Vortex Shedding Frequency

Overview

Engineering reference data for Vortex Shedding Frequency in standard organizations.

Key Formulas

ISO Standard

$ISO \; 9001:2015$

Quality management systems.

ASTM Standard

$ASTM \; E8$

Standard test methods for tension testing.

ANSI Standard

$ANSI/ASME \; B16.5$

Pipe flanges and flanged fittings.

Variables

Symbol	Description	Unit
$ISO$	International Organization for Standardization	—
$ASTM$	American Society for Testing and Materials	—
$ANSI$	American National Standards Institute	—

Practical Applications in Flow Measurement

The vortex shedding frequency principle is widely used in industrial flow measurement, particularly in vortex flow meters. These meters operate by placing a bluff body in the flow path, causing predictable vortex shedding. By measuring the shedding frequency, the flow velocity and volumetric flowrate can be determined.

Core Measurement Formulas

The primary relationship is given by: $f = \frac{Sr \cdot v}{d}$

Where:

• $f$ is the vortex shedding frequency (Hz).
• $Sr$ is the Strouhal number, a dimensionless constant dependent on the bluff body geometry and Reynolds number.
• $v$ is the approach flow velocity (m/s).
• $d$ is the characteristic width (diameter) of the bluff body (m).

From the measured frequency, the volumetric flowrate $q$ can be calculated using the meter's calibration constant $k$: $q = \frac{A \cdot f}{k}$

Where:

• $q$ is the volumetric flowrate (m³/s).
• $A$ is the cross-sectional area of the pipe or the bluff body's effective area (m²).
• $k$ is the meter factor, a design-specific dimensionless constant.

Key Considerations for Accuracy

1. Strouhal Number (Sr): For a given bluff body shape (e.g., a trapezoidal or circular cylinder), $Sr$ is relatively constant over a wide range of Reynolds numbers, making the frequency method reliable. For common geometries, $Sr$ typically falls between 0.18 and 0.22.
2. Reynolds Number Range: The linear relationship between frequency and velocity holds within a specified Reynolds number range for the meter design. Operation outside this range can affect $Sr$ and accuracy.
3. Installation Effects: Upstream flow disturbances, pipe vibrations, or two-phase flow can interfere with clean vortex shedding and require straight pipe runs or flow conditioners.

References

• Original Source