Planar 2D velocity measurements in the cap shock pattern of a thrust optimized rocket nozzle

Abstract

The cap shock pattern is observed in the plumes of over-expanded thrust optimized rocket nozzles at the low pressure ratios that occur during start-up and shut-down transients. These nozzles exhibit both free and restricted shock separation, and the related flow transitions can result in damaging instabilities. Instantaneous 2D planar velocity measurements obtained here with hydroxyl tagging veloci- metry, in which molecular tags are written into the flow by laser photo-dissociation and tracked by laser-induced fluo- rescence, provide a detailed reference data set in the cap shock region. Measured velocity fields are compared to axi- symmetric numerical simulation and are in good agreement at higher pressure ratios. At the low pressure ratios which exhibit the cap shock pattern, the recirculation region dis- cussed in earlier work is predicted by the current numerical simulation but is absent in the experimental measurement.