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Does anybody happen to know why the water flows down the spillway in waves? Best seen right in the first GIF. I am curious what the mechanism behind that is. Is

by danbruc 10y ago

Does anybody happen to know why the water flows down the spillway in waves? Best seen right in the first GIF. I am curious what the mechanism behind that is. Is this a general phenomenon of water flowing down an inclined plane or is it caused by other oscillations, waves on the reservoir, resonances in the outlet system, or something along that line?

jwmerrill 10y ago
Not an answer for this particular case, but there are lots of named hydrodynamic instabilities: https://en.wikipedia.org/wiki/Hydrodynamic_stability#Applications https://en.wikipedia.org/wiki/Hydrodynamic_stability#Applica...
dandelany 10y ago
It seems they are called "roll waves" and happen naturally in this circumstance: http://cee.usc.edu/assets/027/88403.pdf http://cee.usc.edu/assets/027/88403.pdf "The uniform, free surface, turbulent water flows down an inclined, wide, rectangular open channel may become unstable spontaneously, if some sufficient conditions are satisfied. As a result, steady hydraulic conditions are interrupted, including the water depth (or pressure) and flow velocity. When the channel is sufficiently long, the small instabilities on the water surface will develop to a series of intermittent shock-like waves separated by sections of the gradually varying flow."
hexane360 10y ago
It basically signifies a loss of energy. You can think of it in the context of collisions, except with a continuous distribution of fluid instead of a discrete distribution of objects. We know that momentum (mv) is always conserved. In this context, we can replace mass with cross-sectional area, because mass in a given cross-section is proportional to the area. V is flow velocity in length/time. So momentum is flow in length^3/time. Energy is 1/2mv^2. If you lose energy, mv is constant and m*v^2 decreases, meaning that v must decrease and m must increase. In this context, that means that the flow is slower with a larger area. This corresponds to an "inelastic collision". In fluid dynamics this is a hydraulic jump. In this situation, most of the energy loss is near the surface, so you end up with moving hydraulic jumps on top of a laminar flow. This doesn't really happen in natural situations, because riverbeds generally aren't this smooth.
- danbruc 10y ago
  This doesn't really happen in natural situations, because riverbeds generally aren't this smooth. When I first noticed it, I was reminded of waterfalls, especially the way they were portrayed in 8 bit games. [1] It turns out not to be that easy to find a good real world example, at least not without watching hours of waterfall footage, but this one [2] does an acceptable job showing it. I have no idea if this could be caused by the same or at least a similar mechanism. [1] https://www.youtube.com/watch?v=qfOKHuzn1Hs https://www.youtube.com/watch?v=qfOKHuzn1Hs [2] https://www.youtube.com/watch?v=7L3m_3aRiHE&t=190 https://www.youtube.com/watch?v=7L3m_3aRiHE&t=190
1_2__3 10y ago
Thanks for asking; I was always curious myself.

by danbruc 10y ago

jwmerrill 10y ago

Not an answer for this particular case, but there are lots of named hydrodynamic instabilities: https://en.wikipedia.org/wiki/Hydrodynamic_stability#Applications https://en.wikipedia.org/wiki/Hydrodynamic_stability#Applica...

dandelany 10y ago

It seems they are called "roll waves" and happen naturally in this circumstance: http://cee.usc.edu/assets/027/88403.pdf http://cee.usc.edu/assets/027/88403.pdf "The uniform, free surface, turbulent water flows down an inclined, wide, rectangular open channel may become unstable spontaneously, if some sufficient conditions are satisfied. As a result, steady hydraulic conditions are interrupted, including the water depth (or pressure) and flow velocity. When the channel is sufficiently long, the small instabilities on the water surface will develop to a series of intermittent shock-like waves separated by sections of the gradually varying flow."

hexane360 10y ago

It basically signifies a loss of energy. You can think of it in the context of collisions, except with a continuous distribution of fluid instead of a discrete distribution of objects. We know that momentum (mv) is always conserved. In this context, we can replace mass with cross-sectional area, because mass in a given cross-section is proportional to the area. V is flow velocity in length/time. So momentum is flow in length^3/time. Energy is 1/2mv^2. If you lose energy, mv is constant and m*v^2 decreases, meaning that v must decrease and m must increase. In this context, that means that the flow is slower with a larger area. This corresponds to an "inelastic collision". In fluid dynamics this is a hydraulic jump. In this situation, most of the energy loss is near the surface, so you end up with moving hydraulic jumps on top of a laminar flow. This doesn't really happen in natural situations, because riverbeds generally aren't this smooth.

danbruc 10y ago
This doesn't really happen in natural situations, because riverbeds generally aren't this smooth. When I first noticed it, I was reminded of waterfalls, especially the way they were portrayed in 8 bit games. [1] It turns out not to be that easy to find a good real world example, at least not without watching hours of waterfall footage, but this one [2] does an acceptable job showing it. I have no idea if this could be caused by the same or at least a similar mechanism. [1] https://www.youtube.com/watch?v=qfOKHuzn1Hs https://www.youtube.com/watch?v=qfOKHuzn1Hs [2] https://www.youtube.com/watch?v=7L3m_3aRiHE&t=190 https://www.youtube.com/watch?v=7L3m_3aRiHE&t=190

1_2__3 10y ago

Thanks for asking; I was always curious myself.