ABB FS4000, Operating Instruction

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Design and function
18 FV4000 / FS4000 D184B097U02
2 Design and function
2.1 Principles of measurement
2.1.1 Vortex flowmeter
The operating principle of the Vortex flowmeter is based on the Karman street. As the fluid flows
over and under the solid body, vortices are shed alternately above and below. The shedding of
these vortices due to the flow forms a vortex trail (Karman street).
G00680
1 2
Fig. 6: Principle of measurement, FV4000
1 Solid body 2 Piezo sensor
The frequency
f of vortex shedding is proportional to the flow velocity v and inversely
proportional to the width of the solid body d :
d
v
St f
× =
St , known as the Strouhal number, is a dimensionless number which has a decisive impact on
the quality of vortex flow measurement.
If the solid body is dimensioned appropriately, the Strouhal number
St will be constant across a
very wide range of the Reynolds number Re (Fig. 7).
ϑ
×
=
D v
Re
ϑ
= Kinematic viscosity
D = Nominal size of meter tube