Geoscience Reference
In-Depth Information
precision if particles move less than its diameter. It is advisable that the time
between pulses and the size of the interrogation area are such that the movement
of the articles is about 25% of the latter.
-
The type of seeding particles
. They must have a high refraction index and it is
advisable that its density is closer to that of the water. The diameter should be
small but, to avoid errors in the determining the peak coordinates, the projected
particle should occupy an area of 4-9 square pixels.
-
The number of seeding particles per interrogation area
. Flows should be
abundantly and uniformly seeded (see thumb rule above). However, seeding
concentrations should not be too high as to change with the properties of the
fluid or of the flow. Especially delicate are flows with low Reynolds numbers of
fluids with small viscosities (water, for instance) seeded with large concentra-
tions of dense particles. In this case, an undesired density current of seeding
particles may be formed.
-
The use (or not) of overlapping
. Useful for a better visualization of the flow field.
-
The use of windows/filters
. Useful to correct mild in-plane loss-of pairs and to
enhance peak width.
-
The type of subpixel interpolation
. Current algorithms have been discussed by
Westerweel (
1993
), Lourenco and Krothapalli (
1995
), or Kumar and Banerjee
(
1998
), among others.
-
The type of validation algorithm
. In general, algorithms are based on compari-
son with neighboring vectors. They may be based on differentiability of the flow
field and/or threshold criteria (reviews in Nogueira et al.
1997
).
PIV is not a totally nonintrusive measuring technique. Interrogation areas may
smooth out small turbulent scales (one should be aware of anisotropy suppression)
and is prone to noise (from poor image contrast, loss-of-pairs, digital camera, and
electronics) and to spikes (wrong correlation). Data treatment is time consuming
and produces large amounts of data storage. However, PIV is also a very versatile
technique, it can be used as a profiler and a point-wise measuring tool and simulta-
neously, as a visualization tool. The interest in this technique is demonstrated by the
fact that 3D PIV and time-resolved PIV (high time frequencies of image collection)
are among the fastest growing areas of research in measuring techniques. Field
applications are also a growing topic of research.
3 Characterization of Turbulent Flows over Mobile
and Immobile Rough Beds
3.1 Overview
A long established hypothesis is that the entrainment and the near-bed transport of
sediment particles are directly related to organized turbulence. This hypothesis
became fruitful after the pioneer work of Sutherland (
1967
) and benefited from the
