Measuring principle
The Vortex flowmeter is used for measuring the flow velocity of fluids in pipelines. The measuring principle is based on
the development of a Karman vortex shedding street in the wake of body built into the pipeline. In theory, this process
enables measurements to be carried out in turbulent flows with a Reynolds number Re > 3000, but linear measurements
are only possible where Re > 20,000.
The periodic shedding of eddies occurs first from one side and then from the other side of a bluff body (Vortex-shedding
body) installed perpendicular to the pipe axis. Vortex shedding generates a so-called “Karman Vortex Street” with
alternating pressure conditions whose frequency f is proportional to the flow velocity v. The non-dimentional Strouhal
number S (primary head constant) describes the relationship between vortex shedding frequency F. (in Hz.) width b of the
body, and mean flow velocity v (in m/s.)
F
=
b
v
S
*
The flexural vibration of the vortex-shedding body is picked up in the primary head via sensors and analysed in the signal
converter. In the case of gaseous, flowing media, the vibration frequency ranges between 10 and 7000Hz.
To permit the mass rate of flow to be calculated from the volume rate of flow, either medium pressure and temperature or
medium density at the installation location of the flowmeter must be known factors.
Karman Vortex Street
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