I was watching an XKCD “What-If” video recently and Randal off-handedly mentions the title fact as a given. Upon a further Google search I see explanations about why sound moves faster in liquids than gasses but nothing for my specific question. Is there an intuitive explanation for that fact or is it just one of those weird observable facts with no clear explanation

  • Fleur_@aussie.zone
    11·
    2 days ago

    Ahhh in terms of fluid flow? I mean you can move a glass of water as fast as you want (don’t @ me physicists) if the glass is moving at that speed.

    Assuming this is flow.

    Let’s think about a 1 metre long pipe full of water. Let’s make the water start flowing though this pipe by pushing through the water at the start of the pipe. Now as water enters the pipe, water at the other end of the pipe exits. But there’s a time delay between the first new water that enters the pipe and the first water to leave the pipe. (I can explain this further if needed) That time delay is the time it takes the speed of sound in water to travel through the 1m pipe.

    Now let’s consider water moving faster than the speed of sound in water entering the pipe. It’s moving so fast through the pipe that it actually reaches the end of the pipe before the water at the end of the pipe has had enough time to move out of the way. Here flow is effectively dead and the water isn’t really flowing though the pipe anymore, it’s just being pushed through by whatever is moving the water in the first place.

    At the end of the day it’s semantics, you could define flow as being “water moving through a pipe” and in that case yeah, water can “flow” at whatever speed you want if you push it hard enough. Typically though, flow means a steady and consistent stream of fluid. And at supersonic speeds, that stream is anything but steady and consistent.