Geoscience Reference
In-Depth Information
A variety of lasers, using different laser materials, were proposed and used in the
past, like helium-neon, copper-vapor, argon-ion, which are gas lasers, and ruby and
Neodym-Yag, which are solid-state lasers.
In general, light pulses can be obtained with pulsed lasers or with continuous
wave (CW) lasers, combined with a chopping system for producing light pulses
and/or a shuttered recording camera. Actually, the choice of pulsed lasers, with
their ability to produce high-power light with short pulse duration, is predomi-
nant. In particular, in standard PIV setups the commonly used light sources are
actually pulsed solid-state Neodym-Yag (Nd:Yag) lasers. They are made by a Yag
(Yttrium-aluminum-garnet) crystal and the beam is generated by Nd 3+ ions. Nd:
Yag lasers are widely diffused for their high amplification capacity and good
mechanical and thermal proprieties. Now, Nd:Yag lasers are available in compact
packages, with self-contained cooling supplies, easy to manage and positioning.
They have a pulse energy in the range from 2 to 1,000 mJ; even for a standard
type, the pulse energy is around 120 mJ. They emit primarily at 1,064 nm
wavelength and its harmonics, but for safety reasons the laser emission is typi-
cally band pass filtered to isolate the 532 nm harmonics (green light), the only
harmonic perceptible by naked eyes.
The illumination is characterized by two important parameters, which have been
appropriately fixed: the duration of the illumination pulse, named exposure time ,
and the delay time between two consecutive pulses. The exposure time has to be
short enough so that the motion of tracer particles is “frozen” during the pulse
exposure to avoid “streaks” in the image, but not too short in order to guarantee a
good illumination of seeds. The delay time has to be long enough to be able to
determine the displacement of the same particle in the two consecutive images with
a good resolution and without overlapping, but not too long, since the particle could
go out from the light sheet. This is the most adjustable parameter during the
recording phase, since it influences the maximum and the minimum velocity that
can be measured. Moreover, the velocity vectors computed from ( 1 ) are “mean”
values in the time interval D t ( delay time ), so that they can be considered a good
approximation of the “instantaneous” flow values only if the delay time is small
enough. Finally, large D t value increases also the out-of-plane errors, due to the fact
that the particles present in the first image do not appear in the second one, since
they went out of the illuminated sheet. In conclusion, these parameters have to be
carefully fixed, considering the character of the flow under investigation.
The optics, consisting of a combination of mirrors, spherical and cylindrical
lenses, generate a light sheet from the emitted laser beam. In particular, the mirrors
are used to reflect the beam in the desired position, spherical lens expands the beam
into a plane, while the cilindrical lens compresses the plane into a thin sheet. The
modularity of the available optics permits to obtain a light sheet with fixed or
adjustable thickness.
2.1.3 Recording Phase
One of the most important changes in PIV technique was the move from photo-
graphic to digital recording, named Digital-PIV (Willert and Gharib 1991 ),
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