Page 181 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

170 Fundamentals of Ocean Renewable Energy

TABLE 7.4 Record Length Required to Resolve Semidiurnal
and Diurnal Constituents, Based on the Rayleigh Criteria
Constituent Comparison Constituent Record Length (h)
M2 – 13
S2 M2 355
N2 M2 662
K2 S2 4383
K1 – 24
O1 K1 328
P1 K1 4383
Q1 O1 662

set to a few centimetres to record the flow in detail over a limited range. Such
data are suitable for calculating turbulence properties of the flow, for example,
using the structure function technique to calculate turbulent dissipation [12].
A typical time series of backscatter and current speed for a 2-week ADCP
deployment is shown in Fig. 7.9. Note that ADCPs also contain a pressure
sensor, which logs the variation in the water surface. This plot makes both near-
bed (constant) and near-surface (variable) blanking regions clear.

Hull-Mounted ADCP
ADCPs are also frequently deployed in the hull of a ship, looking downwards
through the water column. When the ADCP data are postprocessed, the ship’s
movement can be corrected by using either bottom tracking or the ship’s GPS.
The advantage of using a hull-mounted ADCP is that it introduces spatial
variability into the sampling programme. Therefore, a transect, or a series
of transects, can be built up over time, provided the timescale for collecting
each transect is relatively short compared with the tidal period, and so can
be considered as an instantaneous time slice (e.g. Fig. 7.10). Because data
collected using a hull-mounted ADCP will be characterized by pitch and roll
due to the vessel’s movement, this form of data collection is not reliable for
either (a) quantifying the vertical component of velocity (other than in regions,
such as sills, where this component is very strong), or (b) estimating turbulence
properties. In both of these scenarios, a moored ADCP is required.