Oscillatory flowmeters employ vortex formation and the Coanda effect, both of
which produce a digital or pulse output that comes from the natural dynamics and
physics of the fluid. This natural pulse relates frequency to flow rate and is produced
without moving parts, which makes these flowmeters quite reliable.

The Vortex Shedding Flowmeter

The vortex shedding phenomenon is nothing new. It occurs in nature. The first
recorded observation was by Leonardo da Vinci more than 400 years ago when he
noted the formation of vortex swirls downstream of a rock in a stream of water. At
that time, while interesting to observe, the phenomenon was of no practical value.
It required modern electronics to make some use of this phenomenon.

Operating Principle

When a flowing medium strikes a non-streamlined object or obstruction, it separates
and moves around the object and passes on downstream. At the point of the
contact with the object, vortex swirls or eddy currents separate from the object on
alternating sides. When this occurs, the separation or shedding causes a local
increase in pressure and a decrease in velocity on one side of the object, and a
local decrease in pressure with corresponding increase in velocity on the other
side of the object (see Figure 14-1). After shedding from one side, the process is
reversed and a swirl or vortex is shed on the other side of the object. In this way
the vortex swirls are shed continuously-180 degrees out of phase with each
other. The frequency of the shedding process is proportional to the velocity of the
material flowing past the object known as the bluff body.

An illustration of the vortex shedding phenomenon is a flag waving in the wind.
As the wind passes the flag pole, the pole acts as the bluff body, causing vortices
to be shed. As the vortices are shed by the pole, the vortex swirls past the flag with
the alternating high and low pressure areas on either side of the flag. These alternating
high and low pressure areas cause the flag to wave in the wind.

Another illustration that demonstrates the principle is a canoe paddle being
pulled through a body of water. As the paddle is pulled through the water, a series
of vortices (known as a vortex street) is generated. Vortex swirls being shed on
alternate sides from the edges of the canoe paddle can be seen in the water. The
paddle trying to twist can be felt-first toward one side, then the other - as the
vortex being shed generates the high and low pressure areas at the edge of the paddle
blade. As the vortex street moves downstream, the swirl increases in pressure
and decreases in velocity until eventually the swirl ends and normal flow resumes.