The Study of Transonic Flows by Quantitative Holographic Interferometry

Abstract

Optical flow diagnostics are non-intrusive and therefore suitable for investigating perturbation sensitive transonic flows. Unfortunately, most of these techniques depend on particle seeding and are confined to point measurements. In contrast, holographic interferometry is whole field, does not require seeding, and is an appropriate tool for the study of temporal and spatial properties of unsteady flows. At present, however, holographic interferometry is mainly used qualitatively due to the practical restrictions of data recording, acquisition and processing. This paper addresses some of the limitations that have prevented routine use of holographic interferometry in production wind tunnel testing. Vibration is a frequent problem when capturing interferometric data; however, this paper shows that the associated rigid body movements are easily removed during Fourier transform fringe analysis. A second major limitation arises when attempting to interpret and analyse interferograms from three-dimensional flows. To overcome this problem, a prototype optical system is described which can record multiple interferometric views around the working section. The information extracted from these images may then be used to tomographically reconstruct the three-dimensional density field. Holographic measurements for two-dimensional and three-dimensional axisymmetric flows are presented and compared quantitatively to computational fluid dynamics (CFD) predictions. The experimental and computational results are in general agreement and demonstrate the feasibility of automating the extraction of three-dimensional density data from holographic interferograms.