Laval nozzle flow characterization by Fourier-transform Mach-Zehnder interferometry

The role of the assist gas blown by a nozzle during the laser cutting process of ceramics is very important as a complex flow field is created by the interaction with the material. Flow visualization provides valuable information related with the properties and characteristics of the process in order to clear up misconceptions and as a first step towards the nozzle optimization. Optical methods as Schlieren or interferometry are suitable techniques for this task as well-known non-intrusive full-field methods utilized for analysing fast transient flow phenomena. In this work a Mach-Zehnder interferometer is employed in order to characterize a Laval nozzle by studying the shock wave patterns in a free gas jet as a previous step in order to study the complex interaction of the gas flow against a transparent model of the processed material. The optical phase is extracted from the obtained high-frequency fringe pattern by the Fourier Transform Phase-Difference Method. Disturbing effects are cancelled by proper combination of high-frequency fringe patterns obtained with and without gas flow. The shock wave pattern is analyzed for different geometrical configurations and operating pressures.