Aalborg Vortex Insertion - Manual (Page 6)

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1. GENERAL INFORMATION
1.1 General
Vortex Shedding is not new. Da Vinci observed vortex shedding in 1513. Von
Karman provided an empirically determined formula for it in 1911-1912. One
observes it as eddies downstream of a rock or a piling in a fast moving stream.
Yet, it wasn’t until it was successfully developed and installed in the late 1960’s
that this significant development in the science of flow measurement was given
considerable attention by industry.
1.2 Principles of Operation
With the controlled use of an obstruction installed in a given pipeline, vortices
are generated downstream of the obstruction when fluid or gas flows through
a pipe. A vortex is a swirling low pressure eddy which forms alternately from
side to side of the bluff body. With each eddy, there is formed a corresponding
high pressure pulse opposite the bluff body. (See Figure 1.) These pulses are
directly proportional to the velocity of a given fluid. Each pulse represents given
volumetric units of fluid for a given line size. Since each pulse represents a unit
volume, the output is therefore linear with respect to flow rate.
FIGURE 1
1.3 Sensor Operation
Two pressure sensing devices are mounted internally in proximity to the stainless
steel bluff body. The sensors are piezo electric crystals. The crystals
convert the pressure pulses created by the vortices into voltage signals without
the need of excitation current or voltage. The sensor crystals are never in direct