Sierra 241-VTP, 240-VTP instruction manual Vortex Shedding Frequency, Vortex Frequency Sensing

Chapter 1 Introduction Series 24-HP Instruction Manual

Vortex Shedding Frequency

Von Karman vortices form downstream of a shedder bar into two dis- tinct wakes. The vortices of one wake rotate clockwise while those of the other wake rotate counterclockwise. Vortices generate one at a time, alternating from the left side to the right side of the shedder bar. Vortices interact with their surrounding space by over-powering every other nearby swirl on the verge of development. Close to the shedder bar, the distance (or wave length) between vortices is always constant and meas- urable. Therefore, the volume encompassed by each vortex remains con- stant, as shown below. By sensing the number of vortices passing by the velocity sensor, the Innova-Mass™ Flow Meter computes the total fluid volume.

Velocity sensor
Vortex shedder bar		Vortices
Flow
	Constant
	wave length

Figure 1-2. Measurement Principle of Vortex Flow Meters

Vortex Frequency Sensing

The velocity sensor incorporates a piezoelectric element that senses the vortex frequency. This element detects the alternating lift forces pro- duced by the Von Karman vortices flowing downstream of the vortex shedder bar. The alternating electric charge generated by the piezoelec- tric element is processed by the transmitter’s electronic circuit to obtain the vortex shedding frequency. The piezoelectric element is highly sensi- tive and operates over a wide range of flows, pressures and temperatures.

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