Geoscience Reference
In-Depth Information
of keeping to date with the new developments in the area is the special issues on
PIV, periodicallypublishedon
Experimental in Fluids Journal
.Theyarekeeping
the readers abreast with the most recent applications of PIV in different research
fields.
PIV is widely used in experimental laboratory works, in particular for studying
turbulent flows, but more recently it has been introduced also for field measure-
ments. Another important application area is represented by the specific PIV meth-
ods developed in order to acquire measurements in two-phase flows, such as
mixtures of water with air bubbles or sediments. The main goal of this chapter is
to present a state-of-art of some representative applications of the PIV technique in
hydraulic research.
2 Historical Development and Basic Principle
of PIV Technique
The most rudimental form of PIV could probably be traced far back in history to the
first time a person evaluated the velocity watching leaves floating and moving on
the surface of a flowing stream. From this point of view, the PIV concept is old and
very simple. The velocity is estimated from the movement of tracers present in the
flow, assumed to closely follow the flow. In particular, the velocity vector is
calculated using the velocity definition:
s
!
D
t
!
¼
(1)
t
is the time step, and
!
is the tracer displacement
between two successive observations. In its modern form, a conventional PIV
system has four major components: seeding, illumination, image recording, and
image processing.
Historically, PIV first appeared in literature in the mid-1980s (Adrian
2005
).
Initially, researchers became interested in PIV because of its capability of studying
turbulent flows. By its nature, turbulence is a phenomenon that occurs over a wide
range of physical scales, extending from the largest scale of the flow to the
Kolmogorov scale. So, a successful measurement technique must be able to mea-
sure over a wide range of physical scales in length and velocity. Turbulence occurs
also at high velocity, so tracers must be small enough to follow the flow in presence
of large local and randomly fluctuating accelerations. Moreover, since the tracer
displacement is evaluated considering the particle position in two consecutive
instants, a pulsed light is required. The duration of the illumination light pulse
must be short enough so that the particles are “frozen” during the pulse, without
streaks. The use of small particles, with small light scattering section, and the
necessity of a short duration pulse imply the use of high intensity pulsed laser.
where
s
is a scale factor,
D
