Geoscience Reference
In-Depth Information
function of the ratio of the refractive index of particles to that of the surrounding
medium, the particles' size and shape. In particular, the scattered light intensity
increases with increasing particle diameter; moreover, particles with irregular shape
surface are generally scattered well in all directions. So, a greater relative refractive
index and a larger geometric size both help to improve the signal strength.
Sometimes the natural seeding already present in flow can act as markers. This
solution, acceptable in some cases, such as field applications, can produce problems
in the image analysis phase in many other situations. In fact, if particles of different
sizes are present, as is usually the case, since large particles clearly dominate the
PIV evaluation, it is difficult to give a sure estimate of the effective particle size and
then of the lag velocity. Moreover, a large quantity of particles in a flow can increase
the background noise on the recordings, reducing measurement quality. In almost
all laboratory works, it is desirable to have clear water and add selected tracers in
order to achieve sufficient image contrast and to control particle size. For liquid
flow problems, like hydraulic applications, many solid particles with adequate
characteristic can be found for satisfying all requirements. In particular, metallic
and glass particles of few tens of microns are well suited for seeding water flows.
Other factors to consider are achieving homogeneous particle dispersion and
maintaining particle life time in the flow. A good particle dispersion is related to
how seeds are practically added to the flow. While for gas flows the supply
procedure is very important (Melling
1997
), for liquid flows this can easily be
done by suspending solid particles into the fluid and mixing them in order to get a
good distribution.
The tracer concentration in the flow is a very crucial aspect, which will be
discussed in the section dedicated to image analysis, since it strongly influences the
selection of the processing algorithm.
2.1.2 Light Source and Optics
In PIV applications, the light source is usually a laser (Fig.
2
),
Light Amplification
by Stimulated Emission of Radiation
, because of its ability to emit monochromatic
light with high energy density, which can easily be bundled into thin light sheets. In
some cases, white light source, for example, generated by xenon flash lamps, or
natural light can also be used. Since white light cannot be collimated as monochro-
matic light, these solutions are adopted in particular applications only, like in large-
scale and field measurements.
Every laser can be considered consisting of the following three components:
1. The laser material, which is an atomic or molecular gas, semiconductor or solid
material
2. The pump source, which excites the laser material by the introduction of
electromagnetic or chemical energy
3. The optical resonator, consisting of mirrors arrangements, which allows to
amplify the light energy
