Evaluation of interferometric patterns of supersonic fluid flows by the differential Fourier transform method

The knowledge of the flow structure of fluids is essential in many industrial sectors as it determines the quality and success of processes like welding, cutting or material treatment. The employed fluid flow plays a fundamental role and in all cases to understand the complex pressure and shock waves patterns becomes necessary. The only way to absolutely ensure that the theoretical behavior predicted by numerical simulations fits with the real gas flow behavior is the experimental check. For such application, the optical techniques are very well suited resources due to their non invasive features. Schlieren, shadowgraph and interferometry are the typical techniques used for the study of the mass flow in fluids. In this work, we present the qualitative results obtained from a conventional interferometric technique combined with a new numerical approach used for analyzing the inner structure of supersonic flows provided by nozzles with Laval profile used in industrial oxy-fuel cutting processes. The extraction of the optical phase from the fringe patterns is performed through a Differential Fourier Transform Evaluation. We show that with further numerical processing we can obtain a complete qualitative description of the flow field including the first and second optical phase derivatives (Gradient and Lapacian phase maps).