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E-Zine January 2008Vortex shedding and other fluidic effects are oscillations that occur when fluids pass by an object or obstruction. Examples of these effects in nature include the whistling caused by wind blowing by the branches of trees, the swirls produced downstream of a rock in a rapidly flowing river, and the waving of a flag the wind. Note that in all of these examples, when the flow is slowed, the phenomenon ceases. That is, the whistling stops when the wind dies down, the water flows calmly around the rock when the river is not flowing rapidly, and the flag does not wave in a mild breeze. Fluidic flowmeters are a class of flowmeters that generate oscillations as a result of flow. The number of oscillations can be related to the rate of flow passing through the flowmeter. Vortex shedding flowmeters are a specific type of fluidic flowmeter. Other fluidic flowmeters include designs based upon the Coanda effect and vortex precession. Vortex shedding flowmeters present the flow in a pipe with an obstruction in the general shape of a bluff body or strut. At low flow rates, the fluid simply goes around the bluff body (or strut). As velocity increases, alternate vortices are formed (shed) on each side of the bluff body (or strut) and travel downstream. The number of vortices formed is proportional to the velocity of the fluid, such that doubling the flow will form twice as many vortices. A variety of electronic and mechanical techniques can be used to sense the vortices. The frequency of vortex formation is used to generate a flow measurement signal. Bluff body vortex shedding flowmeter designs have shedder bars that have a width of approximately 20 percent of the inside diameter of the pipe. As a result, the pressure drops associated with these designs are similar. It is advisable to use supplier information to determine the actual pressure drop across these flowmeters. For estimation purposes, one rule of thumb is that the pressure drop from a water flow at 5 meters per second is approximately 400 mbar differential (or approximately 15 feet per second and 5 pounds per square inch respectively). Pressure drop varies as the square of the flow rate such that doubling the flow will result in four times the differential pressure across the flowmeter. The relatively thin strut shedder designs reduce the loss of hydraulic energy across the flowmeter (pressure drop). Reducing the pressure drop across the flowmeter can conserve hydraulic energy in some applications, such as when a pump or fan is controlled with a variable speed drive. Note that, in many installations, installing a flowmeter with a lower pressure drop in place of a flowmeter with a higher pressure drop can cause the pressure drop to be transferred from the flowmeter to the control valve, and result in no energy savings. Excerpted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters ISSN 1538-5280 |
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