Jean Surget

TWO REFERENCE BEAM HOLOGRAPHIC INTERFEROMETRY FOR AERODYNAMIC FLOW STUDIES

SUMMARY Double exposure holographic interferometry with two reference beams makes it possible to obtain, after processing, many different interferometric presentations from a single hologram of an aerodynamic flow field. A double-reference-beam holographic set-up has first been tested in the laboratory, then applied to interferometric gas flow studies in a wind tunnel. This technique is particularly adapted to transient phenomena studies such as aerodynamic flows in shock tubes. Following the good results obtained from the first wind tunnel experiments, a more sophisticated dual-reference-beam holographic bench has been built. This apparatus permits both studies of transparent media (such as aerodynamic flows) and scattering object deformations (such as gas turbine blades). An optical scheme is presented as well as interferograms obtained in the laboratory.

Holographic Interferoaietry Applied To The Metrology Of Gaseous Flows

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).