Page 175 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

164 Fundamentals of Ocean Renewable Energy

where f d is the Doppler shift, f s is the frequency of sound when source and
receiver are both stationary, V is the relative velocity between source and
receiver, and C is the speed of sound, given by
C = fλ (7.2)
where f is frequency and λ is wavelength.
An ADV transmits sound at a fixed frequency, and listens to echoes returning
from sound scatterers in a small volume of water: small particles or plankton
that reflect the sound back to the instrument. Although the sound is scattered
in all directions, a small amount is reflected back to the transducer. The ADV
relies on the assumption that these scatterers move at the same velocity as the
water. Because the ADV both transmits and receives sound, the Doppler shift is
doubled, and so Eq. (7.1) becomes
f d = 2f s (V/C) (7.3)

The Doppler shift only works with radial motion. Although angular motion
changes the direction between source and receiver, it does not alter the distance
separating them, and so does not cause a Doppler shift. Limiting the Doppler
shift to the radial component, Eq. (7.3) becomes
f d = 2f s (V/C) cos A (7.4)
where A is the angle between the relative velocity vector and the line between
the ADV and scatterers.
The reflected acoustic wave is measured by three receiving probes (Fig. 7.4).
The changes in the frequency of the reflected signals are the indicator of
velocities in three directions, corresponding to the three receiving probes. An

FIG. 7.4 A 10 MHz SonTek ADV.