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Thunk Tank

A fun experiment with the hydraulic hammer is done with a coffee can with a dime sized hole in the center of its bottom.

Fill the can about three fourths full of water, while keeping the hole blocked.

Thunk 1
Stir the water to establish a central depression then unblock the hole.

A whirlpool will develop and its air core will snake its way down toward the hole.

Thunk 2
The exiting water flow rate will be a function of the hole size and the pressure differential between the inside and outside at the hole.
When the air core pierces the hole, the cross-sectional area of the exiting water stream will be suddenly reduced.
Thunk 3
The changing momentum of the moving water will produce an "impulse" which will rapidly deform the bottom of the can.

Presto! You've got yourself a Thunk Tank.

You can get a handle on the magnitude of the flow rate change by comparing how long it takes to empty a given volume of water with and without the vortex. To do it without the vortex, don't stir the water. After filling, let the water settle down to a nearly motionless state before unblocking the hole.