HIGH-SPEED COLOR INTERFEROMETRY

Abstract

This paper presents the improvements brought by ONERA in differential and absolute color interferometry. When the optical technique produces differential measurements, a polarized white light source and one or two Wollaston prisms are used. High speed interferograms of the flow downstream of the a circular base are recorded with a camera at 35,000 frames per second with a 750 nanosecond exposure time for each frame. Unfortunately, as the optical technique yields differential measurements, it is necessary to integrate the data which results in some inaccuracy in the measurements. Our latest work has thus led us to develop true color real-time holographic interferometry which allows a direct observation through a reference hologram and makes it possible to take an ultra-high speed movie of the interferogram of a changing phenomenon. High speed interferograms have been obtained around a circular cylinder. Using color in this technique is primordial because it combines the advantages of differential interferometry with those of monochromatic holographic interferometry. Color holographic interferometry generates the achromatic fringe and also provides absolute data throughout the entire field of observation. Results show that this technique is a efficient tool for analyzing and characterizing unsteady aerodynamic wake flows in large field and it could well be the basis for a method of analyzing unsteady 3D flows.