AN EXPERIMENT IN ORDINARY MAGIC · NO. 11

The River's
Handwriting.

Place a stone in moving water. Watch the current learn to write loops, spirals, and alternating signatures. No two moments are quite the same.

A miniature flow laboratory
Runs entirely in your browser
No uploads · no server · no account
01 / The currentDrag in the water to stir it. Try changing the viscosity.
THE CURRENT IS MOVING
FLOW →
↻ DRAG TO STIR THE CURRENT
PERIODIC CHANNEL
Turquoise and coral show opposite directions of rotation.Step 0
FIELD NOTE / 001
A disturbance writes itself into the water.
ω = ∂xuy − ∂yux
78
Reynolds number · approx.
0.000
Wake swirl · mean |ω|
0.000
Downstream sideways motion
02 / Something worth noticing

A stone can become a metronome.

At gentle speeds, water can slip smoothly around a round obstacle. Increase the speed or lower the viscosity, and the wake may shed vortices alternately from its two sides. The resulting pattern is called a von Kármán vortex street.

The remarkable part is that nobody is switching the flow left and right. This rhythm emerges from the equations and the instability of the wake.

Re ≈ UL / ν

Try: Start with One boulder, low viscosity, and a faster current. Then increase viscosity all the way and see how the wake changes. Choose Ink streams to watch colored dye follow the water.

03 / Listening to the wake

One point downstream

MEASURING

A fixed virtual probe records how quickly the water moves sideways. Alternating deflections, when they develop, are the wake's signature rhythm.

Upward driftDownward driftZero

This numerical experiment starts with a brief asymmetric disturbance and uses the D2Q9 lattice Boltzmann method. Its boundaries wrap around, and it is not a high-precision river-flow solver. Particularly turbulent settings can be numerically imperfect.