Wind Tunnel guide

Three looks

Each picture below was made in Wind Tunnel on the cylinder, the textbook body, with only Re changed, after the flow had run for a few seconds.

A wake that lies still

A round post in silver lines, two still eddies behind it — The cylinder at Re 40: two eddies parked behind it, the lines closing again downstream.

Press Cylinder under Island and set Re to 40. The water is thick: it slows along the post's boundary layer, parts behind it and meets again.

The two eddies behind the post turn but stay where they are. Nothing is shed, so the lines downstream run straight.

Setting	Value
Island	Cylinder
Look	Ink
Re	40

Raise Re slowly past 50 and watch the pair start to rock before the first eddy leaves.

A vortex street

Staggered eddies of two colours trailing behind a round post — The cylinder at Re 150 in Vorticity: eddies of one spin, then the other, in two staggered rows.

Set Re to 150 and press Vorticity under Look. Eddies leave the post one at a time, from one side and then the other, each spinning the other way: a vortex street.

Once the beat is steady the reading under the wind adds St, the Strouhal number: about 0.2 for a cylinder, whatever its size.

Setting	Value
Island	Cylinder
Look	Vorticity
Re	150

Press Ink and the same street shows as dye: saffron one way, pearl the other.

A thinner sea

Tight eddies of saffron and pearl dye, curls inside them, trailing behind a round post — The cylinder at Re 1000 in Ink: the eddies roll up tighter, with finer curls wound inside them.

Set Re to 1000. The water is thin now: the eddies roll up tighter, finer curls wind inside each one, and they leave the post more often.

Where the flow shears hardest the Smagorinsky model thickens it a little, so eddies finer than the grid cannot blow it up. Push Re towards 3000 and the street starts to fray.

Setting	Value
Island	Cylinder
Look	Ink
Re	1000

Turn on Tracers to see each speck caught and spun inside the curls.