Hi and welcome back to BrainSnacks in this episode I invite you to discover the supersonic world. When I was a child, I believe that the seconds between a lightning and the thunder corresponded to the kilometres the thunderstorm was away. Well that would mean that the noise made by the thunderstorm would travel at thousand meters per second. Now I know that the sound travels through air at about a third of that speed. Well at least I felt safer when I was little. But not only the sound itself can travel at the speed of sound. Any gas fluid or object that has a velocity above that speed runs super or hypersonic: for example the gas from a volcano eruption, the fluid coming out of a spray can, a supersonic airplane or a spacecraft during the re-entering the atmosphere. And there are some really interesting physics happening there. To understand them, we need some basics which I will explain with the supersonic flow of a gaseous fluid like air. A supersonic flow has a velocity above the speed of sound, which is not a constant but at sea level and 20 degrees Celsius it is approximately 343 m/s. The ratio between the velocity of the flow and the speed of sound is called the Mach number which equals one if the flow is running exactly at the speed of sound. If the Mach number is between one and five the flow is called supersonic and if it’s greater than five it’s called hypersonic. But to understand what happens with the flow at a supersonic speed we have to look at how flows behave in the subsonic speed. Two basic concepts of flow dynamics are the relation between pressure and velocity of a flow and the law of continuity. The first concept can be explained if you think of the pressure as a driving force for the flow. Gas moves from high pressure to low pressure, that means that with a decreasing pressure the flow will accelerate. It’s like when you open a bottle with fizzy water that was shaken. The gas flows out with a tssss, because the pressure inside of the bottle was higher than on the outside. The latter concept can be explained with a flow through a pipe. The mass flow going through the pipe can be defined as dot m equals Rho times u times A with rho us but density, u as the velocity and A as the cross sectional area of the pipe. If the cross sectional area deminishes but the mass flow shall stay constant, the fluid has to accelerate. Rho can be assumed to be constant for low velocities. When we will get to supersonic speeds later, this changes, so keep in mind that the density will play an important role. Let’s stay with that pipe that has a decreasing cross sectional area. Because of continuity the gas inside keeps on accelerating and at one point reaches the speed of sound. Stop!!! Because here some physics reverse. Now the pipe has to increase its for cross sectional area in order to accelerate the flow further. But didn’t we just learn the contrary? This does not make any sense… Well above Mach 1 the flow needs to expand to be able to accelerate. The explanation is not very simple, but I tried to explain it as intuitive as possible and to make a little bit more fun to understand, I created a little comic. A gas is a compressible fluid that means that you can change its density. For example if you have a container filled with air and you decrease the pressure by augmenting the volume the density decreases because the air particles go further apart. Now you have to remember the relationship between pressure and velocity of a flow. With an increasing velocity the pressure drops and with it the density. If the gas flows at a low speed the compressible effects, meaning a decreasing density, are low. When the flow gets very fast, the compressible effects are no longer negligible. At this point the law of continuity comes into play. We look again at the mass flow: when the flow reaches the speed of sound ,the density decreases much faster than the flow can increase its velocity. The only way to have a constant mass flow, is to increase the cross sectional area. This means that a supersonic flow can be created with a pipe that first has a converging part and then a divergent one. This kind of setup is called a de Laval nozzle and it’s used for example for rocket engines. But would it be possible to create a supersonic flow with everyday life objects? Well all you need is a container with the valve that can contain at least twice the atmospheric pressure. If you open the valve then you will get a supersonic flow. That works for example with a spray can or if you open the valve stem of a bicycle tire. Now I hope that you have a basic understanding about how supersonic flows work, I promise you will see or hear the world differently. For example if you listen to the song ‘speed of sound’ by Coldplay, already the title sounds suspicious right? You will hear the following lyrics… meaning that birds fly at 343 m/s, right? The fastest bird on earth is that guy, the peregrine falcon and he can fly at approximately 320 km/h, so approximately 90 meters per second. Not quite the speed of sound, but still respect dude!! Another song that you will hear differently is ‘don’t stop me now’ by Queen. Why let’s see! I want to make a supersonic woman of you!! …Uhm honey, I can’t hear you. I’m moving away from you with the velocity greater than the speed of sound, so what you say will never reach me! I’m sorry it’s because you wanted me to be a supersonic woman… Thanks for watching this BrainSnacks episode and if you like giving your brain little treats, please subscribe and check out my other video about the discovery of Neptune! Bye Bye!!!