8.2. MEASUREMENT TOOLS AND TECHNIQUES
483
7. With different configurations it is possible to measure 1, 2,
or 3 components of the fluid velocity at the same point.
The major disadvantage of the laser Doppler velocimeter is its high cost
which severely limits the number of spatial measurements that can be made
simultaneously. Researchers have worked around this problem by repeating
precisely controlled experiments in order to make LDV measurements at
different locations. Buchhave, George, and Lumley (1979) discussed in
detail the theoretical aspects of laser Doppler velocimetry and the inherent
accuracy and limitations of the measurements.
The above-listed advantages make the laser Doppler velocimeter ideally
suited for two-dimensional flume studies of oscillatory flows and studies of
flow turbulence. The small sampling volume and ability to measure low
velocities allow measurements to be made within the laminar or turbulent
boundary layer. Repeat experiments can provide details of the boundary
layer profile. Turbulence measurements can also be obtained in the wake
of solid objects or during flow reversals. Laser Doppler velocimeters have
been used to measure wave kinematics in the crest. In this case, the laser
beams are not immersed during a portion of the wave cycle; but as soon
as the beams are immersed, high-quality velocity data are obtained. It is
even possible to measure flow within the voids of a model rubble-mound
structure.
In the early days of the laser Doppler velocimeter, institutions built their
own LDV systems, and this required a high level of expertise in optics, theory, and electronics (often a Doctoral candidate or professor performed the
development to support their experimental research). Since that time, the
LDV has evolved to the point that off-the-shelf systems provide outstanding
capability in ready-to-run configurations. Figure 8.3 shows a commercial
backscattering, two-component laser Doppler velocimeter being used in a
wave experiment at the Waterways Experiment Station.
Although the new LDV systems do not require the same level of knowledge needed by the pioneers, successful operation still depends on a thorough understanding of the instrument operation and capabilities. The careless operator can be assured of receiving a lesson in Murphy s Law despite
the sophistication of the newer systems.
Modern LDV systems have many built-in safeguards to protect operators and onlookers from the high-powered laser light. Nevertheless,
Utmost caution must be taken during laser Doppler operations to protect eyes from being PERMANENTLY
DAMAGED by direct radiation from the laser beams.
Fiber optics have been adapted for use with laser Doppler systems. The
transmitted and received light can be relayed over fiber optic cable distances
483
7. With different configurations it is possible to measure 1, 2,
or 3 components of the fluid velocity at the same point.
The major disadvantage of the laser Doppler velocimeter is its high cost
which severely limits the number of spatial measurements that can be made
simultaneously. Researchers have worked around this problem by repeating
precisely controlled experiments in order to make LDV measurements at
different locations. Buchhave, George, and Lumley (1979) discussed in
detail the theoretical aspects of laser Doppler velocimetry and the inherent
accuracy and limitations of the measurements.
The above-listed advantages make the laser Doppler velocimeter ideally
suited for two-dimensional flume studies of oscillatory flows and studies of
flow turbulence. The small sampling volume and ability to measure low
velocities allow measurements to be made within the laminar or turbulent
boundary layer. Repeat experiments can provide details of the boundary
layer profile. Turbulence measurements can also be obtained in the wake
of solid objects or during flow reversals. Laser Doppler velocimeters have
been used to measure wave kinematics in the crest. In this case, the laser
beams are not immersed during a portion of the wave cycle; but as soon
as the beams are immersed, high-quality velocity data are obtained. It is
even possible to measure flow within the voids of a model rubble-mound
structure.
In the early days of the laser Doppler velocimeter, institutions built their
own LDV systems, and this required a high level of expertise in optics, theory, and electronics (often a Doctoral candidate or professor performed the
development to support their experimental research). Since that time, the
LDV has evolved to the point that off-the-shelf systems provide outstanding
capability in ready-to-run configurations. Figure 8.3 shows a commercial
backscattering, two-component laser Doppler velocimeter being used in a
wave experiment at the Waterways Experiment Station.
Although the new LDV systems do not require the same level of knowledge needed by the pioneers, successful operation still depends on a thorough understanding of the instrument operation and capabilities. The careless operator can be assured of receiving a lesson in Murphy s Law despite
the sophistication of the newer systems.
Modern LDV systems have many built-in safeguards to protect operators and onlookers from the high-powered laser light. Nevertheless,
Utmost caution must be taken during laser Doppler operations to protect eyes from being PERMANENTLY
DAMAGED by direct radiation from the laser beams.
Fiber optics have been adapted for use with laser Doppler systems. The
transmitted and received light can be relayed over fiber optic cable distances