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Vortex Shedding and Fluidic Flowmeters
(Part 2 of 4)

By David W. Spitzer

E-Zine February 2008

Click here to review Part 1

Coanda 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.

Fluidic flowmeters (vortex shedding, Coanda effect, and vortex precession) operate linearly within specific constraints. These constraints are functions of fluid velocity and Reynolds number. Both sets of these constraints must be satisfied for the flowmeter to operate properly.

In many vortex shedder and fluidic flowmeter designs, the fluid provides hydraulic energy to operate the sensing system. When the fluid velocity is low, the fluid cannot provide the sensing system with sufficient hydraulic energy, so the flowmeter ceases to operate. Therefore, when the fluid velocity falls below this minimum velocity constraint, the flowmeter will turn off. More sensitive sensing system designs allow measurement of somewhat lower fluid velocities. Fluid density can significantly affect the minimum velocity constraint.

For example, liquid applications typically allow measurement of velocities above approximately 0.3 meters per second (1 foot per second) of water. Liquids with higher densities will operate the sensing system at lower flow rates, so the minimum velocity constraint is lower. Conversely, lower density liquids will increase the minimum velocity constraint.

Click here to read Part 3

Excerpted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters

ISSN 1538-5280

Spitzer and Boyes, LLC
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