Kármán vortex street

When a fluid flows past a blunt obstacle, the flow can separate from its surface and create a wake. Under suitable conditions, rotating eddies are shed alternately from opposite sides of the obstacle. Downstream, these eddies form two staggered rows with opposite senses of rotation: the characteristic vortex street. The pattern is not inevitable in every flow. Its development and regularity depend on factors including the obstacle's shape and size, the speed and properties of the fluid, and the surrounding flow conditions. In laboratory and atmospheric studies, researchers use dimensionless quantities such as the Reynolds and Strouhal numbers to compare flows and describe vortex-shedding behaviour; there is no one frequency law that applies universally across all cases.

A related visible example occurs when prevailing winds pass islands or other elevated land. The terrain disturbs the air flow, and in a moist layer the resulting alternating vortices can organize clouds into a chain of spirals downwind. NASA imagery has documented such patterns in the wakes of Guadalupe Island in the Pacific and Tristan da Cunha in the South Atlantic. NOAA reports satellite observations of these cloud formations around islands in several ocean regions. The clouds make the motion visible, but they are tracers of the airflow rather than the vortices themselves.

Theodore von Kármán's work helped establish the fluid-dynamical understanding of vortex streets, and NASA describes him as first describing the process behind the long, spiralling cloud patterns in 1912. Modern satellites allow researchers to observe the evolution of atmospheric streets across broad areas and over time. A 2020 study using high-resolution satellite winds analyzed the Guadalupe Island case, while a separate 2014 study used numerical simulations to examine periodicity in mesoscale atmospheric streets. That study found a Strouhal-number range of 0.15–0.22 in its simulated cases; this is a result for the conditions investigated, not a universal constant for every vortex street.

Alternating vortex shedding creates fluctuating forces on the obstacle and can produce vibration in structures exposed to wind or flowing water. Engineers study these oscillations when designing bridges, towers, chimneys, pipes, and marine structures. In meteorology, cloud streets provide a way to study wind patterns and how terrain affects atmospheric flow. A vortex street is a wake pattern, not a standing line of permanent whirlpools: individual vortices are carried downstream and evolve as they interact with the surrounding fluid.