Holographic Interferoaietry Applied To The Metrology Of Gaseous Flows

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Abstract
Holographic interferometry is widely used at ONE RA for the detailed analysis of transonic flows, whose great sensitivity to perturbating effects of material probes is well known. The implementation of holographic techniques made it possible to build an interferometer presenting many advantages over the classical Mach-Zehnder instrument as regards the possible extent of the field, but also ease of use and cost of fabrication. Interferograms corresponding to the infinite fringe adjustment provide a visualization of the aerodynamic phenomena. But the quantitative study is prefe-rably carried out from interferograms presenting an initial finite fringe pattern, in order to multiply the measuring points. Processing, which makes use of an entirely automatic process, is ensured on photographic reproductions of the interferograms restituted by double exposure holograms. The negatives are explored by means of a high precision microdensitometer coupled with a data acquisition unit, comprising a minicomputer and a disc memory where the programme and intermediary data are stored. The computer ensures two functions 1 it pilots the displacements of the plate holder of the densitometer according to a preprogrammed path, and 2) it ensures the acquisition and processing of the densitometric data. The results are edited on a printer and on a tracing table. Thanks to this procedure, the processing time is considerably reduced the exploitation time of an interferogram along 25 lines comprising each 60 gas density measurements (i.e. 1500 measuring points) is less than 7 hours. The holographic bench and its acquisition unit are at present used for the systematic study of the shock wave-turbulent boundary layer interaction in tran-sonic flow at the SBA wind tunnel of the ONE RA Fluid Mechanics Laboratory of Chalais-Meudon (near Paris).© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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