STRUCTURE OF UNDEREXPANDED SUPERSONIC JETS FROM AXISYMMETRIC LAVAL NOZZLES

The rainbow schlieren deflectometry is combined with the computed tomography to obtain three-dimensional density fields of a shock-containing free jet issued from a Laval nozzle with a design Mach number of 1.5. Experiments have been performed at a nozzle operating pressure ratio of 4.0, where the nozzle is operated at a slightly underexpanded condition. Multidirectional rainbow schlieren pictures of the jet can be acquired by rotating the nozzle about its longitudinal axis in equal angular intervals and the three-dimensional density fields are reconstructed by two types of image analyses for experimental data. One is a data processing procedure based on the Abel inversion algorithm, and the other is that with the convolution back-projection algorithm. It is found that excellent quantitative agreement is reached between the jet three-dimensional density fields reconstructed from both algorithms. The present rainbow schlieren technique is found to be significantly effective to examine a fine shock-cell structure of the shock-containing jet and makes it possible to provide experimental data with high accuracy and reliability on the jet density field.