ағын · fluid-mechanics lab
Agynhow rivers move
A living river, built from real physics. Ағын is the Kazakh word for a current — the flow of water. Scroll to read the water, or reach in and stir it.
Enter the current →the lab
Stir the river
The whole background is a live flow simulation — thousands of tracer particles carried by a velocity field, solved fresh every frame. Change the current, thicken the water, pinch the channel, or drop in a rock.
Tap the water anywhere to place a rock — drag it to move it. Raise viscosity and the flow calms; lower it (or speed up) and it tears into turbulence.
01 · continuity
Squeeze the channel, speed up the water
Water is very nearly incompressible, so the same volume must pass every cross-section each second. Watch the banks pinch into a throat above — the streamlines crowd together and the flow accelerates exactly where the channel narrows. Try the channel width slider in the lab: pull it up and watch the throat close while the current races through. Same discharge, smaller area, faster water.
02 · bernoulli
Faster flow, lower pressure
Along a streamline the total energy is shared between pressure, motion and height. Where the water speeds through the throat, the ½ρv² term grows — so the pressure P must fall. That low-pressure throat is why a fast current can pull floating debris inward. Real rivers also bleed energy to friction, so it's never perfectly conserved.
03 · two kinds of flow
Smooth sheets, or chaotic eddies
Slow, thick flow glides in smooth parallel sheets — laminar. Push the same water faster, or thin it out, and it shatters into swirling, unpredictable eddies — turbulent. Real rivers snap between the two; there's no lingering in between. Try the switch:
04 · reynolds number
The ratio that decides
The Reynolds number weighs inertia against viscosity. Low Re: viscosity wins and the flow is orderly and laminar. Push it past roughly Re ≈ 2000–4000 and inertia takes over — the stream breaks into swirling, chaotic turbulence. Flip the switch in the section above and watch this number leap across the threshold.
05 · boundary layer
The banks hold the water back
Water sticks to a solid surface — the no-slip condition. Velocity is exactly zero at each bank and climbs to a maximum in the middle, giving the curved velocity profile now drawn across the stream. That thin sheared skin next to each bank — the ripple texture behind this card is a close-up of it — is the boundary layer. Thicker, more viscous water spreads that drag further into the channel.
06 · vortices & wake
The rock leaves a trail of spinning water
Flow can't cleanly rejoin behind a blunt obstacle. Instead it sheds spinning parcels — vorticity — alternately from each side, forming a von Kármán vortex street in the wake. This is what it looks like in the wild: the same alternating shedding tears a mountain rapid to foam. The shedding frequency follows the Strouhal relation; it's the same effect that makes wires hum and flags flutter.
07 · meanders · erosion & deposition
Bends write themselves into the land
A river almost never runs straight for long. The instant it bends, the fastest current — the thalweg — is flung toward the outer bank, undercutting and collapsing it into a steepening cut bank. On the inside of the bend the current slackens, dropping the sediment it carried to build a gently sloping point bar. Erosion here, deposition there — the bend migrates sideways and downstream, and the meander grows.
live schematic — flow accelerates outward through the bend, sediment settles inward
test the current
Four questions on the river
Everything above, distilled. Pick an answer — you'll get the explanation either way.
Score: 0 / 4