486 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
After initial injection of the seeding, time is needed for the seeding
to become more or less uniformly distributed over a wider area. High
concentration of seeding will actually block a portion of the laser light and
the scattered return, making it difficult to obtain reliable results. The
correct amount of seeding for a particular facility is determined largely by
experience.
Acquiring fluid velocities beneath breaking waves using a laser Doppler
velocimeter is hampered by the presence of entrained air bubbles. Anytime
an air bubble passes through either of the laser beams or the sampling
volume, there is a temporary loss of good signal and the resultant processed
velocities will be invalid. If there is a high level of air entrainment at the
measurement point, it will be difficult to discern good data from bad; but if
the occurrence of air bubbles is infrequent, the bad data (called “dropouts”)
will be obvious, and they can be edited from the record.
The development and refinement of laser Doppler velocimetry has had
an important impact on research in the area of fluid kinematics. The LDV
provides the researcher with a valuable tool for making high-quality velocity
measurements that were previously considered impossible, and researchers
have hastened to take full advantage of this capability.
Particle Image Velocimetry
Particle Image Velocimetry (PIV) is a relatively new nonintrusive procedure for quantifying unsteady flow fields (Gray and Created 1988) . In this
technique, nearly neutrally buoyant particles that have been treated with
fluorescent dye are used to “seed” the water. The seed particles are illuminated with a sheet of pulsed laser light, and photographs are taken having
an exposure time exceeding the pulse period of the laser sheet (Wessels,
et al. 1990). Double or multiple exposure photographs will indicate the
short paths travelled by the particles, and techniques have been developed
to extract maps showing the spatial velocity distribution averaged over the
time of the double exposure. Further details are given by Gray and Created
(1988) and Wessels, et al. (1990).
With a laser Doppler velocimeter it is possible to build up a spatial map
of velocities, provided flow conditions can be exactly repeated many times.
Particle Image Velocimetry, on the other hand, provides a quantitative twodimensional “snapshot” of the spatial variations in velocity at a point in
time. This can be very useful for studying complex flow phenomena and
for validating numerical models.
After initial injection of the seeding, time is needed for the seeding
to become more or less uniformly distributed over a wider area. High
concentration of seeding will actually block a portion of the laser light and
the scattered return, making it difficult to obtain reliable results. The
correct amount of seeding for a particular facility is determined largely by
experience.
Acquiring fluid velocities beneath breaking waves using a laser Doppler
velocimeter is hampered by the presence of entrained air bubbles. Anytime
an air bubble passes through either of the laser beams or the sampling
volume, there is a temporary loss of good signal and the resultant processed
velocities will be invalid. If there is a high level of air entrainment at the
measurement point, it will be difficult to discern good data from bad; but if
the occurrence of air bubbles is infrequent, the bad data (called “dropouts”)
will be obvious, and they can be edited from the record.
The development and refinement of laser Doppler velocimetry has had
an important impact on research in the area of fluid kinematics. The LDV
provides the researcher with a valuable tool for making high-quality velocity
measurements that were previously considered impossible, and researchers
have hastened to take full advantage of this capability.
Particle Image Velocimetry
Particle Image Velocimetry (PIV) is a relatively new nonintrusive procedure for quantifying unsteady flow fields (Gray and Created 1988) . In this
technique, nearly neutrally buoyant particles that have been treated with
fluorescent dye are used to “seed” the water. The seed particles are illuminated with a sheet of pulsed laser light, and photographs are taken having
an exposure time exceeding the pulse period of the laser sheet (Wessels,
et al. 1990). Double or multiple exposure photographs will indicate the
short paths travelled by the particles, and techniques have been developed
to extract maps showing the spatial velocity distribution averaged over the
time of the double exposure. Further details are given by Gray and Created
(1988) and Wessels, et al. (1990).
With a laser Doppler velocimeter it is possible to build up a spatial map
of velocities, provided flow conditions can be exactly repeated many times.
Particle Image Velocimetry, on the other hand, provides a quantitative twodimensional “snapshot” of the spatial variations in velocity at a point in
time. This can be very useful for studying complex flow phenomena and
for validating numerical models.