a brief Synopsis
DAEDALUS Informatics
Athens, Greece 01/10/1994*double click on empty space anywhere on page, to access pagetop
4. SEKE - the Wave Energy Conversion Device4.1. Wave Motion Development over Modified Sea-Bed Profiles:A brief overview of recorded experience
A sea wave whose "crest" is moving toward any direction parallel to the free sea level is defined as "running wave". The waves in the open seas are running waves. The propagation speed of running waves depends on their wavelength (L) as well as the depth of the sea. In shallow seas the waves move with the same velocity regardless of their wavelength.. The running wave is not produced by mass transfer of water particles but by transfer of their kinetic energy to the adjacent ones. In this way the sea particles perform circular motions with approximately constant orbital speed. In open seas, where the large depth to the sea bed permits the development of waves of large wavelength and of relatively small height, the motion of the water particles is theoretically circular. In reality, however, a small excess in velocity during the ascent to the wave crest phase produces a relatively small displacement of the sea particles along the direction of the blowing wind. This roughly circular motion, which creates the impression of displaced and running waves, is performed on circles of radii decreasing exponentially with increasing depth. At a depth equal to L/4 the radius of the circular motion becomes equal to zero. Furthermore, in a shallow sea, of depth less than L/4, the circular motion near the surface turns, with increasing depth, into elliptical of increasing eccentricity and with the major axis parallel to the sea bed. The eccentricity becomes equal to one and the motion linear and oscillatory on the sea bed. The momentum of the particles performing these motions is a decreasing function of the depth. In this basically laminar motion of the sea particles, where all perform circular or elliptical motion, transfer of momentum occurs smoothly from each particle to the adjacent ones. The characteristics and process of displacement of the running waves is shown in (1).
Diagram (2), attempts to provide a schematic representation of the wave crest reformation up to the brake of its continuity, as the wave ascends an exponentially modified sea-bed. As is expected, a descent cycle will also follow once the kinetic energy of the wave turns into potential (water level) -minus the resulting friction, its overall velocity reaches zero and the phase of descent begins.
(1) Wave motion and momentum tranfer diagrams. Wave motion in shallow water (2) Wave motion and crest evolution over a gradually modified sea bed
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