Page 174 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 163

FIG. 7.3 A tilt current meter in operation made by Lowell Instruments, LLC.

velocity, causes the float to tilt, and the tilt angle can be directly related to the
current speed.
As discussed in Chapter 3, tidal currents interact with the sea bed to generate
a vertical velocity profile. In addition, the region of the velocity profile near
the surface is influenced by wind. Therefore, it is important to consider what
depth in the water column is representative of the tidal currents at a location,
and this is a factor that must be considered when making measurements in situ
with current meters. Alternatively, acoustic profilers can be used, as described
in Section 7.1.4.

7.1.3 Acoustic Doppler Velocimeter
An acoustic Doppler velocimeter (ADV) is based on the Doppler effect, which
may be a familiar concept to you from the relative change in frequency of a
train whistle or car horn as it travels towards or away from an observer. If an
observer is stationary whilst sound waves pass, n waves would pass during a
time interval t. If the observer was to walk towards the source of sound, more
than n waves would pass during t. Similarly, if the observer was to walk away
from the source of sound, less than n waves would pass during t. The Doppler
shift is the difference between the frequency heard whilst standing still, and
the frequency heard when moving either towards or away from the sound. It is
defined as
f d = f s (V/C) (7.1)