8.2. MEASUREMENT TOOLS AND TECHNIQUES
481
above using time-lapse film exposures. The float trajectories show up as
lines on the developed photographs, and the velocity of each float can be determined as the length travelled divided by the time of exposure. Variations
of this technique can include the use of video, strobe lights, or sequential
photographs taken at uniform time intervals. (See Sharp (1981) for further
details.)
Materials used as the Lagrangian float (or tracer) must be distinguishable in the photograph or video image, be small enough to move at the
same velocity as the local flow, and be large enough to not be affected by
surface tension (Sharp 1981).
A similar photographic technique is used to document flows beneath
the water surface, only in this case the Lagrangian tracers are neutrallybuoyant particles that will be visible in the recorded image. This method
is useful for studying or observing oscillatory flows; but for quantitative
purposes, the method has been replaced by newer techniques.
Laser Doppler Velocimeter
Presently, no other velocity measuring instrument can match the sophistication, capability, accuracy, and measurement range of the laser Doppler
velocimeter (LDV)5.
5 The laser Doppler velocimeter is also referred to as a laser Doppler anemom
(LDA).
The LDV is a nonintrusive instrument system that focuses thin beams
of laser light at a point in the flow, and senses the frequency of light that
is scattered from minute particles being carried along by the fluid. The
particle motion in the flow causes a slight frequency shift (Doppler shift)
in the scattered light relative to the incident light frequency. The Doppler
shift frequency is proportional to the particle velocity, which is assumed to
be moving at the same speed as the flow. This assumption is justified by
the very small sizes of the particles that scatter the light.
Laser Doppler velocimeters consist of four components: a laser, an optical system, a photodetector, and a signal processor. The laser provides
a highly coherent light source, which is needed to measure the relatively
small Doppler frequency shift. Light emitted by the laser is split into two
parallel beams by the optical system and focused by a lens to a point. In
many applications the optical system is located outside the wave flume, and
the laser beams pass through the glass sidewall to a focal point inside the
flume. Different lenses are used to get different focal lengths.
The receiving optics sense the scattered light and the photodetector
converts the light signal into an electrical signal having a frequency that
is proportional to the particle velocity (Sharp 1981). Finally, the signal
481
above using time-lapse film exposures. The float trajectories show up as
lines on the developed photographs, and the velocity of each float can be determined as the length travelled divided by the time of exposure. Variations
of this technique can include the use of video, strobe lights, or sequential
photographs taken at uniform time intervals. (See Sharp (1981) for further
details.)
Materials used as the Lagrangian float (or tracer) must be distinguishable in the photograph or video image, be small enough to move at the
same velocity as the local flow, and be large enough to not be affected by
surface tension (Sharp 1981).
A similar photographic technique is used to document flows beneath
the water surface, only in this case the Lagrangian tracers are neutrallybuoyant particles that will be visible in the recorded image. This method
is useful for studying or observing oscillatory flows; but for quantitative
purposes, the method has been replaced by newer techniques.
Laser Doppler Velocimeter
Presently, no other velocity measuring instrument can match the sophistication, capability, accuracy, and measurement range of the laser Doppler
velocimeter (LDV)5.
5 The laser Doppler velocimeter is also referred to as a laser Doppler anemom
(LDA).
The LDV is a nonintrusive instrument system that focuses thin beams
of laser light at a point in the flow, and senses the frequency of light that
is scattered from minute particles being carried along by the fluid. The
particle motion in the flow causes a slight frequency shift (Doppler shift)
in the scattered light relative to the incident light frequency. The Doppler
shift frequency is proportional to the particle velocity, which is assumed to
be moving at the same speed as the flow. This assumption is justified by
the very small sizes of the particles that scatter the light.
Laser Doppler velocimeters consist of four components: a laser, an optical system, a photodetector, and a signal processor. The laser provides
a highly coherent light source, which is needed to measure the relatively
small Doppler frequency shift. Light emitted by the laser is split into two
parallel beams by the optical system and focused by a lens to a point. In
many applications the optical system is located outside the wave flume, and
the laser beams pass through the glass sidewall to a focal point inside the
flume. Different lenses are used to get different focal lengths.
The receiving optics sense the scattered light and the photodetector
converts the light signal into an electrical signal having a frequency that
is proportional to the particle velocity (Sharp 1981). Finally, the signal