Design of a Schlieren system for low enthalpy hypersonic flow visualization in GHIBLI facility and development of image processing and quantitative analysis codes with preliminary application to sonic free jet

GHIBLI is a 2 MW arc-jet hypersonic facility located at CIRA premises in Capua (Italy), designed for testing candidate TPS materials for re-entry vehicles. Measured data during cold flow tests, i.e. with arc-heater off, showed the achievement of hypersonic conditions at the nozzle exit. Hence, a Schlieren system has been designed to investigate qualitatively and quantitatively such a low enthalpy flow-jet. The apparatus is a classical Toepler’s double lens one. CFD analyses of the free jet were performed to determine the density gradients. On the basis of these results, the limit of sensitivity of the system was determined and the components of the apparatus were dimensioned. A COBLED extended white light source, along with slits made of high reflecting material were experimented. Schlieren images, projected on opaque screen are acquired by a CMOS monochromatic sensor. An image processing code was developed in MATLAB to obtain contrast and clearness enhancement. Quantitative analysis was approached by developing a density-contrast relation, based on schlieren phase-shift effects, modeled under the wave theory of light, and the CMOS tension-charge characteristics. For this purpose, a code named Density from Contrast was developed in MATLAB to measure the luminous intensity of each pixel of captured images and thus compute the density field.