Geoscience Reference
In-Depth Information
Pulsed Laser
Laser sheet
Digital
video
camera
recorder
Cylindrical lens
Synchronizer
Computer
Fig. 2 Scheme of 2D standard PIV system
2.1.1 Tracer Particles
As mentioned before, PIV measures velocities indirectly, evaluating tracer move-
ments. Ideal tracer particles follow perfectly the fluid, do not influence the flow, and
do not interact with each other. For this reason, particle size, density, composition,
and concentration are important factors when selecting seeds for PIV.
A fundamental requirement is that the flow has to be marked by particles
selected in order to avoid significant discrepancy between fluid and particle motion.
A convenient measure for the tendency of particles to follow the flow is the
relation
time
,
t
p
, defined as the time required for a particle at rest to be accelerated within
about 63% of the fluid velocity (Hetsroni
1989
), which can be estimated using the
following expression:
d
p
18
t
p
ΒΌ
r
p
r
(2)
n
where
r
p
and
d
p
are the density and the diameter of the particle,
and
v
are the fluid
density and kinematic viscosity, respectively. Even if the expression (
2
) is strictly
correct only for Stokes' regime (i.e., not high flow velocities), also for larger
Reynolds particle number it still furnishes a valid order of magnitude and a more
precise estimation is not usually necessary. A smaller relation time is associated to a
higher frequency response and to a greater capacity to follow rapid flow fluctua-
tions. The
r
t
p
value has to be compared with eddies' characteristic time of the flow
under study. So, seeding particles should be small and light enough to follow the
flow, but at the same time they have to generate a light signal, strong enough to be
resolved by the imaging device. For this reason, light scattering properties of
particles are very important too. In general, the light scattered by particles is a
