Geoscience Reference
In-Depth Information
LDA techniques have been operational since the 1960s (Yeh and Cummins
1964
; reviews in Tropea et al.
2007
), having been used in the scope of river
hydraulics since the 1980s (Nezu and Nakagawa
1993
; Gyr and Schmid
1997
).
PIV techniques have been applied in hydraulics since the early 1980s
(Lauterborn and Vogel
1984
). Since it requires large storage space and large
RAMmemory, digital PIV techniques have benefited from the advance in computer
architecture, especially since late 1990s. In the context of river hydraulics, PIV has
been growingly used in the last 15 years (e.g., Tait et al.
1996
).
1.2 Objectives and Structure of the Work
The main objective of the present chapter is to show applications of LDA and PIV
techniques in the scope of fluvial hydraulics, within research work conducted at
Instituto Superior T
ยด
cnico, Technical University of Lisbon, Portugal.
The work is divided in three parts. The first is dedicated to the presentation of
basic concepts of LDA and PIV measurements. The second part addresses the
characterization of organized turbulence in open-channel flows over flat rough
mobile beds and over flat rough fixed (armored) beds. Two data sets are compared
to verify (1) if mobile bed flows have the same turbulent structures as flows over
fixed beds and (2) if there are important feedback mechanisms between the organi-
zation of turbulence and the quality and quantity of the sediments moving in the
bed. The third part is devoted to study and characterize the scour mechanism
associated to a vertical cylinder mounted on a mobile bed. The instantaneous
flow velocity in the scour hole in front of the cylinder, for a given instant in the
developing phase of the scour hole, is measured and used to calculate mean and
instantaneous out-of-plane vorticity. The analysis of the downflow and of the vortex
system inside the scour hole allows for the identification of the most relevant scour
mechanisms.
2 Basic Principles of LDA and of PIV Techniques
2.1 Principles of LDA Instrumentation
Laser Doppler Anemometry is a point-wise measuring technique of the velocity of
tracers in viscous fluids. An LDA system features a continuous laser source,
transmitting optics, receiving optics with photo-detector, analog filters, and signal
amplifiers, a signal processing unit and memory unit (Durst et al.
1976
), as seen in
Fig.
1
. Commonly used lasers comprise He-Ne (Helium-Neon), typically at 632.8 nm
(red), Ar-Ion (ions of Argon), normally, in water, operated in the blue or green
