IM-P736-02 EMM Issue 3
16
VLM30 In-Line Vortex Flowmeter
Fig. 2 Measuring principle
Here, the frequency f of vortex shedding is proportional to the medium velocity v and inversely proportional
to the width of the bluff body d.
f = St x
v
d
St, known as the Strouhal number, is a dimensionless number, which has a decisive impact on the quality of
vortex flow measurement. If the bluff body is dimensioned appropriately, the Strouhal number (St) remains
constant across a very wide range of the Reynolds number (Re).
Re =
v x D
Kinematic viscosity
D Nominal diameter of meter tube
2.1 How the Vortex Flowmeter Operates
The operating principle of the Vortex flowmeter is based on the Karman street. As the measuring medium
flows over and under the bluff body, vortices are shed alternately above and below. The shedding of these
vortices due to the flow forms a vortex trail (Karman vortex street).
Bluff body
Piezo Sensor
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