Color Transmission and Reflection Holographic Interferometry Applied to Fluids Mechanics

For analyzing the unsteady wake flows, the authors show the feasibility of real-time true colors holographic interferometry using three different continuous wavelengths and transmission and reflection panchromatic plates. First, the contribution of color in holographic interferometry is demonstrated and then, two optical setups are designed for analyzing the two-dimensional and three-dimensional flows. In the first one, as transmission panchromatic holographic plates are used, the variation in the gelatin thickness does not affect the color interfringes distances. In the second case, the optical setup developed is called "Denisyuk holographic interferometer" because reflection holograms are needed. In this optical setup, the chemical treatment modifies the three interfringes distance if no correction is brought. The problem of controlling the gelatin thickness of the holographic plate has been mastered to increase the diffraction efficiency and the true colors restitution. The development of these techniques began ten years ago and they have been successfully tested for analyzing the unsteady wake flow around a circular cylinder at Mach 0.4.