Holographic Interferometry for Flow and Vibration Analysis

Holographic Interferometry uses the fact that light fields can be stored on photosensitive media when coherent radiation (LASER light) is used. Thus, this technique enables the quantitative comparison between light waves reflected from surfaces before and after stress is applied, as well as the determination of density changes in a flow. The following picture demonstrates the principle of holographic interferometry when used for opaque objects. First a hologram of the object is recorded. To realize this, a small stress is applied; here the receiver of the phone was put on top of the phone and hot water was poured into the cup, then a second hologram is recorded. Both holograms are reconstructed simultaneously. As a result of the interference of both holograms a fringe system is visible. This fringe system can be understood as isolines of surface shift due to the load or due to thermal expansion (isoline level approximately 0.25 micrometer).

Using pulsed laser systems holographic interferograms can be recorded with only very short time delays. The next picture shows a stereoscopic recording of density changes in a transonic turbine cascade flow taking place within 10 microseconds. Due to the small changes in density in most places only one interference fringe is visible.

Especially when using pulsed laser systems digital evaluation of the interferograms is important to enable the averaging of several recordings for obtaining mean values of density distribution in highly turbulent flows. But digital interferogram evaluation also enables the experimentalist to distinguish between density increase and decrease in a pixeled field of data. Two techniques of interferogram analysis are established at the Institute for Thermal Turbomachinery and Machine Dynamics, a "Spatial Carrier Technique" using a two-dimensional Fourier-Analysis of the interference fringe pattern, as well as a "Phase-Stepping" technique where the holographic interferogram is recorded using two reference beams. This type uses a subsequent evaluation of the hologram by digitizing a number of interferograms from this hologram with precisely introduced phase shifts between the reference beams. An overview on these techniques can be found in recent textbooks (se e.g. D.W.Robinson, G.T.Reid, 1993, "Interferogram Analysis", Institute of Physics Publishing, Bristol). For turbine blade cascade applications 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

Vibration analysis using heterodyne holographic interferometry enables multidirectional analysis of vibration and deformation amplitude. The result of such an investigation is the quantitative experimental analysis of the vibration modes in all three-dimensions which can be compared to a numerical finite-element analysis directly. As an example, the following pictures shows the first vibration modes of a gas turbine blade (first bending at 710Hz, first torsion at 1200Hz and second bending at 2340Hz; blade length is 160mm).