The relationship of velocity with flow and frequency can be given as f = S × υ/ d, where S is the Strouhal number. The frequency of the vortex formation and shedding depends on several factors: velocity of the fluid ( υ), width of the shedder ( d), and Reynolds number ( Re). Schematic diagram of vortex type flow meter. The frequency is measured and the flow rate is calculated by the flow meter electronics using the above equation.įig. Since the frequency of the voltage pulse is also proportional to the fluid velocity, a volumetric flow rate is calculated using the cross-sectional area of the flow meter.
The pressure changes are sensed by the piezoelectric crystal and converted into a small electrical signal or voltage pulse representing the vortex shedding frequency. The local pressure near the sensor, located strategically, changes every time a vortex is created or in other sheds.
The sensor type employed is often a piezoelectric crystal, the details of which were discussed earlier in the chapter. The frequency associated with vortex creation or shedding is therefore a dependent variable of fluid velocity only as L is constant and S is fairly constant for a particular wide range of Reynolds numbers and independent of the fluid density and viscosity. Reynolds number can be expressed as Re = υ × D/э, where D is the inner pipe diameter and э is the kinematic viscosity. The Strouhal number (S) is a dimensionless number that defines the quality of the vortex flow rate measurements and bears a relationship with Reynolds number as shown in graph of Figure IV/4.2-4b, which is moderately constant in a long stretch of Reynolds number.
The relationship of velocity with flow and frequency can be given as f = S × υ/d, where S is the Strouhal number. The frequency of the vortex formation and shedding depends on several factors: velocity of the fluid (υ), width of the shedder (d), Reynolds number (Re).
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