The Story behind Instruments Like the Rosemount Vortex Flow Meter

In 1881, Theodor von Karman was born in Hungary. He became well known as a mathematician and physicist and for his work in aerodynamics. However, von Karman also made another important discovery in the early 19th Century. While on a fishing trip to the Southern Carpathians in Romania, he noticed water rushing down mountain streams and realized something amazing was happening. This led to an important principle called the Karman vortex street and eventually useful instruments like the Rosemount vortex flow meter.

What Did Karman Discover?

Water passes over a large rock. The rock in the stream is the source of eddies or vortices. Vortices are little swirling tornadoes or whirlpools and this might not seem like a major discovery but Karman looked deeper into the water. He saw the vortices and realized that a bluff body (partial obstruction) causes fluid passing by, to separate and eventually curve back on itself. He also noticed the space between the vortices remained constant, as long as the bluff body size did not change. But what does this have to do with the Rosemount vortex flow meter?

While the vortex (formed by the rock) moves down the stream it becomes stronger and larger and the vortex eventually sheds or dissipates. This also happens with airflow and you can see vortices moving and shedding as a flag ripples in the wind.

Vortex Meters

A typical vortex meter utilizes components like a bluff body, sensor, transmitter and housing. Vortices formed by gas or liquid are shed in an alternating fashion.

Vortex shedding creates the famous “Karman vortex street”. It contains pressure conditions with frequency proportional to flow velocity. This is the basics behind equipment like the Rosemount vortex flow meter. This principle must also be taken into account when designing buildings and large structures like bridges, for withstanding high winds.

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