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Two dual-beam differential direct-phase-detecting optical interferometers for scanning moving surfaces are described. Two beams from these interferometers are focused ~42 mum apart on moving surfaces, and the difference in their reflected path lengths is measured to provide the surface roughness measurement. These interferometers are exceptionally insensitive to environmental vibrations and to surface physical and chemical factors. Applications discussed include the measurement of the surface roughness of a rotating cylinder and a moving web.

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Experiments and numerical simulations are carried out in the upper-level, undergraduate physics laboratory to examine the highly transient startup flow from a small nozzle. The flow is initiated by the rupture of a Mylar R diaphragm positioned between the nozzle and a pressurized air tank. The studies utilize an approach based on experimental measurements and computational methods. Experimental studies consist of dual-beam heterodyne interferometry and highspeed video (HSV) shadowgraphy. Interferometric measurements at 25 million samples per second relate optical path length changes across the nozzle flow to density variations in the flow. HSV shadowgraphy portrays the initial shock wave and compressible flow features surrounding the nozzle exit at 100,000 frames per second. Finite element computations of the unsteady, axisymmetric, compressible nozzle flow provide a field description of the flow for the full tank-nozzle-exterior domain. The studies show excellent matching between the experimental and computational methods. More importantly, the distinct strengths of the different approaches work together to provide a more comprehensive description of the nozzle flow than can achieved by the individual approaches.