Tidal Current Energy

Tidal Current Energy: Origins and Challenges
Figure 7.4 . Section through trench in seabed east of Stroma and Swona.
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where λ is the wavelength and z is the water depth (as before). If z � λ /2,
then Equation (5) reduces to Equation (6) and wave speed depends on the
wavelength:
c � gλ
2π
In real seas there exist a large number of periodic wave constituents, all
with different amplitudes, frequencies and directions, dispersing over an ever
increasing arc front from their point of origin. The superposition of these waves
generates the randomly varying surface that is often observed, though closer
observation over time will often reveal a small number of substantial, regular
periodic waves that can be analyzed using linear wave theory, which is developed
in detail in Ref. [9] . The linear wave theory is used to generate a range of
water wave properties, applicable to both deep water and shallow water situations
where z � λ /2, which can usefully be applied to wave loading on a slender
tubular structure via the Morison equation [10] .
2.10 . Turbulence
The interaction of tidal currents with the topography that constrains them produces
flow characteristics that are very different from those of the main flow
and capable of inducing large velocity fluctuations, with obvious implications
for any device positioned in the flow current.
Bathymetrical features can also induce large-scale turbulence. Figure 7.4
shows a sectional sketch of a trench within the Pentland Firth that runs horizontally
perpendicular to the dominant flow direction. The proportions are drawn
to scale, indicating that the trench is a substantial feature relative to the depth of
the main flow and that subsequently the flow characteristics close to the seabed
and mid-depth will be influenced for considerable distances downstream.
The flow in the Pentland Firth is highly complex and notoriously difficult
to predict, being very sensitive to meteorological influences, in addition to the
(6)