Abstract
A rainbow schlieren system with a spatial resolution of 20 μm is employed to determine the hue and density profiles in an axisymmetric domain with a field of view of 100 mm in diameter. The rainbow schlieren method is applied for the underexpanded sonic jet from a cylindrical nozzle with larger length-to-diameter ratios. Experiments have been performed in a range of nozzle operating pressure ratios from 2.0 to 5.0. As a result, rainbow schlieren images show jet boundaries, expansion waves, and oblique shock waves in the shock-cell structure by continuous color gradation. Density profiles at any cross-section of the jet flows can be obtained from the Abel inversion. The schlieren images and density contour plots for high nozzle pressure ratios show the archetypal characteristics of shock cell structures appearing repeatedly in the flowfield. Also, an analytical model to predict the flow properties at the exit plane of the cylindrical nozzle is proposed. It is shown that the analytical results are in good quantitative agreement with the experimental ones from the rainbow schlieren.
