Space travel has fascinated so many of us since Kennedy promised we’d get a man on the moon. Scientists and engineers worked tirelessly to introduce humans to the rest of the universe and society has benefited greatly from their work. Space travel investment created many things from freeze dried ice cream and Tang to new, more accurate methods for heat transfer calculations. One of the huge improvements was the study of supersonic flow and the convergent-divergent nozzle. This nozzle has made space flight, supersonic jets and all kinds high speed transportation possible. It operates using some pretty interesting ideas.
Most of us are only familiar with nozzles as they apply to garden hoses or shower heads, but some of the same basic principles apply to these new supersonic devices. A converging nozzle increases the speed of the flow, like when you cover part of the head of the hose to spray the water farther. A diverging nozzle is a little less common but it reduces the speed of the flow, like when you adjust your shower head so it’s not blasting you. A convergent-divergent nozzle combines those two back to back and produces velocities greater than the speed of sound. If you’re thinking, “Wait, if a converging nozzle speeds up the flow and a diverging nozzle slows down the flow wouldn’t they just cancel each other out?” you’re pretty on top of your game. This is where compressibility effects come into play. Most of the nozzles we’re familiar with use water, an incompressible fluid, while these nozzles use gasses which are compressible. Imagine you’re coming back from a trip up in the mountains and you have a totally full water bottle and an empty one. The change in altitude will compress the air in the empty bottle, leaving it crumpled while the full water bottle will remain essentially the same.
Air can usually be assumed to act like an incompressible fluid for low speed, like figuring out how strong of winds will take down a billboard or how fast a fan can move air. When air speeds start to reach the speed of sound pretty neat things start happening but first we need to look a bit at what sound is. Sound waves are pressure waves that pass through air very fast, the important thing here is that they’re waves of high pressure. Now lets get back to the nozzle, our first section is a converging nozzle, this speeds up the flow. Lets say that the air is coming in really fast, like almost speed of sound fast, then as it passes through the nozzle it reaches the speed of sound. This means that any of those sound pressure waves trying to move back through the air will be caught in the throat of the nozzle. Kind of like when a person is walking towards the back of a subway train just as it’s leaving the station. They are moving back, but the train is moving forward, so a person standing in the station would see the person in the train as stationary. Also the air is moving so fast that not all of it can get through the throat as fast as it would like so it starts pushing and shoving like a bunch of college kids trying to get free food. This makes the throat of the nozzle a very high pressure region, so high pressure that the flow keeps accelerating through the diverging portion of the nozzle, where incompressible flows would start slowing down. The pressure of the air forces it out like gas out of a shaken soda bottle.
This simple design requires a deep understanding of the world we live in, and it provides the foundation for almost everything that moves at or faster than the speed of sound.