[Vortex Flowmeters] What is the measurement principle of a vortex flowmeter?

When an obstacle called a vortex shedder bar is placed in the flow, vortices are regularly generated alternately on both sides of the vortex shedder bar due to the viscous effect of the fluid. These vortices are called "Karman vortex street" and are asymmetrically aligned downstream of the vortex shedder bar. The principle that the frequency of vortices is proportional to the flow velocity is applied.

Please refer to the article "Vortex Flowmeters: Measurement Principle and Measured Fluids".