Vortices shedding

Vortex-shedding flow meters typically provide 1% of flow rate accuracy over wide ranges on Hquid, gas, and steam service. Sizes are available from 25 to 200 mm. The advantages of no moving parts and linear digital output have resulted in wide usage in the measurement of steam, water, and other low viscosity Hquids. 

For flow past a cyhnder, the vortex street forms at Reynolds numbers above about 40. The vortices initially form in the wake, the point of formation moving closer to the cylinder as Re is increased. At a Reynolds number of 60 to 100, the vortices are formed from eddies attached to the cylinder surface. The vortices move at a velocity slightly less than V. The frequency of vortex shedding/is given in terms of the Strouhal number, which is approximately constant over a wide range of Reynolds numbers. 

Above Re = 10 the vortex shedding is difficult to see in flow visualization experiments, but velocity measurements still show a strong spectral component at St = 0.2 (Panton, p. 392). Experimental data suggest that the vortex street disappears over the range 5 X 10 < Re < 3.5 X 10 , but is reestablished at above 3.5 X 10 (Schhchting). 

FIG. 6-56 Computational fluid dynamic simulation of flow over a square cylinder, showing one vortex shedding period. (From Choudliuty, et al., Trans. ASME Fluids Div, TN-076[1994].)... 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

Vortex-Shedding Flowmeters These flowmeters take advantage of vortex shedding, which occurs when a fluid flows past a non-streamlined objec t (a Blunt body). The flow cannot follow the shape of the object and separates from it, forming turbulent vortices or eddies at the object s side surfaces. As the vortices move downstream, they grow in size and are eventually shed or detached from the objec t. 

Occasionally, a nonrotational measurement device can generate pulse outputs. One example is the vortex shedding meter, where a pulse can oe generated when each vortex passes over the detec tor. 

VoHex shedding The vortex-shedding frequency of the fluid in cross-flow over the tubes may coincide with a natural frequency of the tubes and excite large resonant vibration amplitudes. 

Acoustic Coupling When the shell-side fluid is a low-density gas, acoustic resonance or coupling develops when the standing waves in the shell are in phase with vortex shedding from the tubes. The standing waves are perpendicular to the axis of the tubes and to the direction of cross-flow. Damage to the tubes is rare. However, the noise can be extremely painful. 

At the fan-tip speeds required for economical performance, a large amount of noise is produced. The predominant source of noise is vortex shedding at the traihug edge of the fan blade. Noise control of aircooled exchangers is required oy the Occupational Safety and Health... 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

Vortex shedding anemometer A device for measuring air velocity by placing an obstruction in a gas flow and measuring the frequency of vortex is formation downstream of the obstruction. 

Differential Pressure Variable Area Uagmeter Mass Row Vortex Shedding Turbine. 3.1.2 Level... 

In flowing water enviroments a tubular rather than a solid rod cantilever anode may be used to give improved resistance to fatigue failure, since the anode design may result in fatigue failure by vortex shedding at high water velocities. 

Fatigue failure due to underdesign or changes in plant operation of cantilever anodes in flowing electrolytes can occur as a result of vortex shedding. However, with proper design and adequate safety factors these failures can be avoided . 

The vibration induced by the fluid flowing over the tube bundle is caused principally by vortex shedding and turbulent buffeting. As fluid flows over a tube vortices are shed from the down-stream side which cause disturbances in the flow pattern and pressure distribution round the tube. Turbulent buffeting of tubes occurs at high flow-rates due to the intense turbulence at high Reynolds numbers. 

Helical strakes (strips) are fitted to the tops of tall smooth chimneys to change the pattern of vortex shedding and so prevent resonant oscillation. The same effect will be achieved on a tall column by distributing any attachments (ladders, pipes and platforms) around the column. 

Pagni, P.J., Pool fire vortex shedding frequencies, Applied Mechanics Review, 1990, 43, 153-70. 

Vortex-induced separator, 22 62-63 Vortex-induced vibration, 11 756 Vortex meters, mass flowmeters, 20 681 Vortex patterns, 11 755 Vortex precession meters, 11 668 Vortex shedding, 11 756 Vortex shedding meters, 11 668-669 Vortices, superconductor, 23 824-825 Voyage charters, 25 327 VP Sandoflam 5085, 11 491 V-type inks, 14 326 Vulcanizable silicone rubber, 25 129 Vulcanizable silicones, properties and applications of, 22 594-595, 596-597 Vulcanizates EPDM, 10 713 EPM/EPDM, 10 715 ethylene-acrylic elastomer, 10 698,... 

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