Comprehensive optical diagnostics of complex flow fields

Abstract

This paper describes a series of promising optical diagnostic techniques to provide a comprehensive acquisition of the optical information from various flow fields under investigation. A simultaneous application of the two or even several diagnostic techniques which combines a CFD code calculations yield more accurate measurements of the density and/or the density gradient distributions in complex flow fields. Traditionally qualitative data of the density gradient in a high speed compressible flow have been obtained by schlierengraphy. A white light source and a laser two optical arms schlieren optical schemes were constructed on the base of a conventional schlieren system. The optical image of a model was focused on a photo film and projected on a screen. A CCD- camera was used to catch the focused image in real time, to display it on a control monitor and to record it on a video tape recorder. An acoustically stable holographic variable shear interferometer, which has been easily constructed on the base of the conventional schlieren system allowed to record a signal hologram during the wind tunnel run by using a continuous wave laser light source. Signal and comparison holograms have then been post-processed by the dual hologram technique to display shearing interferograms. A holographic Moire deflectometry method is presented. The holographic recording provides a 'freezing' of the distorted signal wave on a hologram to analyze it a posteriori by the Moire technique. A novel holographic Moire interferometer is described and its application to testing a small size axisymmetric supersonic air jet is shown. An interference method with enhanced sensitivity and compensating for optical aberrations, which is based on rerecording holograms is briefly described. It would provide a reliable acquisition of the phase information from wide range of weak phase objects including high speed low density flow fields. The method has been demonstrated by mapping the density field of the supersonic air jet as a representative weak phase object. The numerical analysis of a 2-D flow field was also carried out by means of computational fluid dynamics (CFD) methods.