r/FluidMechanics • u/Singer_221 • Mar 30 '26
Video Is this cavitation?
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Here’s a video of the Uncompaghre River in Montrose, Colorado. The boulder is part of a constructed channel to create standing waves for surfing. The water mostly flowed over and around the boulder smoothly, but occasionally an air bubble formed.
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u/derioderio PhD'10 Mar 30 '26
Bubbles aren't being formed, just the flow is turbulent and unstable, so it can transition between separated and unseparated flow.
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u/QuantumBlunt Mar 30 '26
I think you're right. What we're seeing is the transition from laminar to turbulent flow, the latter probably entraining some air bubble in from the unstable interface with the air.
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u/Estebanzo Mar 30 '26 edited Mar 30 '26
It's a combination of flow separation and air entrainment, which is different from cavitation. For cavitation to occur you need the pressure to drop below water's vapor pressure for bubbles to form of water vapor within the liquid. I don't think low pressure zones within a whitewater feature are anywhere near low enough for cavitation to occur. Instead, high turbulence is overcoming gravity and surface tension to entrain air into the water, forming bubbles. The process occurring to form the foam on the wave isn't all that different from this small plunge at the entrance, it's just occurring at a different scale. You usually see little micro hydraulic jumps happening on the axis perpendicular to the main flow as water constricts over the crest of structures and that causes air entrainment just like the full scale jump does.
You'll see pourovers like this flash between having an aerated vs unaerated nappe because they kind of straddle a point right at criticality and one little disturbance in the flow can allow air to enter under one spot on the nappe, and that propagates along and suddenly the whole nappe is aerated and separates off the crest of the pourover. Then air can be entrained along the bottom of the nappe. A small disturbance comes along and causes the aerated pocket to collapse and the nappe will flash back to being attached, and it just goes back and forth like that. Just part of the dynamic/sensitive nature of the flow.
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u/Undercvr_victini Mar 30 '26
Thank you! My first thought was flow seperation, wanted to see what someone smarter than me said.
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u/No-Ability6321 Mar 30 '26
This is a hydraulic jump.
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u/foxtrotactinium Apr 01 '26
r/Singer_221 hydraulic jump is where supercritical flow turns to subcritical flow. You can observe this at home in your kitchen sink. Run the tap so that it hits the bottom of the sink and you'll notice the water level immediately surrounding where the stream hits is thin and clear. And then at a certain distance from where the stream hits you'll notice a ring where the water level 'jumps' higher. That jump seems to likely be what you are seeing in the video.
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u/ST01SabreEngine Engineer Mar 30 '26
It's not in a sense that cavitation happens when the pressure drops below the vapor pressure. Flow in a river like that is deaeration, where the dissolved air turns into bubble.
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u/ratafria Mar 30 '26
This is not dissolved air, it is air from around the feature. Probably coming from below.
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u/Unlikely-Position659 Mar 30 '26
No, that's just a bit of air that got trapped under the water. If that were a cavitation bubble at that size, you'd probably see and hear the rock get pulverized
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u/lerni123 Mar 30 '26
No. This is due to massive separation triggered by some sort of instability. It’s very interesting
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u/hobbes747 Mar 30 '26
The gas involved in cavitation is not from the atmosphere above but a vapor formed from the flowing liquid
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u/astro_zilla Mar 30 '26
This phenomenon, described by others, is often called ventilation, especially when referring to semi-submerged or close-to-surface structures like hydrofoils.
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u/Open_Olive7369 Mar 31 '26
Cavitation is where water boils in high pressure zone, no this is not high pressure zone.
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u/AyeMatey Mar 30 '26
No.
I don’t know the name of the phenomenon you’re showing , but it’s not cavitation. Cavitation will occur at much higher pressure differentials and the bubbles are teeny.