Design of a Matched Index of Refraction Facility for Flow Studies with Mixing and Complex Geometries
A matched index of refraction facility has been constructed at Texas A&M University to enable optically transparent measurements in a test section capable of laminar, transition, and turbulent flow regimes. The index matching is achieved by recirculating Drakeol® 5 oil through a closed loop system via a centrifugal pump whose heat generation does not adversely affect the refraction. Quartz is used as the solid material inside the test section to enable the index matching. Design of the system is discussed, followed by preliminary results for three rectangular slot flows that coalesce into a single channel. Particle Image Velocimetry was employed to capture the velocities in the mixing region. This configuration allows for simultaneous measurement of the flow 0.35 hydraulic diameters upstream and 3.9 diameters downstream of the outlets. The Reynolds numbers of the three channels are
in the range of 1800-20000, spanning multiple flow regimes and promoting different flow physics. Results in the turbulent regime closely resembled the interaction of three confined slot jets. The non-turbulent regime shows the fluid stalling to approximately half of its inlet velocity, prompting recirculation zones near the outlet. The dominant energy contributions and momentum dissipation in the stalling region come from shear and spanwise stresses, whereas streamwise stresses dominate in other locations. It was also observed that the shear stresses decrease with increasing bulk velocities, while the spanwise stresses remained relatively constant. The ability to simultaneously measure upstream and downstream of the outlets enables further investigation into understanding the complex interaction of the momentum exchange that occurs as a result of the geometry, flow regime, and the effect of
pressure in these areas. This data also enables a straightforward exercise to help validate models of turbulence and transition for such mixing scenarios.