Measurement of the Density Inside a Supersonic Jet Using the Background Oriented Schlieren (BOS) Technique

Shock waves are usually modeled as sharp discontinuities in pressure, density and temperature. A Background Oriented Schlieren (BOS) system was set-up to determine the actual widths of the density gradients in these regions of a supersonic jet. The technique consists on a comparison between a reference image of a dot matrix and another image of the same matrix deformed by the flow of a transparent gas. The apparent displacement of the dots in the second image, due to changes in the index of refraction, can be related to changes in the density. The flow is generated when compressed air passes through a straight nozzle with a diameter of 4 mm. Density measurements along the centerline are presented for three Mach numbers: 2.79, 1.89 and 1.79.