This technique enables quantitative analysis of density gradients in flows, based on a Fourier analysis of interferograms and a specially designed interferometer first proposed by a PhD student at TU Graz and published in
Pretzler, G., Jäger, H., Neger, T., 1993, "High-accuracy differential interferometry for the investigation of phase objects", Meas.Sci.Technol.4,649-658.
It was then applied to turbine flows in Woisetschläger, J., Pretzler, G., Jericha, H., Mayrhofer, N., Pirker, H.P., 1997, “Differential interferometry with adjustable spatial carrier fringes for turbine blade cascade flow investigations", Exp.Fluids, Vol.24.
This technique enables a real time interferometric observation of density gradients with adjustable sensitivity and high stability against vibration noise.
The movie below shows a real time observation of a transonic flow (M=1.2) through a linear turbine blade cascade as observed through the differential interferometer. Continuous air is supplied by the compressor. In the trailing edge region you can observe shock oscillations caused by shedding vortices at the turbine blade trailing edge. In this high speed recording a slight overall movement of the carrier fringe system can also be observed. No special vibration insulation or acoustic shielding was used for this recording.
See also: Woisetschläger, J., Jericha, H., 1996, "Heterodyne Laser Interferometry for Cascade Flow Investigations", 13th Symposium on Measuring Techniques for Transonic and Supersonic Flows in Cascades and Turbomachinery, ETH Zürich, Optical Measurements, paper 19