8.2. MEASUREMENT TOOLS AND TECHNIQUES
489
ceiver location closer to the transmitter. Multiple receivers can be arranged
with a geometry that allows two or three orthogonal velocity components
to be resolved. Acoustic transit time current meters do not require any
special “seeding” of the flow with small particles.
Acoustic Doppler current meters operate on the same principle as
the laser Doppler velocimeter only sound waves are used instead of light.
Acoustic waves are transmitted and the sound waves are scattered by small
particles moving with the flow. The acoustic frequency measured at the
receiver will contain a frequency shift due to the movement of the reflective particles. This Doppler frequency shift is then converted to voltage,
and eventually to velocity by application of calibration factors. Different
geometrical arrangements can be used to resolve one, two, or three velocity
components.
Acoustic Doppler current meters generate sound waves having wavelengths between 0.1 mm and 2 mm, depending on the size of the small particles in the flow. For a bistatic6 acoustic Doppler current meter, sampling
volume is a function of the acoustic wavelength, acoustic beam width, and
beam intersection range. Consequently, the volume over which the velocity
is averaged ranges on the order of several mm3 to several cm3 (Wessels, et
al. 1990). This sampling volume is much larger than the sample volume of
a laser Doppler velocimeter. However, the lower frequencies of ultrasonic
waves eases the signal processing requirements substantially. Maximum
sampling frequency for acoustic Doppler current meters is a function of the
speed of the pulse and the distance travelled, but typical operating sampling rates can range up to 30 Hz. Sometimes it is necessary to seed the
flow with special particles.
This current meter features two intersecting acoustic beams.
Both types of acoustic current meters are relatively inexpensive, and
current meter calibration is based on the geometry of the instrument transmitter and receiver and the speed of sound in the fluid. Therefore, these
current meters do not require any calibration procedure beyond entering a
value for speed of sound in the model fluid. Calibrations should be checked
if there is reason to suspect the instrument geometry has been inadvertently
altered.
Some acoustic current meter configurations may be somewhat intrusive
to the flow, and this should be considered when orienting the instrument
to the principle flow direction. The reliability of acoustic current meters
is hampered by the presence of air bubbles in the flow. The speed of the
sound wave is changed by bubbles, resulting in errors in velocity estimation.
Air bubbles forming on any surfaces which serve to reflect the ultrasonic
pulse are also a problem for the same reason.
489
ceiver location closer to the transmitter. Multiple receivers can be arranged
with a geometry that allows two or three orthogonal velocity components
to be resolved. Acoustic transit time current meters do not require any
special “seeding” of the flow with small particles.
Acoustic Doppler current meters operate on the same principle as
the laser Doppler velocimeter only sound waves are used instead of light.
Acoustic waves are transmitted and the sound waves are scattered by small
particles moving with the flow. The acoustic frequency measured at the
receiver will contain a frequency shift due to the movement of the reflective particles. This Doppler frequency shift is then converted to voltage,
and eventually to velocity by application of calibration factors. Different
geometrical arrangements can be used to resolve one, two, or three velocity
components.
Acoustic Doppler current meters generate sound waves having wavelengths between 0.1 mm and 2 mm, depending on the size of the small particles in the flow. For a bistatic6 acoustic Doppler current meter, sampling
volume is a function of the acoustic wavelength, acoustic beam width, and
beam intersection range. Consequently, the volume over which the velocity
is averaged ranges on the order of several mm3 to several cm3 (Wessels, et
al. 1990). This sampling volume is much larger than the sample volume of
a laser Doppler velocimeter. However, the lower frequencies of ultrasonic
waves eases the signal processing requirements substantially. Maximum
sampling frequency for acoustic Doppler current meters is a function of the
speed of the pulse and the distance travelled, but typical operating sampling rates can range up to 30 Hz. Sometimes it is necessary to seed the
flow with special particles.
This current meter features two intersecting acoustic beams.
Both types of acoustic current meters are relatively inexpensive, and
current meter calibration is based on the geometry of the instrument transmitter and receiver and the speed of sound in the fluid. Therefore, these
current meters do not require any calibration procedure beyond entering a
value for speed of sound in the model fluid. Calibrations should be checked
if there is reason to suspect the instrument geometry has been inadvertently
altered.
Some acoustic current meter configurations may be somewhat intrusive
to the flow, and this should be considered when orienting the instrument
to the principle flow direction. The reliability of acoustic current meters
is hampered by the presence of air bubbles in the flow. The speed of the
sound wave is changed by bubbles, resulting in errors in velocity estimation.
Air bubbles forming on any surfaces which serve to reflect the ultrasonic
pulse are also a problem for the same reason.