Instantaneous and Average Structure of a Supersonic Underexpanded Jet

The instantaneous and average structure of a supersonic underexpanded jet is studied numerically and experimentally. The photos obtained in experiments with different exposure times and the Pitot pressure measurements are compared with the results of the numerical simulation performed using an implicit large-eddy method. We note that the jet flow instability, disturbance growth, and transition to turbulence lead to the situation, in which the instantaneous flow structure can be considerably different from the average structure. The flow pattern observable in the calculations is in good agreement with that presented in the experimental photos obtained with short exposure times. Both calculated and experimental data indicate that an important role in the jet flow dynamics is played by large-scale vortex structures that exist against the background of small-scale turbulence. The calculated and experimental Pitot pressure distributions are similar with each other, up to a certain distance from the nozzle exit section. Further downstream, the experimental and calculated Pitot pressures start to increase rapidly but the calculations predict the onset of this growth at a greater distance from the nozzle than it is observable in the experiments.