine wave in
Deep water
tkxtx a ,
Regular Wave –
contours of equal pressure
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a
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iii
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i txktx
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ii
i
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i
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2( )
i
i
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2i aiS d
0
( )nnm S d
0 2
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2 2
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4 2
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010/1
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0
1.5
0.4
5.2
mh
mh
mh
So,
Average Wave Height: ĥ
Significant Wave Height (Average of highest 1/3 wave heights): h⅓
Average of highest 1/10 wave hts:
h1/10
Effects of waves on ship
operation
Motions – discomfort
Slamming
Green sea on board
Speed loss
Mechanical failure
Instantaneous loss of stability
Structural stresses and vibration
