A real life problem, just for fun

[catkin]

Homework Statement

Around one of our buildings we have an "ant channel" of roughly square cross section (say 20 cm x 20 cm) with fish in it to control the mosquito larvae. The surface of the water carries particles giving a good indication of any surface movement.

Late one afternoon we observed the surface moving relatively quickly (say 3 cm/s). Suspecting a leak we followed the moving surface water and were amazed to find that the moving surface gave way abruptly to a still surface. Watching a little longer we observed that the interface between moving and still areas of the surface was itself moving -- at about half the velocity of the moving surface.

It appeared as if there was a discontinuity in the flow of water.

Homework Equations

None.

The Attempt at a Solution

I think I can explain the phenomenon. If no one gets it, I'll give clues.

 

[nova-ex]

Hm. I'm not sure. But can I hazard a guess? There's a change in the depth of the water?

 

[ogb p]

Based on the information provided, it seems like the fish in the "ant channel" are causing the surface water to move as they swim around. This movement of the water creates a discontinuity in the flow, where the surface water is moving at a faster velocity than the still water below. This can be explained by the Bernoulli principle, which states that as the velocity of a fluid increases, its pressure decreases. So the fish swimming at the surface are creating a lower pressure area, causing the surface water to move faster. As the surface water moves, it creates a disturbance in the still water below, causing the interface between the two to move as well. This is known as a boundary layer, where the velocity of the fluid changes rapidly near a surface. It's possible that the fish are creating this boundary layer, which is why the interface between the moving and still water is also moving. This is just a hypothesis and would require further investigation and experimentation to confirm.