If a bluff (non-streamlined) body is placed in a flow, vortices detach themselves at regular intervals from the downstream side as illustrated in the diagram below:
In flow measurement the vortex shedding frequency is usually a few hundred hertz (Hz). At Reynolds numbers over 103 the volumetric flowrate, Q, is directly proportional to the observed frequency of vortex shedding f, i.e.
Q = Kf
Where K is a constant determined by the pipe and obstruction dimensions.
The vortices manifest themselves as sinusoidal pressure changes which can be detected by a sensitive diaphragm on the bluff body or by a downstream ultrasonic beam.
Related: How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter
This flowmeter can work at low Reynolds numbers, has excellent turndown (normally 15:1) , no moving parts and minimal head loss.
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