When a moving fluid meets a blunt body, it cannot always stay wrapped around it. On the rear side the flow breaks away, leaving a churning low-pressure wake. Watch how a cylinder sheds vortices alternately from each side — the Kármán vortex street.
Flow does not have to stay attached to the body it meets. Around the front of a blunt shape the fluid speeds up and the pressure falls, and everything is fine. But on the rear half the surface curves away and the pressure climbs again. A boundary layer that has already been slowed by friction may not have the energy to keep pushing uphill against that rise, and at the separation point it lifts off the surface.
The free stream sweeps in from the left and meets the cylinder at a stagnation point. Attached flow accelerates around the front. At the two separation points on the rear shoulders the streamlines detach, and behind the body a separation bubble of slow recirculating fluid sits at low pressure. Further downstream the wake carries a speed deficit, and from its edges the upper and lower vortices peel off alternately — the Kármán vortex street.
Separation raises drag sharply and spoils the lift of a wing once it stalls. The alternating vortices are worse than a nuisance: each one nudges the body sideways, and if that nudge matches its natural frequency the vibration can build until something breaks — so cables, chimneys, bridge decks and heat-exchanger tubes all have to be designed around it.
Uniform flow arrives from the left and runs straight into the cylinder. Right at the front the fluid is brought to a halt at the stagnation point, where the pressure is highest.
Squeezed around the curved shoulders, the flow speeds up and the pressure drops. As long as the pressure keeps falling, the boundary layer stays stuck to the surface and follows it easily.
On the rear half the surface falls away and the pressure climbs. The slowed fluid near the wall now has to push into rising pressure — an adverse gradient. It has little energy left, so something has to give.
At the separation points the boundary layer leaves the surface. Behind the body the fluid reverses and swirls in a low-pressure separation bubble, while the streamlines no longer follow the cylinder.
Downstream lies the wake, a band of slow, turbulent fluid. Its edges roll up into vortices, shed alternately from the top and bottom of the body. The fixed pattern they leave behind is the Kármán vortex street.
It is the moment a boundary layer stops following a surface and breaks away from it. It happens when the pressure starts rising along the flow: the slowed fluid near the wall no longer has enough energy to push forward against that rising pressure.
The flow splits at the front and accelerates around the sides, where the pressure drops. On the rear half the surface curves back in and the pressure rises again. The boundary layer, already slowed by friction, cannot climb that pressure hill, so it detaches from the surface.
The wake is the disturbed region trailing behind the body, where the fluid has lost speed and the pressure is lower. It is filled with slow, recirculating fluid and enhanced turbulence, and it is the main source of the drag on the body.
As the separated flow rolls up, it alternately peels off a vortex first from one side of the body and then from the other. The result is a staggered, repeating trail of swirling vortices called the Kármán vortex street.
Each shed vortex pushes the body sideways, and if that pushing matches the body's natural frequency the vibration can grow dangerously. This is why chimneys, cables, bridge decks and heat-exchanger tubes are checked for vortex-induced vibration.