The
particle image velocimetry (PIV) technique (Adrian, 1988) is a nonintrusive
velocity measurement technique using laser-sheet flow visualization.
Tracer particle images in the flow are used as raw data for
the PIV analysis and the direction and displacement of the particle
movements are directly related to the flow velocity. PIV has
received the utmost favorable attention in measuring flow velocity
fields for the past 15 years; primarily because of its breakthrough
advantage of the whole-field velocity vector mapping capability
whereas almost all other velocimetry techniques are based on single-point
measurement probes. Two-dimensional flow phenomena can be easily
detected through this whole-field measurement.
The use of a cross-correlation CCD camera (Keane and Adrian,
1993), in synchronization with a double oscillator Nd:YAG laser
source, have been the major technical milestones for torday’s wide
acceptance of the use of PIV in diverse fluid mechanics areas.
The primary purpose of the present study is to design and implement two different Particle Image Velocimetry (PIV) systems and use them to individually study stationary turbulent air flows and unstationary turbulent water flows. The first PIV system consists of Nd:YAG pulsed laser sheet illumination and two-layer CCD camera recording. For studying stationary turbulent flows, a two-pass square channel with a sharp bend will be used. From this experimental study, the detailed turbulent flow characteristics were revealed by PIV measurements. Also the results were carefully analyzed and interpreted to derive a correlative explanation between the present flow field data and the former heat transfer data (Ekkad and Han, 1997), qualitatively and quantitatively. The second PIV system uses Copper-vapor laser sheet illumination and high-speed cinematographic image recording. This innovative PIV technique has been applied to comprehensively map non-periodical unstationary turbulent flows. The post-processing based on the temporal correlation allows further enhancing the accuracy of the final velocity vector field.
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Particle Image Velocimetry (PIV) is an optical diagnostic method that nonintrusively measures the full-field flow velocity vectors. PIV has been developed together with coherent high power light sources, image recording technologies, digital computers, digital storage media, and statistical analysis algorithms. In extracting statistical data, PIV generally follows five-step-procedures
Particle |
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PIV |
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Post- |
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Instantaneous |
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Statistical |
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The present PIV system (149KB in PDF) consists of a Spectra Physics Model PIV-400
Nd:YAG double resonant tube laser providing frequency-doubled (wavelength
= 532 nm) pulsed emissions of 400 mJ/pulse via a Q-switching module
with a pulse duration of 10 ns. The time delay between the
two successive pulses was varied from 5 to 40 µs depending
on the tested flow conditions. The seeding particles were generated
by means of a Laskin type nozzle, which blows compressed air into
the midst of corn oil forming air bubbles containing oil vapor that
condenses to tiny oil droplets as the bubbles emerge to the oil-air
interface and collapse. The mean diameter of droplets
was measured to be about 1 µm using the Malvern laser diffraction
droplet analyzer (Barth, 1984), with their specific gravity of 0.918,
and refractive index of 1.464 (First, 1992). A combination of two
cylindrical lenses and a spherical lens collimated the laser light
sheet of approximately 1-mm thickness at the measurement regions.
A LaVision FlowMaster-3 software system was used for the image recording,
time synchronization control between the laser and the CCD camera,
and data processing. The cooled full frame interline transfer 1280×1024×12
bit CCD camera was used for recording particle images. The 1280
×1024 imager CCD chip has a 8.70 mm x 6.96 mm dimension and
the size of each pixel, dr = 6.8 ×6.8 µm. The
field-of-view of PIV images was set as (lx = 114.3 mm) ×(ly
= 114.3 mm) for the main flow measurement (the xz-plane) and (lx
= 50.8 mm) ? (ly = 50.8 mm) for the secondary flow measurement (the
yz- and the xy-plane). A Nikon 55 mm AF lens with f# = 4.0 was attached
to the CCD camera with its magnification Mo = 0.061 and 0.137 for
the main and the secondary flow measurement, respectively.
The
test section geometry (31KB in PDF) was adopted from the previous heat transfer study
of a U-channel (Ekkad and Han, 1997). (a) in figure shows
a smooth wall U-channel of a 50.8-mm square flow passages (Dh =
50.8 mm), and (b) shows a rib-roughened wall U-channel of the same
dimension. Both channels were made of 12.7-mm thick Plexiglass
the inner wall surfaces, except for the optical access areas for
the laser sheet and camera imaging, were painted with the non-gloss
black to reduce the laser light reflection from the glossy Plexiglass
surfaces. The tested mean flow velocity Ub = 3.49, 8.73, and
16.00 m/s corresponding to Re = 12,000, 30,000, and 55,000, respectively.
The flow separator of Dh/4 thickness has 90 degree sharp edges
providing 180 degree flow turning. The rib turbulator, attached
only to one side and the total of 19, was made of a Plexiglass rod
of 0.125 Dh × 0.125 Dh cross section with 1.25 Dh intervals.
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Smooth-Wall U-Channel
The mean secondary flow development of the Dean-type counter-rotating vortices (91KB in PDF)
Ribbed-Wall U-channel
The mean secondary flow development of the Dean-type counter-rotating vortices (93KB in PDF)
Reynolds Effect
The secondary flow development of smooth-wall at the Reynolds numbers of 12,000 (92KB in PDF)
The secondary flow development of smooth-wall at the Reynolds numbers of 55,000 (92KB in PDF)
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