A difficulty common to an optical diagnostic technique applied to fluid dynamics studies (e.g., flow visualization with a high speed camera and particle image velocimetry, PIV) is the refraction of light passing through test section walls. Refraction through a cylindrical pipe wall can generate phenomena such as hidden regions and multiple images. The method of choice to eliminate refraction problems in liquid flows is to match refractive index, which can be accomplished by placing an appropriate liquid around the curved solid surface.  It is necessary to construct a box with flat walls (RIM box) around the test section with curved inside and outside walls. When the box is filled with a quiescent index matching liquid, the flow can be viewed with minimum refraction errors from directions perpendicular to each face of the RIM box.

Since the material of construction of the outer pipe of annular test section of the flow loop is acrylic, the RIM box is also made of same acrylic. The refractive index (RI) of acrylic is 1.49. The RIM liquid selected for the current application is glycerin (RI ~ 1.49).