eFunda: Introduction to Vortex Flowmeters
engineering fundamentals Vortex Flowmeter
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Overview

Vortex flowmeters, also know as vortex shedding flowmeters or oscillatory flowmeters, measure the vibrations of the downstream vortexes caused by the barrier placed in a moving stream. The vibrating frequency of vortex shedding can then be related to the velocity of flow.

Further Information
When a fluid flows steadily over an isolated cylindrical solid barrier and the Reynolds number is great than about 50, vortices are shed on the downstream side. The vortices trail behind the cylinder in two rolls, alternatively from the top or the bottom of the cylinder. This vortex trail is call the von Karman vortex street or Karman street after von Karman's 1912 mathematical description of the phenomenon.

The Karmen street has two significant influences on the principle of operation of vortex flowmeters:

  1. The frequency of vortex shedding is definite and is related to the Reynolds number (flow velocity, viscosity of fluid, and the diameter of the cylinder).
  2. The frequency of vortex shedding is the same as the vibrating frequency of the cylinder induced by the flow.

If the density and viscosity of the fluid are known and the diameter of the cylinder is given, the frequency measured at the cylinder can be used to represent the flow velocity.

Dimensional anaylsis shows that the frequency of vortex shedding fv is governed by the Strouhal number of the vortex pattern

where Dc is the diameter of the cylinder or width of the barrier and V is the flow velocity.

The Strouhal number is a dependent variable of the Reynolds number:

However, experimental investigations show that the Strouhal number is about constant across a wide range of the Reynolds number (102 ~ 107). This yields

The Strouhal number is about 0.18 for a cylinder at a Reynolds number range 300 ~ 107.

Notice that the flow velocity profile, the shape of the bluff (barrier), and the ratio of the cross section area of the bluff to the pipe will all affect the Strouhal number. One would need to incorporate a correction factor when applying the above formula.

Assume that the bluff is a rod across the diameter of the pipe, the width of the bluff is w that is equivalent to Dc in the above formula, and the diameter of the pipe is D. The flow velocity at the bluff is

For incompressible pipe flow, at any given moment, the flow rate is constant throughout the pipe, that is, . Thus, the ratio of upstream flow velocity V to the bluff region flow velocity Vbluff is

where A is the cross section area of the pipe, and Amin is the cross section area of the pipe with the obstructing bluff.

From the above equation, the flow velocity V can be written as

and the volume flow rate Q is

In industrial applications, a K factor is usually introduced to compensate for the non-uniform profile of the pipe flow. The volume flow rate then becomes

The Strouhal number S can be determined experimentally. Some common Strouhal numbers are listed below for reference purposes. The bluff is either a cylinder or a square column.

   w/D      S   
0.10.18
0.30.26
0.50.44

The above formulas assume a steady-state upstream flow. Disturbances on the upstream side may affect the vortex shedding frequency and result in measurement errors.

Common Specifications

Common specifications for commercially available vortex flowmeters are listed below:

  Fluid Phase:

Score Phase Condition
 Gas  Clean 
  Dirty 
 Liquid  Clean 
 Steam  Saturated 
  Superheated 
 Liquid  Corrosive  
  Dirty 
: Recommended
: Limited applicability

  Line Size: Inline model: 10 ~ 1200 mm (0.4 ~ 48 inch)
Insertion model: 75 mm (3 in) and up
  Turndown Ratio: 10: 1

Pros and Cons

  • Pros:
  - Low to medium initial set up cost
  - Not much maintenance needed when used in clean flow conditions
  • Cons:
  - Low to medium pressure drop due to the obstruction in the flow path

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