Geoscience Reference
In-Depth Information
Turbulent Flow Hydrodynamics and Sediment
Transport: Laboratory Research with LDA
and PIV
Rui M.L. Ferreira
1
Introduction
1.1 Laser Doppler Anemometry and Particle Image Velocimetry
Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) are
indirect techniques to measure flow velocities. Both require the existence of small
particles in the moving fluid, ideally in concentrations that do not change the
properties of the fluid, and both measure specific statistics of the velocity of such
particles, herein called tracing particles or seeding. Both techniques employ lasers,
directly in the case of the former, making use of the Doppler effect as the laser light
is scattered back to the receiver, and indirectly, in the case of the latter, as the laser
is used to illuminate seeding particles in a plane section of the flow. Both techniques
are among the least intrusive techniques to measure flow velocities. In fact, their
intrusiveness is solely the result of seeding the flow with particles that may affect
flow or fluid properties; in general, no material parts of the instrumentation are
needed near the flow region under measurement.
LDA techniques are based on complex chemical and electronic units and very
sensitive optical systems with little software complexity; on the contrary, PIV
techniques, albeit requiring complex laser producing units, rely heavily on software
for the analysis of raw data. In what concerns the study of water flows, both
techniques are ideal for controlled research environments, as laboratories where
the flows are confined in flumes, generally glass-walled, with optimal visualization
conditions. Relatively to more intrusive techniques such as Acoustic Doppler
Velocimetry, both techniques are applied with greater difficulty in field conditions,
as the equipment is less robust and, in the case of the LDA, relying on complex
cooling systems and demanding power sources.
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