How do the Vortex Flow Meters work?

The working principle of the vortex flow meter:

If a non-streamlined vortex-generating body (bluff body) is set in the fluid, two rows of regular vortices will be generated alternately from both sides of the vortex-generating body. This kind of vortex is called Karman vortex street. When the fluid flows through the vortex-generating body set in the flow field at a certain flow rate, a pair of alternating and neatly arranged vortex columns (vortex streets) are generated downstream of the column, first on one side of the column, and then Generated on the other side of the cylinder. This theory of vortex generation was first proposed by Karman, named Karman vortex street, and gave the relationship between frequency and flow velocity, and the coefficient was named Strohal number.