Geoscience Reference
In-Depth Information
quantities, for example, surface velocities are converted to depth integrated velo-
cities by applying a 1/7 power law or other vertical distributions for computing the
flow discharge (Fuijta et al.
1998
; Hauet et al.
2008
). Muste et al. (
2004
) suggest
also the possibility of evaluating flow depth layer variation.
LSPTV technique is also used so that a small number of seeding particles is
required. In particular, Admiral et al. (
2004
) compare the performances of LSPIV
and LSPTV, applying both techniques to a large-scale model, demonstrating that,
although both methods predict the overall distribution quite well, LSPTV works
better near boundaries and in places with high velocity gradients. A particular
configuration, named space-time image velocimetry (STIV), was recently devel-
oped for acquiring measurement with poor or no seeding (Fuijta et al.
2007
a).
It takes advantage of the specular reflection of the ambient light on the small waves
present on the free surface.
3.3 Different PIV Setups
Classical 2D-PIV method is only capable of recording the velocity vectors in the
light sheet plane and the out-of-plane velocity component is lost. For high three-
dimensional flows, stereoscopic PIV (SPIV) has been developed (Nishino et al.
1989
; Prasad and Adrian
1993
; Gaydon et al.
1997
). Using principles of the
stereoscopic vision, SPIV reconstructs through equations the three-component
velocity fields captured from two cameras. The equations have to be calibrated to
account for the geometrical and optical characteristics of the stereoscopic system
(Fouras et al.
2007
; Grizzi et al.
2010
).
Recent extensions of SPIV include dual-time SPIV (Perret et al.
2006
) and
multilayer PIV. In this latter method, more light sheets are generated by splitting
the laser beam using beam splitter (Sanjou and Nezu
2009
). Holographic PIV
is another method for recovering the third velocity component (Coupland and
Halliwell
1992
).
4 Conclusions
The PIV techniques are actually widely used in hydraulics. Their main advantage is
the possibility of nonintrusive velocity field measurements, with an elevate preci-
sion. Standard 2D-PIV methods are not able to measure the third velocity compo-
nent, but now stereoscopic PIV is a well established extension of the traditional PIV
in case of three-dimensional flow fields.
There are still some concerns related to the high prices and the operators' safety.
Now more advanced systems are still prohibitively expensive, but standard PIV
systems can be actually found to affordable price. For this reason, this technique is
becoming very diffuse in hydraulic laboratories. The operators need to be well
