Karman vortx street - from Wikipedia.org
A vortex flow meter works based on the principle of 'Kármán vortex street' demonstrated in the figure on left side. This principle essentially means that when an obstruction is placed in path of a flow (similar to a flagpole in the wind), it can produce a series a vortices alternating from each side of the obstruction as seen in the figure. The frequency of alternating of these vortices is proportional to the flow rate being obstructed.
The vortex type flow meter uses a small rod called 'shredder bar' or 'bluff bar' to shred vortices from both sides in an alternating manner. As shown in the vortex flow meter schematic, ultrasonic type transmitter and receivers are used downstream the 'shredder bar' to send ultrasonic signals crossing the path of these vortices. These signals are used to measure the frequency of vortices for changing sides. These signals are sent to a software to be converted to fluid velocity and flow rate readings.
Use of only a small 'shredder bar' or 'bluff bar' in the path of the flow, means very low pressure drop across the flow meter. It also allows high sensitivity of the readings at low flow rates, meaning large turn down ratios.