Large Matched-Index-of-Refraction (MIR) Flow Systems for International Collaboration In Fluid Mechanics Page: 2 of 11
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Large Matched-Index-of-Refraction (MIR) flow systems for international
collaboration in fluid mechanics
Donald M. McEligot, University of Idaho, Idaho Falls, Idaho 83402 USA, DonaldM@uidaho.edu
Stefan Becker, Uni. Erlangen, D-91058 Erlangen, Germany
Hugh M. McIlroy, Jr., Idaho National Laboratory (INL), Idaho Falls, Idaho 83415-2200 USA
Abstract ---- In recent international collaboration, Idaho National Laboratory (INL) and Universitdt Erlangen (UE)
have developed large MIR flow systems which are ideal for joint graduate student education and research. The benefit
of the MIR technique is that it permits optical measurements to determine flow characteristics in complex passages and
around objects to be obtained without locating a disturbing transducer in the flow field and without distortion of the
optical paths. The MIR technique is not new itself others employed it earlier. The innovation of these MIR systems is
their large size relative to previous experiments, yielding improved spatial and temporal resolution. This report will
discuss the benefits of the technique, characteristics of the systems and some examples of their applications to complex
situations. Typically their experiments have provided new fundamental understanding plus benchmark data for
assessment and possible validation of computational thermal fluid dynamic codes.
Index terms ---- Fluid mechanics, international collaboration, optical techniques, refractive-index-matching
In recent international collaboration, INL and Uni. Erlangen have developed large MIR flow systems [Stoots et al., 2001;
McIlroy and Budwig, 2007] which can be ideal for joint graduate student research. The benefit of the MIR technique is
that it permits optical measurements, such as particle image velocimeters (PIV) and laser Doppler anemometers (LDA),
to determine flow characteristics near surfaces, in passages and around objects having complicated geometries without
introducing an intrusive probe into the flow field and without distortion of the optical paths. One way to eliminate
optical interference of LDA systems is by employing suitable transparent solid materials together with fluids that possess
the same refractive index as the solid material itself. In this way, the solid disappears optically (and therefore has no
influence on the laser beams) but maintains its full mechanical influence on the flow. With a transparent model of
different refractive index than the working fluid, the light rays of optical measuring instruments can be refracted in such
a manner that measurements are either impossible or require extensive, difficult calibrations. Without refractive index
matching LDA beams may not cross to form the measurement control volume at the desired focal length, if they cross at
Before the INL and Uni. Erlangen developments, no matched-index-of-refraction (MIR) flow facility existed that
permitted the study of flow past complex geometries requiring high Reynolds numbers, fine spatial resolution and larger
scale. Examples of such geometries include heat exchanger and nuclear reactor tube bundles and boundary layers. A
demonstration of the benefits of refractive-index-matching is shown in Figure 1 with a test model for an idealized ribbed
annulus [McCreery et al., 2003].
THIS EXAMPLE DEMONSTRATES THE BENEFITS OF REFRACTIVE-INDEX-MATCHING WITH A TRANSPARENT MODEL HAVING CURVED
INTERFACES (FLOW THROUGH A HORIZONTAL RIBBED ANNULUS OF 164 MM DIAMETER [MCCREERY ET AL., 2003]). HORIZONTAL STRUCTURAL
RODS ARE STEEL AND THEREFORE OPAQUE; THE RIGHT END IS PLASTIC, WHICH IS NOT MATCHED.
The MIR technique itself is not new; Corino and Brodkey  employed it to measure turbulence structure in a
circular tube. The innovative advantage of the INL and UE systems is their large size, leading to improved spatial and
temporal resolution compared to others. Earlier, most experiments with index matching were small, with characteristic
lengths on the order of five cm or less. No index-matched flow facility existed that permitted reasonably-sized flat plate
boundary layers to be installed and, hence, provide the basic test facility to study laminar-to-turbulent boundary layer
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McEligot, Donald M.; Becker, Stefan & Hugh M. McIlroy, Jr. Large Matched-Index-of-Refraction (MIR) Flow Systems for International Collaboration In Fluid Mechanics, article, July 1, 2010; Idaho. (digital.library.unt.edu/ark:/67531/metadc1015100/m1/2/: accessed February 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.