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What Is Vortex Shedding? | Process Engineering Glossary
What Is Vortex Shedding?
Vortex shedding is a flow phenomenon in which fluid flowing past a bluff, non-streamlined obstacle alternately sheds swirling vortices from opposite sides of the object, creating an oscillating pattern in the wake. This alternating vortex pattern generates a periodic, alternating force on the obstacle itself, with the shedding frequency directly related to flow velocity, obstacle size, and fluid properties through a well-established relationship known as the Strouhal number.
Vortex shedding is both a nuisance to be managed, since it can cause vibration and fatigue in piping and structures, and a useful phenomenon deliberately exploited in vortex flow meters, which measure flow rate by detecting the shedding frequency behind a deliberately placed bluff body.
Applications of Vortex Shedding
Vortex Flow Meters
Vortex flow meters measure flow rate by placing a bluff body in the flow path and detecting the vortex shedding frequency downstream, which relates directly to fluid velocity.
Piping and Structural Vibration Analysis
Vortex shedding-induced vibration is a specific concern for piping spans, stacks, and other slender structures exposed to cross-flow, requiring vibration analysis to avoid resonance with the structure’s natural frequency.
Thermowell and Instrument Design
Thermowells and other instruments inserted into flowing pipe streams are specifically evaluated for vortex shedding-induced vibration risk, since resonance can lead to fatigue failure over time.
Benefits of Knowing Vortex Shedding
Enables accurate, moving-part-free flow measurement. Vortex flow meters provide accurate flow measurement without moving parts, offering reliability advantages in many services.
Prevents vibration-induced fatigue failures. Understanding vortex shedding risk supports designing piping spans and inserted instruments to avoid damaging resonance conditions.
Provides a predictable basis for vibration risk assessment. The well-established Strouhal number relationship gives engineers a reliable basis for predicting shedding frequency and assessing resonance risk.
Limitations to Consider
Resonance risk depends on accurate natural frequency data. Assessing vortex shedding-induced vibration risk requires accurate knowledge of the structure’s natural frequency, which isn’t always straightforward to determine precisely.
Vortex flow meters have specific application limits. Vortex meters require a minimum flow velocity to generate a reliably detectable shedding signal, limiting their suitability for very low flow applications.
Multiphase or highly turbulent flow can complicate shedding behavior. Clean, predictable vortex shedding assumes relatively simple single-phase flow, and multiphase or highly disturbed flow can complicate both meter accuracy and vibration predictions.
Vortex Shedding FAQ
How does vortex shedding relate to flow regime concepts?
While flow regime describes overall multiphase flow patterns, vortex shedding describes a specific single-phase phenomenon occurring around bluff obstacles within a flow.
Why is vortex shedding relevant to fluid mechanics and Bernoulli’s equation applications?
Vortex shedding is a more complex flow phenomenon than the idealized flow Bernoulli’s equation describes, requiring more advanced fluid mechanics treatment to fully characterize, relevant also to pump discharge piping vibration analysis.
How does vortex shedding compare to the Venturi effect in flow measurement applications?
Vortex meters and Venturi meters both measure flow using a physical flow disturbance, though the Venturi effect relies on a pressure-drop relationship rather than an oscillating shedding frequency, both grounded in appropriate line sizing and relevant to avoiding water hammer and critical flow considerations in the same piping system.
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