Flow-Visualisation Lab / test section 07

Kármán

The wake you cannot see, dyed and lit. Push a steady stream past a cylinder and, above a critical speed, the flow stops rejoining — it peels into a von Kármán vortex street, counter-rotating eddies shedding in alternation and drifting downstream.

OBSTACLE  circular cylinder
MEDIUM    dyed water flume
METHOD    discrete-vortex
STROUHAL  St ≈ 0.20
KÁRMÁN
Live wake render unavailable — the street sheds in software.
LIVE · VON KÁRMÁN STREET
Re 135
fs 2.6 Hz
λ/d 4.3
Shedding
Free-stream speedRecrit ≈ 47
135
Reynolds no.
01

The instability

A slow stream slides around a cylinder and closes neatly behind it — flow in, flow out, symmetric and dull. Nothing sheds. The wake is a quiet standing pair of eddies pinned to the back face.

Raise the speed and that symmetry becomes impossible to hold. The boundary layer separates from each shoulder; the two shear layers behind the body feed each other until, past a critical Reynolds number near 47, one side wins. A vortex rolls up, detaches, and its departure lets the other side roll up in turn.

The result is a street — a staggered double row of counter-rotating vortices, shed first from the top, then the bottom, then the top again, marching downstream at a fixed cadence. The shedding frequency is not arbitrary: it locks to the Strouhal relation, rising in exact step with the flow speed.

fs  =  St · U / d   shedding frequency
Re  =  U · d / ν      flow regime · onset at Re ≈ 47
  1. A
    Attached
    Below onset the stream wraps the cylinder and rejoins. Dye parts, curves, and closes — a symmetric, silent wake.
  2. B
    Separation
    Past onset the boundary layer lets go at each shoulder, feeding two free shear layers into the wake.
  3. C
    Roll-up
    One shear layer curls tighter than the other and pinches off a discrete vortex, breaking the symmetry.
  4. D
    The street
    Shedding alternates top and bottom at the Strouhal cadence, laying a staggered double row down the channel.
02

Reading the wake

Colour is the sense of the spin. Each eddy is dyed by the direction it turns — the street reads as a two-tone braid.

Vortex #5AA9E0
Shed from the top shoulder, turning one way. The dye caught in its core reads cool blue as it drifts down-channel.
Counter-vortex #E07A5A
Its opposite, shed a half-cycle later from the bottom, spinning the other way. Warm orange marks the reversed sense.
Dye rake #93AEBA
A comb of streaklines released upstream. Straight in the free stream, they wind into the eddies that catch them.

Test-section record

Cylinder 07

Filed under bluff-body wakes · two-dimensional flow visualisation

Body
Circular
cylinder, span-uniform
Regime
Laminar
Re 20 – 190
Onset
Re ≈ 47
shedding threshold
Strouhal
St ≈ 0.20
fₛ = St·U/d
Street
Staggered
h/λ ≈ 0.28
Method
Discrete-vortex
+ semi-Lagrangian dye
Tracer
Dye rake
vorticity-coloured
State
Shedding
Re 135