Supersonic airflow in a rocket nozzle
The blog Fuck Yeah Fluid Dynamics posted some stills from this video recently. The images were fantastic, but I didn't totally understand what I was seeing. Thankfully, FYFD blogger (and aerospace engineering Ph.D. student) Nicole Sharp was kind enough to answer my questions.
What you're looking at is a rocket engine. The video shows what happens to airflow in the engine as it goes from subsonic to supersonic. In the video and the pictures, you can see a dark red line moving down the tunnel. That's the edge of the shockwave that marks the boundary between subsonic air and Mach 1. You should also pay attention to the little black vortices that whirl away from the edge of the engine wall. Those are pretty important.
The whole goal of an experiment like this, Sharp told me, is to learn more about shockwave/boundary layer interactions. The boundary layer is a very small region, right against the walls of the engine. It's the point where friction and viscosity matter to the air flow.
"Say I've got an airplane wing and air moving over it at 800 mph," Sharp said. "If I am a meter away from that wing the air doesn't care. It's not impacted by flow. But right at the surface of the wing you have friction, so the air can't just slide along surface. This is the region where the velocity of air changes from 0 to 800 miles per hour. It's millimeters or centimeters, but it's where you have a huge change in velocity."
It's important to study what happens in that boundary layer because this is where turbulence happens. In fact, those vortices you see are turbulence, generated when the impact of the shockwave hits the engine wall. That turbulence can cause pressure fluctuations, Sharp said. And those pressure fluctuations can have real, physical impacts on the structural integrity of the engine.