Vortex Flowmeter Working Principle
The Karman vortex street principle relies on inferring flow velocity by using the frequency of vortices generated when a fluid flows through a triangular prism of a specific shape.
When a fluid in a pipeline passes through a vortex generator (triangular prism), local acceleration generates vortices (as shown in Figure 1). These vortices split into two alternating columns, a pattern known as a Karman vortex street. The interlaced separation of the vortices generates pulsating pressure in the wake behind the prism. The detection probe behind (or inside) the prism is subjected to this tiny pulsating pressure, causing alternating stress on the piezoelectric crystal element embedded in the probe, generating an alternating charge signal. The detection amplifier processes this signal through switching, amplification, filtering, and signal shaping, then outputs a voltage (or current) pulse signal with the same frequency as the vortex separation frequency, or, after conversion, an analog current signal proportional to the vortex separation frequency. This allows the flow velocity of the fluid medium to be detected by measuring the vortex frequency signal, and the flow rate to be calculated from the medium's flow velocity.
The vortex separation frequency is directly proportional to the average flow velocity of the fluid and inversely proportional to the width of the bluff body. The relationship is:
f = St × v / d
Where:
f: vortex separation frequency (Hz)
St: Strouhal number (a dimensionless constant that remains essentially constant over a wide flow range)
v: average flow velocity (m/s)
d: characteristic width of the bluff body (m)
The frequency signal generated by the vortex is detected by a sensor (such as a piezoelectric crystal or capacitive probe) and calculated by a processor to obtain the instantaneous and cumulative flow rates.
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