The vortex-shedding flowmeter is a relatively new type of instrument which is rapidly
gaining in popularity and is being used as an alternative to traditional differential
pressure meters in more and more applications. The operating principle of the instrument
is based on the natural phenomenon of vortex shedding, created by placing an
unstreamlined obstacle (known as a bluff body) in a fluid-carrying pipe, as indicated
in Figure 16.11. When fluid flows past the obstacle, boundary layers of viscous, slowmoving
fluid are formed along the outer surface. Because the obstacle is not streamlined,
the flow cannot follow the contours of the body on the downstream side, and the
separate layers become detached and roll into eddies or vortices in the low-pressure
region behind the obstacle. The shedding frequency of these alternately shed vortices
is proportional to the fluid velocity past the body. Various thermal, magnetic, ultrasonic
and capacitive vortex detection techniques are employed in different instruments.
Such instruments have no moving parts, operate over a wide flow range, have a low
power consumption, require little maintenance and have a similar cost to measurement
using an orifice plate. They can measure both liquid and gas flows and a common inaccuracy
figure quoted is š1% of full-scale reading, though this can be seriously downgraded
in the presence of flow disturbances upstream of the measurement point and a
straight run of pipe before the measurement point of 50 pipe diameters is recommended.
Another problem with the instrument is its susceptibility to pipe vibrations, although
new designs are becoming available which have a better immunity to such vibrations