Video credit: Phred Petersen (RMIT University, Melbourne, VIC) with assistance from Phil Taylor (Phantom Camera Field Applications Engineer)
This is a slow-motion (50,000 fps) video of a toy bottle rocket “launching” off of its pad. Using Schlieren photography techniques, the differences in flow density appear as a range of different colors. As the pressurized fluid (probably water; this is not specified by the authors) exits the bottle, it quickly encounters the much lower atmospheric pressure. The fluid mechanics are complex, but the end result is a shockwave is produced some distance from the nozzle. The pattern repeats itself several times, producing the standing wave pattern seen above. Eventually, the fluid pressure equalizes to the surrounding pressure, and no more shockwaves can be seen. Although at much lower pressure, this demo illustrates a well-known phenomena- “shock diamonds”- seen in the supersonic exhaust of rocket engines and some jet engines.