Sri Lanka
free · no sign-in · nothing uploaded
Save
1920 × 1080 · H.264 · 20 s
For a seamless loop, record at least 20 s and cut on a shed.
4K asks a lot of the graphics card; if it stutters, choose 1080p.
The link carries the island, the wind, Re, the look and the colours.
Island
Famous vortex streets, the ones satellites photograph:
Wind
Re 150: a clean vortex street.
Look
Colour
Draw
Drag raises land · shift-drag sinks it
What this is
Wind Tunnel blows a steady wind past a real island and lets you watch the wake it leaves: the twin trail of eddies that satellites see downwind of Guadalupe, Jeju or Sri Lanka. The sea is a lattice-Boltzmann fluid computed on your graphics card, thousands of steps a second; the islands are real coastlines. Turn the wind, raise the Reynolds number, draw an island of your own or open a silhouette, and save a still or a video at 1080p or 4K. It is free, needs no sign-in, and nothing is uploaded.
How to use it
- Pick an island or a preset under Island, or Draw and drag in the frame to raise land; Open a silhouette turns any picture's dark shape into an island. The famous vortex streets, Guadalupe, Jeju, the Canaries and the rest, can sit beside their NASA satellite photo: turn on Satellite photo.
- Turn the wind with From and raise Re: below about 47 the wake lies still, above it the eddies peel off left and right in turn. Explain the picture labels what you are seeing. Ink, Paper, Vorticity, Cloud and Speed show the same sea five ways, in a palette or your own colours under Colour.
- Choose 1080p or 4K and the seconds, then press PNG for this moment (with a transparent sea if you like) or MP4 to record the flow from now; Copy a link sends the same sea, wind and colours to someone else.
Real islands meet the wind at a Reynolds number in the billions; the air's own turbulence acts like a thicker fluid, which is why their wakes look like the ones here.
Keys: Space pause · R reset · E or S PNG · [ ] fewer or more steps · 1 to 5 looks · T tracers · X explain the picture · C satellite photo · L copy a link · ? this guide.
Coastlines from Natural Earth (public domain).
Read the guide
Words: vortex street · Reynolds number · Strouhal number · lattice Boltzmann · vorticity · WebGPU
Common questions
What is a von Kármán vortex street?
The double row of eddies a steady flow sheds behind a blunt body, one from each side in turn, each spinning the other way. Theodore von Kármán showed in 1911 why the staggered row is the one that lasts. Clouds draw it downwind of islands such as Guadalupe and Jeju; in Wind Tunnel you watch it form from the first eddy.
What does the Reynolds number change?
Re is the island's width times the wind's speed divided by the water's viscosity: how far the flow's push outweighs its stickiness. Low, two eddies sit still behind the island. Past about 50 they peel off in turn and the street forms; past several hundred it turns ragged. In Wind Tunnel, Re sets the viscosity; the wind's speed stays the same.
Can I download 4K video, and is it free?
Yes. Choose 4K or 1080p and 5, 10, 20 or 30 seconds, then press MP4: the flow is recorded from that moment as an H.264 MP4 at 3840 × 2160 or 1920 × 1080, made in the page and saved straight to your device. It is free, needs no sign-in, and nothing is uploaded.