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Introduction to the ADFM Velocity Profiler™
The ADFM divides the return signal into discrete regular intervals that cor-
respond to different depths in the flow. Velocity is calculated from the fre-
quency shift measured in each interval. The result is a profile, or linear dis-
tribution of velocities, along the direction of the beam. Each of the small
black circles in
Figure 1-24
represent an individual velocity measurement in
a small volume known as a depth cell.
The directions of the velocity profiles in
Figure 1-24
are based on the ge-
ometry of the ADFM's transducer assembly.
Figure 1-35
shows a side view
of the transducer assembly. The profiles shown in
Figure 1-24
are gener-
ated from velocity data measured by an upstream and downstream beam
pair. The data from one beam pair are averaged to generate Profile #1, and
a beam pair on the opposite side of the transducer assembly generates Pro-
file #2.
Since Doppler measurements are directional, only the component of veloc-
ity along the direction of transmit and receive is measured, as shown in
Figure 1-35.
Narrow acoustic beams are required to accurately determine
the horizontal velocity from the measured component. The narrow acoustic
beams of the ADFM insure that this measurement is accurate. Also, the
range-gate times are short and the depth cells occupy a small volume - cyl-
inders approximately 5 centimeters (2 inches) long and 5 centimeters (2
inches) in diameter. This insures that the velocity measurements are truly
representative of that portion of the flow. Potential bias in the return energy
spectrum due to range dependent variables is avoided. The result is a very
precise measurement of the vertical and transverse distribution of flow ve-
locities.
Flow Velocity Vector
Vector Component
Figure 1-3.
ADFM Beam Geometry
The velocity data from the two profiles are entered into an algorithm to de-
termine a mathematical description of the flow velocities throughout the
ADFM Technical Manual (January 2000)
page 1-5
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