This photograph is actually a still shot from a video by Tim Dodd “The Everyday Astronaut” of the recent SpaceX Starhopper 150 meter test. The whole video can be found here: https://www.youtube.com/watch?v=lsoS6C0uzGY&t=288s?rel=0. The flow visualization that I want to highlight here is the creation of mach diamonds or pressure diamonds by the rocket engine. In this case, the mach diamonds are being created by a pressure gradient from the outside atmosphere to the supersonic flow of methane exhaust from the rocket. The rocket engine is designed to be more efficient at a slightly higher altitude, meaning the exhaust pressure is lower than optimal at sea level. This causes the supersonic flow to be “pinched” in by the atmospheric pressure until it is concentrated into the first mach diamond. This is the top bright triangle or cone that we can see in the exhaust. The methane flow then expands and contracts again to create the repeating pattern of mach diamonds.
Photography credit: Tim Dodd, “The Everyday Astronaut” on YouTube and Twitter.
Experiment credit: SpaceX, they were not testing flow visualization in their experiment, but for our purposes it is an experiment using combustion to visualize the mach diamonds created by a pressure gradient between supersonic flow and the surrounding atmosphere.