A simple laser interferometer for wind tunnel flow visualisation

A simple interferometer design based on a conventional wind tunnel schlieren system is described. This new interferometer arrangement employs a laser light source, a lens which splits off the reference beam after test beam expansion and a lens and Lloyd mirror to recombine the two beams. The reference beam passes through the test section but is contracted to a narrow waist and displaced well away from the model location. The design combines a number of characteristics which render it particularly useful for wind tunnel tests. These characteristics include: simplicity, optical robustness, low vibration sensitivity, modest coherence requirements and ease of interferogram analysis. The main disadvantage is that only half the total field of view can be recorded on a single interferogram. Test results, confirming the above advantages, are presented from an interferometer based on the 406 mm aperture schlieren system fitted to the Aeronautical Research Laboratories transonic wind tunnel.