Todor von Karman discovered that when an obstacle is placed in the path of a fluid, the fluid is unable to adhere to an object downstream and separates periodically from one side and then from the other. The fluid is separated by a slow motion in the boundary layer in the body from the downstream side and turns into a vortex.
He also found that the distance between vortices was constant regardless of the flow velocity. The distances between the vortices are constant and only a function of the diameter of the pole d. Therefore, the faster the wind, the faster the vortices will form and the wavelength will not change.
Strouhal later determined that as long as the Reynolds number of the fluid was between 20,000 and 7,000,000, the ratio between the width and distance of the vortex was 0.17. This number is called the strouhal number.
Therefore, if the width of the vortex is known and a detector determines the frequency of the vortex, the speed of each substance can be determined according to the following equation:
Flow velocity = (f * d) / (0.17) = kfd
In the construction of a flowmeter based on the karman principle, the manufacturer usually chooses an obstacle width that is a quarter of the diameter of the pipe. As long as it is not obstructed or clogged, as long as the Reynolds number of the tube is large enough to produce vortices. As long as the detectors are sensitive enough to detect these vortices, the result will be a flowmeter that is sensitive to flow velocity. Other are not sensitive.
When the vortex forms on one side of the body, the velocity of the fluid on the other side increases and the pressure decreases. On the other hand, the speed decreases and the pressure increases, so it causes a change in pressure on both sides of the body. When the next vortex is formed by the opposite side, all the effects are reversed. As a result, the velocity and pressure distributions in the vicinity of the body change with a frequency similar to the changes in the frequency of vortex formation.
Various detectors can be used to measure one of the following:
- Oscillating flow on both sides of the body
- Oscillating pressure difference on both sides of the body
- Flow path created in the body
- Oscillating pressure or flow at the rear of the body
- Presence of free vortices downstream of the body
A flow sensitive detector can be with a heated thermistor element or a spherical magnetic shuttle. Pressure sensitive detectors use metal fins or diaphragms. The pressure applied to the diaphragms can be converted to variable capacity or variable traction in a piezoelectric resistor or induction sensor. The pressure applied to the fins can be easily converted into an electrical signal through any of the aforementioned sensors. Instead, velocity components in free vortices downstream of the fuselage can be used to modulate an ultrasonic beam. Depending on the specifications of the flowmeter sensor system, it is suitable for gaseous liquids or both.
Early detector designs were very sensitive to clogging and required frequent maintenance.
These devices were later replaced by non-clogging units and were solid-state designs.
Most of these designs are still sold on the market and are considered by users who do not care about quick and easy access to the detector or the reliability and sensitivity of heat transfer or ultrasonic detectors. There still seems to be a tendency for detectors to be modularly cheap and interchangeable so that they can be replaced quickly if needed. Several vortex detectors on the market today can be easily replaced.
In this design, the detector is a liquid-filled capsule with a piezoelectric crystal in the center that detects pressure changes from the vortices as it passes through the filling liquid.
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