This measurement principle is based on the fact that downstream of a barrier formed vortices (vortices) in the fluid, in a closed pipe and in an open channel. It is possible to observe this phenomenon, for example, vortices ("zone of turbulence") formed downstream of the bridge abutment.
The frequency of shedding of the vortices on either side of the pillar (solid body) is proportional to the average velocity of fluid flow and, therefore, the flow rate. Already in 1513, Leonardo da Vinci described the formation and vortex shedding behind a stationary obstacle in a fluid stream.
Figure: Left: vortex shedding behind a bridge abutment. Right: Photo taken from a satellite that is sensitive to the vortices formed in the cloud layer by the effect of a volcanic peak (arrow).
In 1878, Strouhal studying a scientific description of the vortices formed behind solid barriers. Their studies revealed that a cable stretched across a stream of air oscillate. Found that the frequency of this oscillation is proportional to the velocity of air stream. We observe this phenomenon in our own car or house, the wind climbed to produce passes through a slit is due to vortex shedding, and increases or decreases as the speed changes. This phenomenon is called "wind tone."
The Strouhal number used in this context describes the relationship between the frequency of vortex shedding, flow velocity and diameter of the solid (see Figure below):
St = f * d / v
St: Strouhal number
v: Flow velocity
f: Frequency of vortex shedding
d: Diameter of the solid
Figure: Principle of Vortex flowmeters measure.
d = Diameter of the solid, f = vortex shedding frequency, v = velocity of fluid
L = distance between two vortices
The physicist Theodore von Kármán laid the theoretical basis for the measurement of flows with vortex flowmeters in 1912, when he described what has been called "zone of turbulence". His analysis of the double row of vortices formed behind a solid body in a fluid flow revealed a fixed relation between the transverse distance (d) separating the two rows and the longitudinal distance (L) separation between vortices in the same row. If, for example, the obstacle is cylindrical, this ratio is 0.281. Thus, for a uniform pipe diameter, the volume of each vortex is constant. If we assume that the vortices are the same size regardless of the different conditions of performance, then count the number of vortices per unit time directly gives an estimate of the flow.
Formation of vortices and solid geometry:
The fluid reaches its maximum speed at the widest part of the solid, and from that point it loses some of its speed. The flow tries to rid the body contour (a) instead of embroidery. Beyond the point (a) the pressure drops and reflux occurs, and ultimately, vortices (b). These vortices are shed alternately on each side of the solid body and transported by the fluid (nomogram vortex shedding frequency
Figure: Training and vortex shedding
Solid obstacles Vortex flowmeters vary by manufacturer. They come in rectangular, triangular, spherical, delta or more specifically, for the various proprietary models. In each model, the Strouhal number should be kept constant for the whole range of measured values, in other words, this whole field of measurement values, the vortex shedding frequency to be independent of pressure, temperature and density. In this field of measured values of constant Strouhal number (Re> 20,000) Vortex flowmeters work (see Figure below).
The solid obstacles in the form of delta show a nearly ideal linear behavior and have proven particularly reliable. NASA engineers have undergone solid-body model exhaustive studies. The accuracy of the measurement with this geometry can be vL ± 1%, and reproducibility is around 0.2%.
Features of Vortex flow meters are usually defined in terms of "parameter K". This parameter represents the number of vortices that are detected per unit time (pulses per unit volume). The manufacturer gets the parameter K for the calibration of the device and includes this information on the instrument nameplate. This parameter depends on the geometry of the solid and the size of the pipe.
Example:
In a Vortex flow meter (DN 50 / 2 ") whose parameter K is 10 pulses per liter, each pulse corresponds to a volume of 0.1 liters, whether the fluid is water, steam or other fluid.
Ill.: Strouhal number (Str) for various solid obstacles junction of the Reynolds number (Re), a = Solid body in the form of delta, b = Solid body in the form of a sphere.
