Determination of the scalar index of refraction field with BOS in a supersonic jet

In this work, the scalar field of the refractive index of an axisymmetric supersonic jet was measured with the Background Oriented Schlieren (BOS) technique. The jet was generated by passing compressed air through a straight nozzle with a diameter of 4 mm. An apparently stationary shock pattern had been visualized previously with shadowgraphs and Rayleigh Scattering. However, the use of BOS allowed a precise measurement of the width, a few tenths of a millimeter, of the shock waves. This result is important because usually, when they are modelled, shock waves are considered infinitely narrow discontinuities.

Additionally, real images obtained with schlieren and shadowgraph techniques were compared with images derived from the BOS data. The correspondence is excellent for schlieren but not for shadowgraphs, due to numerical errors.