Titan Discuss Flow Meter Vortex Shedding
If you place an obstruction (such as a bluff body) in a fluid stream vortices will be formed downstream of the impediment. If the arrangement is symmetrical the swirls will be shed from alternate sides of the obstruction. The frequency of the vortices will be directly proportional to the fluid characteristics and the velocity of the fluid stream.
Flow Meter Vortex Shedding in Liquids & Gases
This flow meter vortex shedding phenomenon is true for both gases and liquids. Interestingly the flow meter vortex shedding effect can be seen to occur in everyday life. The flapping of a flag is probably the most common example of this effect. Other common examples of the effect include the rattling of wires on yacht masts or the “keening” heard near power lines in high winds. Many metal chimneys or vent pipes have spiral elements on the outer surface these are there to break up the vortex shedding and prevent the tube vibrating. There are numerous methods for detecting the frequency of the vortexes varying from strain gauges on the obstruction in the fluid stream to ultrasonic detection of variations in fluid movement further downstream.
This illustration shows the alternate shedding of fluid known as the Karman Vortex Street effect. The effect may be defined as follows:
Where: f= Karman vortex frequency
St= Strouhal number
D= Width of the bluff body
Putting an obstruction in a fluid conduit and measuring the frequency of the induced oscillations is a relatively simple thing to do and there are a large number of ways to detect the induced disturbances in the flow stream. Generally flow meters based upon ‘Vortex shedding’ require flow in the turbulent region so are most suitable for gases and low viscosity liquids. Currently this technology has had most success in the measurement of steam flow and corrosive or cryogenic liquids where other metering methods cannot handle the operational envelope. Being fluidic devices, the pipes can be blown dry with high velocity air with no damage to the meter.
To discuss specific flow meter requirements or flow meter vortex shedding, please contact Titan here