When most people think of supersonic speeds, they might picture a sonic boom or military jets. But one of the coolest things that happens when the sound barrier gets broken is what shows up in jet engine exhaust. I’m talking, of course, about shock diamonds, and you don’t have to be a fighter jet to make them.
By Avery Thompson
In case you’re unfamiliar, shock diamonds, also called mach diamonds or mach disks, occur when jet exhaust travels faster than the speed of sound in atmosphere. Due to complex factors, supersonic exhaust tends to exhibit repeating wave patterns, as seen in this video:
Essentially, shock diamonds are formed because of pressure differences between the exhaust jet and the surrounding air. Initially, the exhaust is at a higher pressure than the air, which causes the exhaust jet to expand. However, it expands too much and the pressure drops, causing the jet to collapse and form the diamonds. These shock diamonds raise the pressure of the jet again, repeating the process.
But there’s nothing about shock diamonds that’s exclusive to jet aircraft. Shock diamonds can form in any supersonic jet stream, even if that jet is made of water. Even a simple bottle rocket, using water and pressurized air, creates supersonic exhaust and shock diamonds. You can see those bottle rocket shock diamonds in this video by Vision Research:
Perhaps the coolest thing about shock diamonds is that you don’t have to travel to the nearest military airfield to spot them. You can witness the exact same effect in your own backyard for only a few dollars, provided you have a high-speed camera to capture them.
This article was originally written for and posted by Popular Mechanics USA.