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AUR Product Highlights
- AUR Studio LDV Burst Processor
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Flow measurements today: Advanced Laser-Doppler
Velocimetry provides crucial details missed by PIV
May 2010
© Applied University Research, Inc.
AUR's Advanced laser-Doppler velocimetry (LDV)
systems provide flow information unavailable from widely-used Particle
Image Velocimetry (PIV) systems.
PIV can be an enticing technique for rapidly viewing instantaneous
snapshots of flow behaviors in some applications.
At face value, what practitioner would turn down the opportunity to
obtain thousands of velocity component measurements and velocity
profiles from a pair of
digital images? But as with all measurement techniques, and PIV as the
prime example, "the devil is in the details". Let's take a
look at some issues with PIV and ways that AUR's Advanced LDV
systems, capable of producing three-orthogonal-component velocity
profiles from one measurement record, can improve the
information you obtain in flows.
Near-surface flow:
Where vorticity originates
High
Reynolds
number (HRN) Flows are those of interest in
engineering applications. They are characterized by flow energy
distributions over several decades of spatial and velocity scales.Vorticity transport determines the
behavior
of HRN flows. Understanding vorticity transport phenomena yields
information for skin friction and base drag predictions, convective
heat transfer performance, flow noise prediction, flow modeling, and
passive and active flow control. In many HRN flows with solid
boundaries, vorticity flux in the system occurs exclusively on the
boundary itself. Measuring the near-wall region, with its attendant
issues of laser flare and large velocity gradients, is critical to
understanding the dynamics of the flow.
- PIV: Laser
flare
on surfaces, large velocity dynamic range and large velocity
gradients greatly limit PIV near-surfaces in HRN flows. While
limited developments have lessened the severity of these issues in few
reported studies in low Reynolds number flows, HRN near-wall flows
remain inaccessible to PIV.
- AUR's
Advanced LDV: Optimized laser beam configurations, smart signal
processing, small measurement volumes, and even position-measuring
probes make AUR's LDVs ideally suited for this application.
Dynamic Range: Measurements
in High Reynolds Number Flows
- PIV: According
to
Prof. Ron Adrian (2005),
currently of Arizona State University, PIV exhibits a dynamic range less
than
200:1--equivalent to a two-digit voltmeter (for flow velocity).
- AUR's Advanced LDV: LDV
uncertainty scales on the instantaneous measurement velocity, not on
system setup parameters as with PIV. Dynamic ranges in velocity
of 13,000:1 are reasonable.
Three-velocity-component
measurements
- PIV is,
fundamentally, a two-velocity-component instrument. Three-velocity-component PIV measurements
are twice as uncertain out of the plane for stereo PIV. Holographic and
tomographic PIV has only proven useful in low Reynolds number flows to
date.
- AUR's
Advanced LDV offers many configurations for measuring instantaneous
three-component velocity vectors and all components may be measured
with equally low uncertainties.
Statistical information in flows
- PIV: Turbulent flows require many
thousands of samples to obtain second-order or greater Eulerian
statistics. The demands of obtaining and processing 10's or 100's of
thousands of high resolution images to obtain a single 'snapshot' of
the flow statistics are significant.
- AUR's Advanced LDV: It is possible to
obtain high quality statistics using the AUR Studio LDV Burst Processor
in real time. Turbulence
structure data may be quickly and automatically reduced from Doppler
signals to distributions of statistics in minutes.
AUR's Advanced LDV products
fill the void
AUR's Advanced LDV products provide resolved
three-component velocity statistical measurements in high Reynolds
number flows, even with 10's of microns of a surface. While PIV is
seemingly attractive for the amount of information containing in a
single pair of digital photos, AUR's Advanced LDV systems provide
information in regions of interest in flows that are just not
accessible by PIV. Contact us
and we will specify an Advanced LDV system for the flow velocity
measurements you need to make your program a success.
Reference
Adrian, R J 2005 "Twenty years of particle image
velocimetry," Exp. Fluids 39, p. 159-169.
About AUR:
Applied University Research, Inc. develops advanced technologies for
flow velocimetry and control. Founded by Dr. Roger L. Simpson, a
prominent researcher in turbulent flow mechanics and instrumentation
for over 35 years and past president of AIAA, AUR leverages unique
capabilities and knowledge to deliver solutions to flow problems.
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Applied University Research, Inc.
Ph: 540.797.0643
FAX: 866.223.8673
605 Preston Ave.
Blacksburg, VA 24060
http://www.aurinc.com
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Research
Facilities:
6580 Valley
Center Drive
Suite 317, Bay 160
Radford, VA 24141
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