Wind Tunnel guide

Help

Short answers to what people ask most, and to what goes wrong.

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.

It says it needs WebGPU

The sea is computed on your graphics card through WebGPU. Chrome and Edge on a computer have it, and so does Safari 26; other browsers may not yet. Nothing else on the page needs it.

It runs slowly

Press [ for fewer lattice steps each frame; the sea moves more slowly but every frame comes sooner. A laptop on battery may also slow its graphics card down.

A 4K video stutters or takes long

A 4K frame is four times a 1080p one. Choose 1080p, or a shorter length; the picture on screen holds still while a video records, so nothing is lost.

The flow started again by itself

At a very high Re, or after a sharp edge is drawn, the flow can break. The page notices and starts the island again rather than show a black sea; the reading says so. Lower Re a little, or smooth the land you drew.

A silhouette made no island

The picture needs a clear dark shape on a light ground, or a shape on a transparent ground. A photograph with no clear shape may give a speck or nothing; a drawing or a logo works best.