Fluidic flowmeters are a class of flowmeters that generate oscillations as
a result of flow. The number of oscillations can be related to the rate of flow passing
through the flowmeter. Vortex shedding flowmeters are a specific type of fluidic flowmeter.
Other fluidic flowmeters include designs based upon the Coanda effect and vortex precession.
Vortex shedding flowmeters present the flow in a pipe with an obstruction
in the general shape of a bluff body or strut. At low flow rates, the fluid simply goes
around the bluff body (or strut). As velocity increases, alternate vortices are formed
(shed) on each side of the bluff body (or strut) and travel downstream. The number of
vortices formed is proportional to the velocity of the fluid, such that doubling the
flow will form twice as many vortices. A variety of electronic and mechanical techniques
can be used to sense the vortices. The frequency of vortex formation is used to generate a
flow measurement signal.
Bluff body vortex shedding flowmeter designs have shedder bars that have a
width of approximately 20 percent of the inside diameter of the pipe. As a result, the
pressure drops associated with these designs are similar. It is advisable to use supplier
information to determine the actual pressure drop across these flowmeters. For estimation
purposes, one rule of thumb is that the pressure drop from a water flow at 5 meters per
second is approximately 400 mbar differential (or approximately 15 feet per second and 5
pounds per square inch respectively). Pressure drop varies as the square of the flow rate
such that doubling the flow will result in four times the differential pressure across the
flowmeter.
The relatively thin strut shedder designs reduce the loss of hydraulic
energy across the flowmeter (pressure drop). Reducing the pressure drop across the
flowmeter can conserve hydraulic energy in some applications, such as when a pump or
fan is controlled with a variable speed drive. Note that, in many installations,
installing a flowmeter with a lower pressure drop in place of a flowmeter with a
higher pressure drop can cause the pressure drop to be transferred from the flowmeter
to the control valve, and result in no energy savings.
TCoanda effect fluidic flowmeters contain passages or other hydraulic
mechanisms that allow a portion of the downstream fluid to be fed back near the inlet of
its fluidic oscillator. By impacting the incoming fluid, the feedback flow causes the
main flow to preferentially attach itself to the opposite surface of the flowmeter.
This increases the opposite feedback flow and forces the main flow away from that
surface. This process repeats and causes flow in the feedback passages to oscillate
in proportion to flow, such that doubling the flow will create twice as many
oscillations. A variety of electronic and mechanical techniques can be used to
sense the feedback flow oscillations. The frequency of feedback flow changes is
used to generate a flow measurement signal.
In vortex precession fluidic flowmeters (often called swirl flowmeters), a
static element is used to impart rotation to the incoming fluid and cause the fluid to form a
vortex downstream that resembles a cyclone. The downstream portion of the vortex rotates around
the axial centerline of the pipe. In other words, looking through the flowmeter in the downstream
direction, the downstream portion of the vortex is rotating in a circle at the pipe wall. A vortex
breaker is installed at the outlet of the flowmeter body to stabilize the vortex and to keep it from
propagating downstream where it can disturb the process or other hydraulic devices, such as control
valves. The speed with which the vortex rotates is proportional to the flow rate, such that doubling
the flow will cause the vortex to rotate twice as many times. A variety of electronic and mechanical
techniques can be used to sense number of vortex rotations. The frequency of vortex rotation is used
to generate a flow measurement signal.
