PIV Visualization Utilizing a Matched Refractive-Index Method

Introduction

With the development of computers and their surrounding equipment, the simulation of complicated flow structures around aircraft will further become easier and cheaper by applying computational fluid dynamics. However, in order to judge whether the flow field obtained is reasonable or not, turbulent models and/or numerical schemes should be selected based on the comparison with experimental results.1

On the other hand, three-dimensional measurement of unsteady flow structures especially around obstacles with complicated geometry is still difficult due to some problems. For instance, where a three-dimensional flow structures around obstacles is visualized by a PIV technique, it is extremely difficult to grasp the whole flow-structure including the flow behind the obstacles even if transparent materials are used, because the difference of the refractive index between the working fluid and the transparent material causes distortion in the image.2

Therefore, in this chapter, I introduce a special visualization technique to match the refractive index of the working fluid with that of the transparent material that is called “matched refractive-index PIV measurement” and show some complicated flow fields visualized by this technique.3

PIV visualization utilizing a matched refractive-index method:

  • Refractive-index adjustment of NaI solution
  • PIV measurement with fluorescence particles

1. Refractive-index adjustment of NaI solution

Where the whole three-dimensional flow structure around obstacles is visualized by a PIV technique, it is necessary to match the refractive index of the working fluid with that of the obstacle material.

This research employs a sodium iodide solution (NaI solution), which is easy to handle and chemically stable, as the working fluid. This solution is deliberately chosen in order to be able to adjust the refractive index of the working fluid to that of the acrylic obstacle with the index of 1.49.4

Normally the refractive index of this solution is not so sensitive to temperature change so that the refractive index of the NaI solution is adjusted by changing its concentration.

Matched refractive-index experiment using NaI
Fig. 1 Matched refractive-index experiment using NaI

Fig. 1 shows a light path difference caused by refraction, where a YAG laser used in the PIV measurement is irradiated to an acrylic cylinder of 30mm in diameter fixed at a center in a 10cm square acrylic box filled with the NaI solution at 30 degrees Celsius. The light path difference, δ, is measured at a location of 660mm from the back of the cylinder.

The difference decreases with the increase in NaI concentration and reaches zero at 61.6wt%. That means that the refractive index of the NaI solution completely corresponds with that of the acrylic cylinder at this concentration. In actual visualization experiments, a refractive index at this concentration under visible light, which is 1.485, was always checked by using a portable refractometer before each experiment, because the change in the refractive index might be caused by deposition of NaI crystals onto the pipe wall and/or volatilization from the solution.5

2. PIV measurement with fluorescence particles

The PIV utilized in this experiment is a double-pulse YAG laser system manufactured by Japan Laser Corporation. The laser output is 25mJ@532nm and the maximum oscillatory frequency is 30Hz. In the PIV measurement, a time series of tracer particles’ images in a sheet laser is taken with a high-speed camera, and, then, a two-dimensional flow structure is quantitatively visualized from the movement of the tracer particles.

The time interval of the double pulse and the tracer concentration is adjusted depending on the flow conditions. To process the obtained particle images, a cross-correlation scheme is adopted to get spatially dense velocity information. Furthermore, melamine fluorescence resin particles with 1~20 μm diameter are utilized as the tracer particles.6

The specific gravity of NaI solution at the above-mentioned concentration is relatively close to that of this tracer, so that buoyancy influence can be ignored. When this fluorescence particle is irradiated with the YAG laser, it causes excitation in the fluorescent agent which emits light of 580nm wavelength. By taking only this newly emitted light into a CCD camera with an attached filter lens, it makes it possible to obtain a clearer particle image than the usual tracer image, because the diffused reflection light of the laser observed on the pipe wall surface and on the acrylic sphere surface can be completely removed simultaneously.

References:

  1. Mulder, M. ed., 2011. Aeronautics and astronautics. BoD–Books on Demand.
  2. Lindken, R. and Merzkirch, W., 2002. A novel PIV technique for measurements in multiphase flows and its application to two-phase bubbly flows. Experiments in fluids33(6), pp.814-825.
  3. Yuki, K., Hasegawa, S., Sato, T., Hashizume, H., Aizawa, K. and Yamano, H., 2011. Matched refractive-index PIV visualization of complex flow structure in a three-dimentionally connected dual elbow. Nuclear engineering and design241(11), pp.4544-4550.
  4. Narrow, T.L., Yoda, M. and Abdel-Khalik, S.I., 2000. A simple model for the refractive index of sodium iodide aqueous solutions. Experiments in fluids28(3), pp.282-283.
  5. Bernabei, R., Belli, P., Montecchia, F., Nozzoli, F., Cappella, F., Incicchitti, A., Prosperi, D., Cerulli, R., Dai, C.J., He, H.L. and Kuang, H.H., 2008. Possible implications of the channeling effect in NaI (Tl) crystals. The European Physical Journal C53(2), pp.205-213.
  6. Kuebelbeck, A. and Riedel, J., Merck Patent GmbH, 2008. Nanoscale Fluorescent Melamine Particles. U.S. Patent Application 11/914,045.