Wind Tunnel guide

Overview

Wind Tunnel is a fluid flow simulator in your browser: it blows a steady wind past a real island and shows the wake it leaves, the vortex street, eddies shed in turn from either side, the pattern satellites photograph in the clouds downwind of islands.

A dark sea of silver lines bending round Sri Lanka, saffron and pearl eddies trailing downwind
Sri Lanka in the south-west monsoon at Re 150, in Ink: saffron and pearl eddies peeling off in turn, the silver lines bending round them.

The sea is a lattice Boltzmann fluid: every cell of a grid holds nine populations of particles, the D2Q9 lattice, that stream to their neighbours and collide, thousands of steps a second, in compute shaders on your graphics card through WebGPU.

The islands are real coastlines from Natural Earth (public domain), turned so the wind always blows from left to right; the needle beside From shows where north lies in the picture.

Land is a wall the water cannot cross. What streams into it is sent straight back (bounce-back), so a thin, slow boundary layer forms along each shore, and where it leaves the shore the separated water behind the island turns back on itself.

The Reynolds number decides what happens next. Low, two eddies sit still behind the island. Past about 50 they peel off one at a time, each spinning the other way, and the street forms. Higher still it turns ragged.

The reading under the wind gives the steps a second, the relaxation time τ that sets the viscosity, and, once the shedding is steady, the Strouhal number measured behind the island.

Ink lets dye into the flow in thin streaklines and colours it by which way it spins; Vorticity shows the spin itself; Paper, Cloud and Speed are three more ways to read the same sea.

Save this moment as a PNG, or record 5 to 30 seconds of the live flow as an H.264 MP4, at 1920 × 1080 or 3840 × 2160. It is made in the page, free, with no sign-in, and nothing is uploaded.

Open Wind Tunnel Start here

In this guide

  1. StartWhat you need and your first wake in five steps.
  2. Every controlEvery control in the panel, one line each.
  3. Three looksOne body at three Reynolds numbers.
  4. Keep your workThe PNG, and the MP4 at 1080p or 4K.
  5. KeysEvery shortcut, and what each one does.
  6. HelpWhat to do when something goes wrong.
  7. Use it forFive real uses, each with a link that opens it ready.
  8. IslandsTen famous islands: the photo, the reason, the simulation.