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Unformatted text preview: s can also be used in unconfined flow situations, such as winds or
ocean currents. They can be compact, even microsize with two or three component directions. Figure 6.32 illustrates a handheld wind velocity meter which uses a sevenbladed turbine with a calibrated digital output. The accuracy of this device is quoted
at 2 percent. Vortex flowmeters. Recall from Fig. 5.2 that a bluff body placed in a uniform crossflow sheds alternating vortices at a nearly uniform Strouhal number St fL/U, where
U is the approach velocity and L is a characteristic body width. Since L and St are constant, this means that the shedding frequency is proportional to velocity
f (const)(U ) (6.120) The vortex meter introduces a shedding element across a pipe flow and picks up the
shedding frequency downstream with a pressure, ultrasonic, or heat-transfer type of
sensor. A typical design is shown in Fig. 6.33.
The advantages of a vortex meter are as follows:
5. Absence of moving parts
Accuracy to 1 percent over a wide...
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- Spring '08