Refractive Index Matching (RIM) Test Rig

The Refractive Index Matching (RIM) test rig at the University of Bristol is a specialised closed-circuit flow facility designed for high-resolution measurements within and around complex geometries. It operates on the principle of refractive index matching, where the working fluid—an aqueous ammonium thiocyanate solution—is carefully adjusted to match the refractive index of the solid test models, which are 3D-printed from PMMA. This eliminates optical distortions such as refraction, making complex geometries appear optically "invisible" and allowing uninterrupted passage of light through the system. As a result, advanced optical diagnostics such as Stereo Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) can be applied with high accuracy.

The rig features a transparent acrylic test section measuring 1800 mm in length with a 100 mm × 100 mm cross-section, supported by a 40-litre reservoir, a constant head tank, and a 2 kW centrifugal pump for precise flow control. The system is constructed primarily from non-corrosive materials including stainless steel and PA12-SLS, chosen for their chemical resistance and long-term stability. To ensure uniform inflow conditions, the RIM facility incorporates flow conditioning elements, including a perforated cylinder, honeycomb, mesh screens, a 5:1 contraction, and vortex suppressors at the outlet. These components reduce turbulence intensity and promote flow uniformity across the test section.

The facility supports experiments over a wide Reynolds number range, from laminar to turbulent regimes, with flow velocities ranging from 0.05 m/s to 0.5 m/s. Time-resolved PIV measurements can be acquired at rates up to 10 kHz, enabling the capture of transient flow phenomena. The RIM test rig is particularly suited to exploring flow in porous and complex structures, making it a vital experimental platform for validating numerical models and advancing fundamental fluid dynamics research.