Under certain conditions vortices regularly form downstream of an object (bluff body) placed in a flowing fluid and shed on alternating sides of the stream. This phenomenon is commonly called a Karmen vortex street. The resultant vortex frequency is proportional to the fluid velocity and hence the flow rate can be calculated by counting the number of vortices being produced. This is achieved placing a pressure transducer, thermistor, or ultrasonic sensor just behind the bluff body which picks up the high and low pressure and velocity fluctuations as the vortices move past the sensor.
Vortices will not be shed regularly in medium to high viscosity fluids. Typically, a minimum Reynolds number of 10,000 is required to obtain optimum performance from a vortex flow meter.
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