Vortex flow meters use an obstruction, known as a bluff body, in the flow stream to create downstream vortices which are alternately formed on either side of the bluff body. As these vortices are shed from the bluff body, they create alternating low and high pressure zones that oscillate at particular frequencies directly proportional to the velocity of the fluid. The flow rate can be calculated from the fluid velocity. A basic analogy would be Air: a flag post is an obstruction to the air flow and vortices will form downstream of it. These vortices are visualized by means of a flag.