Vortex Shedding Frequency Calculator (Strouhal Number)
By Santaji GadepatilinUpdated October 29, 2026
Fluid flowing past a bluff body sheds vortices alternately from each side at a frequency proportional to velocity — the principle of the vortex flow meter and the cause of thermowell vibration. This vortex shedding frequency calculator uses the Strouhal relation f = St·V/d — see how a vortex shedding flowmeter works and vortex flow meter troubleshooting.
How to use the vortex shedding frequency calculator
Enter the flow velocity, the width of the bluff body (or the diameter of a cylinder such as a thermowell) and the Strouhal number. You get the shedding frequency. Add the pipe internal diameter to see the volume flow and an approximate K-factor in pulses per litre.
Vortex shedding formula
f = St × V ÷ d
Q = V × π D² ÷ 4 · K = f ÷ Q
Example
Water at 3 m/s past a 20 mm bluff body (St = 0.2) in a 100 mm pipe:
- f = 0.2 × 3 ÷ 0.020 = 30 Hz
- Q = 3 × π × 0.1² ÷ 4 = 84.82 m³/h
- K ≈ 30 ÷ 23.56 L/s = 1.273 pulses per litre
Shedding frequency for St = 0.2
| Velocity | d = 10 mm | d = 20 mm | d = 40 mm |
|---|---|---|---|
| 0.3 m/s | 6.0 Hz | 3.0 Hz | 1.5 Hz |
| 1 m/s | 20.0 Hz | 10.0 Hz | 5.0 Hz |
| 3 m/s | 60.0 Hz | 30.0 Hz | 15.0 Hz |
| 5 m/s | 100.0 Hz | 50.0 Hz | 25.0 Hz |
| 10 m/s | 200.0 Hz | 100.0 Hz | 50.0 Hz |
| 30 m/s | 600.0 Hz | 300.0 Hz | 150.0 Hz |
Practical notes
- Vortex meters need a minimum Reynolds number (typically about 10,000–20,000) to shed regularly — below that they cut off.
- Typical sizing velocities: liquids 0.3–7 m/s, gases and steam about 2–80 m/s.
- Pipe vibration at frequencies near the shedding frequency can cause false counts; see the K-factor calculator for pulse-to-flow scaling.
Frequently asked questions
What is the vortex shedding frequency formula?
f = St × V ÷ d, where St is the Strouhal number (about 0.2 for cylinders), V the velocity and d the width of the body facing the flow.
Why is the vortex meter linear?
Over a wide Reynolds number range the Strouhal number is nearly constant, so the frequency is proportional to velocity and therefore to volume flow.
Why does vortex shedding matter for thermowells?
If the shedding frequency approaches the thermowell's natural frequency the well resonates and can break by fatigue. ASME PTC 19.3 TW requires checking this frequency ratio.
Is the calculated K-factor exact for my meter?
No. The bluff body partly blocks the pipe, raising local velocity, and real meters are flow-calibrated. Use the meter's stamped K-factor for measurement.