What Causes Vortex Rings in Water When You Drag a Plate?

TL;DR

Dragging a plate through still water creates half-ring vortex formations that travel across the pool without dissipating. These vortices form due to differences in fluid speed and maintain their shape because of angular momentum, resulting in distinct dark spots on the water's surface, shaped by the balance of gravity, centripetal forces, and surface tension.

Transcript

[SQUEAKING] [POP] Check this out. I'm just gonna drag this plate through the water in the pool. That's it. But look what forms-- perfect black circles on the bottom of the pool. And it's so weird. They don't die. They just keep going right next to each other. I sped this footage up because the circles took 3 minutes to cross the pool and didn't see... Read More

Key Insights

• 💁 Vortices are circular movements in fluids formed by a difference in fluid speed.
• 💦 Half-ring vortices are formed when water is forced to move around an object, creating connected dips on the water's surface.
• 😋 Half-ring vortices stay together because they are connected at the ends, forming a half vortex ring.

Questions & Answers

Q: What are vortices and how are they formed?

Vortices are circular movements in gas or liquid. They are formed when there is a difference in fluid speed, creating a shear force that starts the fluid spinning.

Q: Why do the half-ring vortices stay together?

The half-ring vortices stay together because they are connected at the ends. The movement of water around the whole plate creates a curved vortex line, resulting in a half vortex ring.

Q: Why are half-ring vortices rarely seen?

Half-ring vortices are rarely seen because they occur on the boundary of two media. Most vortices either close into a loop or end at a boundary, making half-ring vortices less common.

Q: How do the dark spots form in the water?

The balance between gravity, centripetal forces, and surface tension creates a funnel-shaped lens. This lens focuses sunlight away from the center, creating a dark circle surrounded by a bright ring.

Summary & Key Takeaways

• Moving a plate through the water forms little vortices, which are gas or liquid moving in circles, caused by a difference in fluid speed.

• The vortices are able to travel across the pool without dissipating due to their angular momentum and only lose energy through friction.

• The half-ring vortices are formed when water is forced to move around the plate, creating connected dips on the water's surface.