314
9 Experimental Methods in Fluid Mechanics
manufactured by Datawell, Haarlem, the Netherlands, the transfer function
shows that the buoy response is essentially fiat for wave periods between 5 and
10 s. Some attenuation is observed for periods between 10 and 25 s (Massel,
1996a).
When a moored wave rider follows the waves, the force on the mooring line
will change. This force is produced by a change in buoy's immersion. The wave
rider buoy does not follow the wave surface if the wavelength is less than 5 m
(the wave period is less than 1.8 s). To avoid measurement of unwanted accelerations due to roll and pitch of the buoy, the sensitive axis of the accelerometer
is mounted on a stabilized platform. To keep a moored wave rider at the correct
position, a rubber cord is used as a part of the mooring system. The stiffness
of the rubber cord allows the buoy to follow waves up to 20 m. A buoyancy of
approximately 900N keeps the wave rider from submerging under the combined
action of an 18 m wave height and a 1m/s current.
Buoy displacement records are internally filtered at a high frequency cut-off
of 0.6 Hz. Transmission of data is to Argos satellite or through standard 27-40
MHz radio-link to shore. The buoy measures heave in the range ±20 m with 1
cm resolution for wave period 1.6-20 s. For the directional period buoys, the
direction is measured with a resolution of 1.5°.
For large wave buoys, the response of the buoy itself must be taken into
account when calculating the wave parameters, especially those related to directional spreading. The buoy response depends on mooring constraints which
are varied by a number of factors, including current and wind speed (Steele
et at., 1992; Tucker, 1989; Gnanadesikan and Terray, 1994). A comprehensive
comparison of various types of wave buoys is given by Allender et al. (1989).
In recent years, new methods of ocean wave measuring, based on satellite
techniques have been developed. We will describe them in the next section.
9.3 Remote Sensing Techniques
9.3.1 Introduction
The launch of oceanographic satellites has provided a new and extensive set of
data on the state of the ocean waters. These observations are of special importance in the study of the deep oceans, which comprise the vast majority of
the globe, where reliable observations are almost nonexistent. Oceanographic
remote sensing is usually subdivided into environmental satellites or more specialized satellites. The most important satellites designed for oceanographic applications include SEASAT, GEOSAT, TOPEX/POSEIDON, RADARSAT, ERS-I,
ERS-2 and SeaWiFS. Although SEASAT suffered a disappointingly short life of
106 days, it attracted great interest within the oceanographic community. The
first successful space mission specifically designed for studying the circulation of
the ocean waters was TOPEX/POSEIDON, launched on August 1992. To meet
the stringent measurement accuracy required for ocean circulation studies, a
number of instruments have been installed on the satellite platform, such as a
9 Experimental Methods in Fluid Mechanics
manufactured by Datawell, Haarlem, the Netherlands, the transfer function
shows that the buoy response is essentially fiat for wave periods between 5 and
10 s. Some attenuation is observed for periods between 10 and 25 s (Massel,
1996a).
When a moored wave rider follows the waves, the force on the mooring line
will change. This force is produced by a change in buoy's immersion. The wave
rider buoy does not follow the wave surface if the wavelength is less than 5 m
(the wave period is less than 1.8 s). To avoid measurement of unwanted accelerations due to roll and pitch of the buoy, the sensitive axis of the accelerometer
is mounted on a stabilized platform. To keep a moored wave rider at the correct
position, a rubber cord is used as a part of the mooring system. The stiffness
of the rubber cord allows the buoy to follow waves up to 20 m. A buoyancy of
approximately 900N keeps the wave rider from submerging under the combined
action of an 18 m wave height and a 1m/s current.
Buoy displacement records are internally filtered at a high frequency cut-off
of 0.6 Hz. Transmission of data is to Argos satellite or through standard 27-40
MHz radio-link to shore. The buoy measures heave in the range ±20 m with 1
cm resolution for wave period 1.6-20 s. For the directional period buoys, the
direction is measured with a resolution of 1.5°.
For large wave buoys, the response of the buoy itself must be taken into
account when calculating the wave parameters, especially those related to directional spreading. The buoy response depends on mooring constraints which
are varied by a number of factors, including current and wind speed (Steele
et at., 1992; Tucker, 1989; Gnanadesikan and Terray, 1994). A comprehensive
comparison of various types of wave buoys is given by Allender et al. (1989).
In recent years, new methods of ocean wave measuring, based on satellite
techniques have been developed. We will describe them in the next section.
9.3 Remote Sensing Techniques
9.3.1 Introduction
The launch of oceanographic satellites has provided a new and extensive set of
data on the state of the ocean waters. These observations are of special importance in the study of the deep oceans, which comprise the vast majority of
the globe, where reliable observations are almost nonexistent. Oceanographic
remote sensing is usually subdivided into environmental satellites or more specialized satellites. The most important satellites designed for oceanographic applications include SEASAT, GEOSAT, TOPEX/POSEIDON, RADARSAT, ERS-I,
ERS-2 and SeaWiFS. Although SEASAT suffered a disappointingly short life of
106 days, it attracted great interest within the oceanographic community. The
first successful space mission specifically designed for studying the circulation of
the ocean waters was TOPEX/POSEIDON, launched on August 1992. To meet
the stringent measurement accuracy required for ocean circulation studies, a
number of instruments have been installed on the satellite platform, such as a